A Review of the Impact of Extreme Weather Events on Freshwater, Terrestrial and Marine Ecosystems

ABSTRACT

Extreme weather events (EWEs), including floods, droughts, heatwaves and storms, are increasingly recognised as major drivers of biodiversity loss and ecosystem degradation. In this systematic review, we synthesise 251 studies documenting the impacts of extreme weather events on freshwater, terrestrial and marine ecosystems, with the goal of informing effective conservation and management strategies for areas of special conservation or protection focus in Ireland. Twenty-two of the reviewed studies included Irish ecosystems. In freshwater systems, flooding (34 studies) was the most studied EWE, often linked to declines in species richness, abundance and ecosystem function. In terrestrial ecosystems, studies predominantly addressed droughts (60 studies) and extreme temperatures (48 studies), with impacts including increase in mortality, decline in growth and shift in species composition. Marine and coastal studies focused largely on storm events (33 studies), highlighting physical damages linked to wave actions, behavioural changes in macrofauna, changes in species composition and distribution, and loss in habitat cover. Results indicate that most EWEs lead to negative ecological responses, although responses are context specific. While positive responses to EWEs are rare, species with adaptive traits displayed some resilience, especially in ecosystems with high biodiversity or refuge areas. These findings underscore the need for conservation strategies that incorporate EWE projections, particularly for protected habitats and species.

INTRODUCTION

Evidence is mounting of accelerating pressures on ecosystems and biodiversity, including protected habitats and species, due to pollution and habitat degradation and fragmentation from a range of land-use and other anthropogenic drivers including extreme weather events. Disturbances to ecosystems by climate change have been described in terms of a press (long term) and pulse (extreme events) framework (Harris et al. 2018). Climate change brings a suite of additional pressures to the existing multi-stressor environment that has the potential to alter ecosystems at every level of functioning (Díaz et al. 2019; Weiskopf, et al. 2020). Climate change in Ireland has led to increased average air temperatures, with an average rise of nearly one degree in a century (Nolan 2015; Nolan and Flanagan 2020), altered rainfall distribution patterns (Walther et al. 2021; Stroh et al. 2023) as well as increased frequency of extreme weather events (Gleeson et al. 2013a; 2013b; Donnelly 2018). Variable impacts on biodiversity related to the more gradual climate change have been reported from Ireland and Great Britain (Sharkey et al. 2013; Martay et al. 2017; Donnelly 2018). In Ireland, the main impacts so far recorded include a shift towards an earlier onset of spring activity in plants, bird species, and insects, and possible ramifications for bird species abundance and spatial distribution (Donnelly et al. 2015; Donnelly 2018).

Irish marine ecosystems have also experienced adverse impacts from climate change such as surface temperature increases of c. 0.5°C over the past decade, sea level rise of 2–3mm per annum since the early 1990s, ocean chemistry changes linked to increased uptake of CO₂, acidification of off-shore surface waters with an overall reduction in pH of 0.02 units per decade, and the presence of harmful algal species throughout the year (Nolan et al. 2023). Due to its location on the edge of the Atlantic, Ireland is vulnerable to extreme sea levels, with wave and storm surges increasing threats to coastal communities, including notable wave overtopping events recorded in Dublin and Galway. Overall these disturbances have impacted marine life, such as shifting the distribution and abundance of key plankton, fish and seabird species (Nolan et al. 2023).

In addition to the more general press nature of climate change, pulse pressure from extreme weather events such as droughts, extreme temperature anomalies, flooding, and storms, have been shown to have far reaching consequences on ecosystems. Consequently vertebrate and invertebrate populations globally have experienced resource bottlenecks leading to extinction events and significant declines in population levels (Maron et al. 2015). The authors also identified droughts (amphibians, mammals and reptiles) and storms (birds) as providing particular challenges for certain taxa (Maron et al. 2015). Regional extinctions and large reductions in population size and species richness were also reported in a global review of conservation implications of extreme events by Maxwell et al. (2019). However, this study also reported positive outcomes—generally species responding to increased resource availability in the aftermath of floods (Maxwell et al. 2019). Similarly, Neilson et al. (2020) reported both negative and positive impacts on species from extreme events and specifically noted that some species compensated either behaviourally, physiologically or demographically in the face of extreme weather, reducing potential impacts by up to 86%. These studies highlight the diverse impacts extreme events can have on species and ecosystems and the need to understand potential risks to Irish ecosystems of important conservation value such as Special Areas of Conservation (SACs) and Special Protection Areas (SPAs). SACs under the Habitats Directive (92/43/EEC) and SPAs focussing on wild birds (nesting, breeding and overwintering) as required by the Birds Directive (79/409/EEC), are both part of the Natura 2000 network for protection of priority habitats (e.g. raised and blanket bogs) or species (e.g. pearl mussel). These are central to local and global efforts to protect representative habitats and threatened species.

To our knowledge, the existing global research has not been assessed to determine the potential impacts of extreme weather events on Irish ecosystems and identify knowledge gaps. In this paper we

1. 1. Synthesise the reported impacts of extreme weather events, as opposed to more gradual climate change, on the biota of aquatic and terrestrial habitats with specific relevance to Special Areas of Conservation (SACs) and Special Protection Areas (SPAs).

2. 2. Discuss which impacts are relevant to a temperate climate and the habitats in Ireland. While the focus is mainly on temperate regions, we have included papers from other regions that may shed some light on the likely impact of extreme weather events on the species groups of interest in Ireland.

METHODOLOGY

DETERMINING KEY TERMINOLOGY AND DEFINITIONS

Terminology fundamental to undertaking the literature review was identified in advance. This was divided into two categories:

• • Environmental/ecological – words/phrases related to species, communities and ecosystems affected by extreme weather events. These mostly referred to broad-scale population and ecosystem dynamics (Table B.1) that experienced changes as a result of said events.

• • Meteorological/climatic – words/phrases related to extreme weather or climate events or episodes under consideration (Table B.2). These generally impacted localities or regions and were distinct from (but could still be influenced by) those prevailing climatic conditions in said localities or regions.

A final list of terminologies was agreed between the team members and definitions for included words/phrases identified from published literature and other relevant sources were compiled (e.g. Intergovernmental Panel on Climate Change (IPCC 2023), United Nations (UN 2023)). These compilations informed the construction of working definitions (Tables B.1, B.2).

Extreme events can be defined in various ways (Table B.3) and can take many forms. However, of relevance to this review are the meteorological events that are exacerbated by anthropogenic climate change, and which present specific risks to SACs and SPAs. These are likely to include heat waves, forest and peatland fires, snowstorms, severe frosts, drought, intense precipitation, inland and coastal flooding and storm surges (Stocker et al. 2013; Herring et al. 2018).

LITERATURE SEARCH

Environmental/ecological and meteorological/climatic terms and phrases identified as being of key importance were used to construct a search matrix, grouping terms depending on the type of event, the ecological measure or index that was impacted and the level at which this was studied (from population to ecosystem), creating a total of 840 possible combinations (Table 1).

Based on an initial scoping search, Scopus (Elsevier) and Web of Science (WoS; Clarivate) were identified as the most relevant databases. The searches were undertaken between 18/07 and 27/07/2023, inclusive. The majority of the 840 search combinations were used to obtain results from both literature databases (1540 searches in total). Some combinations were eliminated immediately during the initial searches due to very low levels of relevance of the returned results.

Table 1.

Matrix of search terms used in this review.

Fig. 1.

Flow chart of inclusion and exclusion process for the primary literature review as well as the secondary search focussed on Irish ecosystems following the PRISMA framework (Page et al. 2021). The final figures includes all studies cited in the results for this review.

In total, 191,336 results (n = 90,483 (Scopus), 100,853 (WoS)) were retrieved from multiple searches (Fig. 1). The results for each search combination were assigned a ‘relevancy score’ from 0–5, based on how many of the top 10 results were deemed as having inclusion potential through review of their titles. Only results from search terms that obtained a score of 3 or higher were included. This narrowed the number of search terms under consideration to 264 and reduced the number of potential publications to 109,158 (n = 47,916 (Scopus), 61,242 (WoS)). These were exported to Rayyan (Ouzzani et al. 2016), a web application for organising literature lists for screening, which was used to determine inclusion and exclusion of individual studies. The duplicate detection feature in Rayyan detected 96,011 potential duplicates, with similarity scores of 95% or higher.

Upon further review of the remaining results it was decided that only results including the terms ‘extreme weather’ or ‘extreme event’ would be considered for eligibility, isolating 1,182 documents. Those deemed not relevant or where aspects of the investigation were not consistent with the review criteria (arbitrary ‘extreme’ values, focusing on climatic values vs. isolated events, large scale or global studies not relevant to Irish ecosystems etc.) were excluded from the review. To ensure studies with specific relevance to Irish ecosystems were included, a secondary search was conducted using the search term (Ireland OR Irish) AND (‘Extreme’) AND (‘Cold spell’ OR ‘Drought’ OR ‘Flood’ OR ‘Heat wave’ OR ‘Heatwave’ OR ‘Storm’) AND (‘Aquatic’ OR ‘Carbon sequestration’ OR ‘Carbon storage’ OR ‘Conservation’ OR Ecolog* OR Ecosystem OR ‘Greenhouse gas’ OR ‘Terrestrial’). This resulted in the identification of a further 28 documents. After screening and removal of duplicates already included in the original search, 11 additional references were identified for inclusion. Six studies were added during the data extraction phase through consultation with researchers with specific expertise in ecosystems of relevance to Irish SACs/SPAs. Thus, the final number of studies from which information was extracted to form this review was 251 (Fig. 1).

The included studies were organised in table form with data extracted on type of study (modelling vs experimental), type of ecosystem(s), community(ies) studied, protected species/habitat/area studied, types of extreme event(s) studied, nature of extreme event(s) assessed, impact of extreme event(s), management option(s) recommended, and knowledge and research gaps identified. The resulting analysis is presented in the following sections in a narrative way organised into sections and subsections as follows: ecosystem type - extreme event type - community impacted.

RESULTS

IMPACTS ON FRESHWATER ECOSYSTEMS

Freshwaters are included here because of their relevance to protected species such as salmon, otter and many bird species. Impacts may be direct or indirect through changes, in for example, invertebrate communities, which are essential food resources for many species. Although the potential impact of climate change on freshwater ecosystems is widely covered in the published literature, there are relatively few papers that focus specifically on extreme weather events. Many of the results from studies of extreme events, particularly those from field observations, report an interplay between several associated stressors. For example, drought may lead to elevated water temperatures, or flooding may increase flows and inputs of nutrients and sediment in the case of rivers, or reduce water retention times and alter stratification profiles in lakes. Ecological responses depend on the nature and timing of the extreme weather event or succession of different events, the ecological elements investigated, and the habitat characteristics.

Flooding and extreme pecipitation events

Extreme flooding can negatively impact all biological elements from phytobenthos to fish, such as the salmon. In terms of invertebrates the key impacts include: 1) Reduction in abundance, density and richness rather than complete species loss (Feeley et al. 2012; Piniewski et al. 2017); and, 2) Changes in community structure in terms of taxa representation (de Eyto et al. 2016; Piniewski et al. 2017) and trait characteristics (Chiu and Kuo 2012; Feeley et al. 2012 Piniewski et al. 2017).

A study in an Irish humic lake ecosystem showed that flooding can lead to destabilisation of the water column and changes in carbon cycling, depressing primary production and increasing carbon emissions to the atmosphere for extended periods after an extreme precipitation event (de Eyto et al. 2016). However, the extent of the impact depends on the timing of the extreme event, with more drastic effects reported for a summer extreme precipitation event as opposed to a winter event (de Eyto et al. 2016; Kelly et al. 2020). Bacterial communities in this lake were shown to be relatively resilient and had returned to pre-disturbance conditions within two weeks after the extreme event (Hoke et al. 2020). In a systematic review of literature on the impact of storm events (including extreme precipitation) on phytoplankton, Stockwell et al. (2020) found that the responses were highly variable and context dependent, making it difficult to draw general conclusions from the complexity of interactions.

Feeley et al. (2012) is among the few studies on effects of extreme flooding on rivers in Ireland. They reported up to 85% reduction in the abundances of species following a high intensity rainfall event in 2011. Although numbers were reduced, the diversity of some groups such as Plecoptera and Ephemeroptera were unaltered and thus provided for recovery in abundances. In addition to reduction in abundances, Trichoptera, Coleoptera and Diptera did lose species. Similar results were reported by Giller et al. (1991) for another severe flooding event in Ireland and from high gradient cold water streams in North America by Snyder and Johnson (2006) and Mundahl and Hunt (2011). These studies suggest physical habitat damage is the primary cause of the detected macroinvertebrate impacts from scouring and downcutting of the stream bed, stripping most instream vegetation and causing the deposition of additional boulders, coarse and fine substrates (Fig. 2). However, biological and geomorphic (e.g. channel incision and bank scouring) responses can differ between catchments due to differences in channel bed structure, gradient, and catchment vegetation cover (Robertson et al. 2015). Furthermore, changes in water chemistry may also need to be taken into account, for example, an influx of nutrients to rivers (Wu and Xia 2019), ponds (Shabarova et al. 2021), and lakes (de Eyto et al. 2016). Protected species such as the pearl mussel (Margaritifera margaritifera L.) may be particularly vulnerable to extreme flow that can dislodge adults (e.g. Hastie et al. 2003) and associated high sediment that smothers juvenile habitat (Baldan et al. 2021), although moderate flow can clear the gravels of fine sediment.

In terms of traits, shifts to r-selected taxa, i.e. fast-growing, small body size, and with an opportunistic strategy, have been reported, however functional diversity was retained, thus conveying resilience (Woodward et al. 2015). This has also been reported for diatom assemblages (O’Driscoll et al. 2012). In contrast, K-selected taxa had traits that conveyed resistance to flooding disturbances (Chiu and Kuo 2012; Woodward et al. 2015). Reduced invertebrate abundance from flooding can have subsequent impacts on reproduction and survival of aquatic birds such as the dipper (Hong et al. 2016), but there is little evidence to support impacts at population level (Sánchez et al. 2017).

Fig. 2.

Stream bed and banks before (A) and after (B) the extreme flooding event described by Feeley et al. (2012). Figure originally published as Fig. 2 in Feeley et al. (2012) and reproduced here under author copyright permission.

The potential and time for ecological recovery is a key issue in terms of risk assessment and adoption of potential mitigation measures (de Eyto et al. 2016). While some studies have found evidence of recovery between six months and three years after the flooding event (Snyder and Johnson 2006; Mundahl and Hunt 2011; Behan 2014; de Eyto et al. 2016), some impacts on specific species (Trichoptera, Behan 2014), total abundances (Mundahl and Hunt 2011), or habitat and trophic structures (Snyder and Johnson 2006), can remain visible in all cases, and may persist for decades (Snyder and Johnson 2006). Feeley et al. (2012) proposed that repeated flooding events would possibly reduce local recolonisation sources and thus, conceivably limit the potential for recovery to pre-event conditions, at least in the short term. In addition, the antecedent conditions of lake ecosystems can influence the impact of, resilience to and resistance to extreme storm events (Stockwell et al. 2020; Thayne et al. 2023). Timing of extreme flooding events was also noted as an important factor in a study on Irish turlough ecosystems (Morrissey et al. 2021). The increased frequency of occurrence of extreme flooding events in late winter in lowland karst catchments is predicted to delay the growing season for wetland grasses and flora in local turlough ecosystems, likely resulting in the loss of some habitats (Morrissey et al. 2021).

The review by Piniewski et al. (2017) highlighted that fish responses to flooding were generally not significant and in contrast to invertebrates. While richness and abundance values decreased by 70% and 32% for invertebrates, these metrics actually increased by 31% and 11%, respectively, for fish. However, in the case of salmonids, eggs and juveniles can be flushed from benthic gravels during high flows, with the actual impact depending on the degree of washout and where the displaced eggs and juveniles end up (Smialek et al. 2021). High flow events can also lead to reduced juvenile recruitment (MacDonald, 2005). de Eyto et al. (2016) also noted the importance of timing of an extreme precipitation event in relation to its potential impact on protected Atlantic Salmon (Salmo salar L.) smolts. Extreme precipitation occurring in April and May could disrupt zooplankton assemblages, a primary food source for smolts in Lough Feeagh at this time of the year.

Severe flooding and high water may allow fish to move further upstream in river systems (e.g. Cochran and Stagg 2011). Vincenzi et al. (2017), working on marble trout, reported that fish spawned at an earlier age post flash floods and exhibited faster growth. They considered this to be a transient response most likely due to lower fish densities, and less competition with older fish during the spawning season. However, they emphasised the potential loss of within-population genetic diversity and therefore the long-term sustainability of this fish population. The timing of a flood event may influence the level of impact on fish populations, for example, a flood event in the wet season as opposed to a dry season (Pires et al. 2008). Research on Irish fish species has generally focussed on the effects of climate change (e.g. Connor et al. 2018; O’Briain et al. 2019; Kelly et al. 2022; Barry et al. 2023) rather than the specific impacts of extreme weather events.

Drought

More than three quarters (15 out of 19) of the studies on drought are from Mediterranean or continental climate countries. A Portuguese study by Calapez et al. (2014) showed that drought reduced invertebrate abundances, in particular EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa, but Diptera and Coleoptera were less affected, suggesting that they may be resistant to desiccation or can survive in moist parts of the stream bed or river banks. Although diatoms recovered faster in this study, the community had not returned to its characteristic pre-drought condition one year after the event. The authors noted that good habitat conditions lessened the impact on macroinvertebrates and highlighted, in particular, the role of habitat heterogeneity and the occurrence of species refugia in enhancing the resistance and resilience of communities to drought (Calapez et al. 2014). Bertoncin et al. (2019), working on a pond in Brazil, also reported increases in the abundances of tolerant macroinvertebrate taxa but overall homogenisation of the communities due to the effect of the drought on the habitat characteristics. The authors noted that predation by fish may be a contributing factor. The potential for increased or decreased habitat heterogeneity due to extreme drought was discussed by Cardoso et al. (2022), working on phytoplankton and zooplankton but in shallow, semi-arid lakes in Brazil. They noted an increase in habitat heterogeneity within and between lakes which can increase β diversity but within lake diversity can be reduced.

Among the few studies that focus on single species groups is Dorić et al. (2023), who sampled Chironomidae over a 14-year period in protected tufa barrier lakes in Croatia when both drought and flooding occurred. While taxon richness and abundances did not change over the study period, there were changes in community and trophic structure as different species took advantage of the differing environmental and habitat conditions associated with drought or flooding. The protected pearl mussel, as a stationary species, is considered to have low resistance to the effects of extreme drought. A severe drought in Portugal in 2017 led to mortality of mussels affecting overall abundances and potentially, the recruitment of juveniles in subsequent years (Nogueira et al. 2021). Similar impacts were reported from Scotland (Cosgrove et al. 2021). High water temperatures and associated low oxygen concentrations together with the loss of wetted habitats contributed to the mortalities (Baldan et al. 2021). Drought also has the potential to impact the protected white-clawed crayfish (Austropotamobius pallipes), although apart from a reference to drought among the other impacting stressors on this species by Alonso et al. (2000), no further studies on extreme weather impact were retrieved.

Droughts are particularly challenging for biota in saline waters due to increased salinity as reported by de Necker et al. (2021), resulting in the loss of species that are intolerant to elevated salinity levels. Corixids (water boatmen) and hydrophilids (beetles) appeared to be the most tolerant to the drought-induced highly saline conditions. Much of the aforementioned work on macroinvertebrate response to drought highlights the variable species-level tolerance to the adverse conditions.

In terms of impacts of drought on fish, the different life history stages are likely to vary in their vulnerability and response. Drought can clearly reduce the available habitat, often to isolated patches with increased crowding in areas, together with possible water quality challenges such as elevated water temperatures and reduced oxygen concentrations. It is only in recent years that studies of extreme drought impacts on fish have emerged and these are, therefore, limited (Piniewski et al. 2017). As noted by Piniewski et al. (2017), only one study reported a reduction in abundances and none detected changes in richness and diversity. One of the more recent papers by Meijer et al. (2019) on a critically endangered fish species largely restricted to drying-prone waterways in New Zealand recorded a catchment-wide switch from adult-dominated populations to populations composed of juveniles in response to a two-year drought. Reduction in abundance appears to depend on the type of species. Data analysis by Mahardja et al. (2021) on fish in the San Francisco estuary highlighted declines in abundances of pelagic species whereas littoral species were more resistant to drought. They proposed a resilience assessment framework based on abundances and body size distribution. Impacts on recruitment appear to depend on how and where the eggs are deposited. For example, pelagic-broadcast spawners are particularly vulnerable (Perkin et al. 2019).

Drought induced low flows can reduce the window for upstream spawning migration of salmonids (e.g. Kastl et al. 2022 in relation to Coho salmon). In Ireland the risk will depend, in part, on when these species (Atlantic salmon and sea trout) enter Irish rivers. In the case of salmon, the late spring and summer run would be potentially vulnerable periods. Loss of habitat in drought is also expected to impact the protected sea lamprey (Hume et al. 2021). It is likely that brook and river lamprey may face similar issues but Wang et al. (2021), citing Rodríguez-Lozano et al. (2019), noted that juvenile lamprey, here again a protected species, may survive in saturated sediments for up to 22 days. Thus, the duration of the drought is a key factor influencing the immediate and longer-term impacts.

Heat waves and extreme temperature anomalies

Water temperature regulates a range of ecosystem processes, including decomposition and metabolic activity, from growth to reproduction, of aquatic organisms each of which has a thermal tolerance range. Changes in water temperature can also alter physicochemical conditions, e.g. reduced oxygen saturation. Movement of species and changes to warm-water dominated species as water temperatures increase (ramp pressure) is well reported and takes place over variable timescales (Perrin et al. 2022). Superimposed on this are heat waves in certain years which can result in more immediate responses. For example, EPT communities can be restructured due to changes in richness and abundance of certain species, presumably tolerant to the changing conditions, as detected in a long-term study on a stream in Germany (Dietrich et al. 2023). The authors noted that repeated heat events led to a long-term decrease in abundance. Interestingly, they showed that extreme cold events led to increases in EPT abundance which the authors attributed to delayed and thus concentrated hatching.

Molluscs have also been shown to be vulnerable to heat waves. Cosgrove et al. (2012) estimated that 90% of extant pearl mussel populations are at risk to projected climate-change driven high temperatures. Mouthon and Daufresne (2015) investigated the response of mollusc communities in the River Saone (Eastern France) to the European heat wave of 2003. It resulted in a reduction in species density (gastropods by 85.5% and bivalves by 64.0%) and richness, and incomplete recovery eight years later. The authors acknowledged that anthropogenic pressures may have been a contributing factor at some sites. Another factor influencing recovery following temperature extremes is whether or not a species was eliminated totally by the event and the event’s impact on available food resources, referred to by Seifert et al. (2015) as ecological legacy effects. The specific site characteristics, such as geology, soil type and riparian shading, can influence the resilience of Irish stream habitats to heat waves (Kelly and Kelly 2024). Favourable site conditions provide important climate refugia for salmonid populations (Kelly and Kelly 2024).

The influence of lake morphology on thermodynamics, stratification and trophic status (seasonal species succession patterns), and thereby the response of cladoceran communities to a heat wave was investigated by Anneville et al. (2010). They went on to caution the extrapolation of ecological results from one waterbody to another. Increases in cyanobacteria biomass can also be linked to high water temperature and stability during heat waves (Calderó-Pascual et al. 2020). Jennings et al. (2022) noted that while impacts of extreme events such as heat waves on physical lake properties are now well understood, ecological impacts are highly complex and require further study.

Compound events

Many studies of extreme weather events involve coupled stressors, e.g. flooding can be accompanied with high or low temperatures, or storm winds can equally be accompanied with extremes in precipitation and/or temperature. For the purpose of this study, we define compound extreme events as the occurrence of multiple extreme events simultaneously or in close sequence at a single or multiple locations (Leonard et al. 2024; Kopp et al. 2017; Zscheischler et al. 2018; Hao and Singh 2020; Alfroz et al. 2023). These can cause greater havoc than an individual or a single extreme of the same or higher magnitude (Zscheischler et al. 2018; Alfroz et al. 2023).

In this context, studies of lake systems appear more prevalent in the literature and in these, inputs of nutrients, suspended solids and/or other contaminants, can make it difficult to attribute the responses to a particular extreme in weather conditions.

Impacts on lakes can relate to the effects of wind on mixing of the water and position of the thermocline. Wood et al. (2017) investigated the effects of large rainfall events (between March and April 2014) on cyanobacteria assemblages in Lake Rotorua, New Zealand. Higher inputs of nitrate during wet events were observed whereas drier periods led to increased dissolved reactive phosphorus and low dissolved inorganic nitrogen concentrations. Changes in the species assemblages were dynamic and difficult to relate to specific changes in water chemistry, in part due to an interplay with water temperature. Also working on lake cyanobacteria in Germany, Kasprzak et al. (2017) reported that an extreme weather event (drop in air temperature from 19 to 14°C, together with an increase in wind speed) ‘triggered a cascade of coupled physical, geochemical and biological processes’ (Kasprzak et al. 2017, p7) that resulted in increased cyanobacteria blooms followed by reduced water clarity due to calcite crystal precipitation which persisted for several weeks. The authors highlighted the significance of the results for other clear-water lakes.

Inputs of nutrients from terrestrial origin can be accelerated by extreme precipitation events. For example, a study of nine lakes in the Northeast USA by Klug et al. (2012) following a tropical cyclone that was accompanied by high winds and heavy precipitation highlighted the impact of influxes of terrestrially derived carbon. These inputs led to elevated turbidity, with values remaining high for more than eight months following the storm. Net production in many of the lakes shifted from autotrophy to heterotrophy. Coupled with this was elevated total phosphorus, which the authors predicted could lead to algal blooms in the future if the storm derived TP was retained within the lake (Klug et al. 2012). Lofgren et al. (2014) examined long-term monitoring data from two Swedish forested headwater catchments affected by both bark beetle infestation and heavy storms. Their study revealed that extended periods of heavy storm as well as other environmental disturbances, lasting more than five years, led to elevated levels of nitrate and ammonium concentrations in the watersheds, mostly attributed to the infestation of bark beetles that occurred after the heavy storm event. Calderó-Pascual et al. (2020) reported that lake mixing induced by a storm occurring during a heat wave led to a three-fold increase in total zooplankton biomass and a reduction in phytoplankton biomass, though the changes were relatively short lived and zooplankton biomass declined again once the lake restabilised aided by the ongoing heat wave.

In response to drought and flooding events, heteroptera of soda pans showed spatial and temporal variability in species assemblages (Cozma et al. 2020). The mobility of species in this group was shown to contribute to their ability to quickly recolonise the pans when conditions became favourable. Thus, in terms of species resilience, assessment of dispersal ability needs also to be considered.

Overall, responses to flooding and drought are likely to be species specific and dependent on a combination of environmental conditions, including but not limited to, the intensity and timing of the extreme event being considered, habitat changes, the availability and size of refugia, together with the morphological, physiological and behavioural characteristics of individuals (e.g. Magoulick and Kobza 2003).

IMPACTS ON TERRESTRIAL ECOSYSTEMS

This section deals with studies that were undertaken across all land-based biomes and ecosystems (amphibians were included due to their inhabitation of both land and water), with the exception of forests (dealt with in Section 3.3) and coastal systems (in Section 3.5).

Droughts

Within those studies which investigated terrestrial plant taxa, most focussed on drought-related impacts (Table B.4). Natural droughts were associated with increased regional mortality among six tree species inhabiting three US habitats (Gitlin et al. 2006), as well as a reduction in soil moisture content and an increase in distance to groundwater from the surface in some areas of a Scottish highland catchment (Soulsby et al. 2021). The associated changes to the water table during drought can have significant impacts on peatland carbon dynamics, as shown through the effect of drought on dissolved organic carbon and particulate organic carbon exports from an Irish peatland catchment (Ryder et al. 2014), as well as changes to CO₂ and CH4 fluxes from Irish bog mesocosms under drought (Estop-Aragonés et al. 2016). The magnitude and direction of impact on CO₂ and CH4 fluxes varied greatly between sites and might depend on site characteristics such as soil properties or nitrogen inputs (Estop-Aragonés et al.2016). These processes could have severe implications for peatland carbon stores under future climate scenarios (Ryder et al. 2014), which are of particular relevance to Irish protected habitats. Droughts also significantly impact grassland carbon cycling, leading to decreased primary production (Wigley-Coetsee and Staver, 2020) and either an increase or a decrease in soil CO₂ emissions depending on the soil type and prevailing soil moisture conditions at the site (Zou et al. 2025). Drought also decreased aboveground biomass (Kahmen et al. 2005; Wigley-Coetsee and Staver 2020; Qu et al. 2023), and increased belowground biomass (Kahmen et al. 2005), mycorrhizal fungi biomass (Walter et al. 2016), competition for resources (Grant et al. 2014), and invasibility (Kreyling et al. 2008) in grasslands.

Legacy effects from drought can cause longer term changes to ecosystem resistance and resilience. For example, a mesocosm experiment utilising native and invasive plants, as well as pre-conditioned ‘drought-experienced’ and ‘naïve’ soils, showed that drought conditions can reduce invasive plant shoot biomass but increase overall biomass in ‘drought-experienced’ soils (Yang et al. 2022). A further study reported that greenhouse plants grown in ‘drought-experienced’ soils produced more shoot biomass (De Long et al. 2019). Similarly, Backhaus et al. (2014) found increased drought resistance in plant communities previously exposed to recurrent mild drought stress, possibly due to soil biotic legacies.

Extreme drought events can also affect soil properties, nutrient availability, and microbial community composition (De Long et al. 2019; Hammerl et al. 2019; Qu et al. 2023). Such dynamics can lead to a decline in plant community diversity, ultimately resulting in the collapse of the ecosystem (Bartha et al. 2022), though Kahmen et al. (2005) found an increase in species diversity with drought. Grassland ecosystems have shown some resistance and resilience to extreme drought conditions with differences observed between annual and perennial plant communities (Bartha et al. 2022; Dodd et al. 2023).

In terms of effects on fauna, drought has been documented in a Swedish study to lead to local extinction of Euphydryas aurinia, a protected butterfly species in the Habitats Directive, yet population size rebounded to 130% of the pre-drought figure in one patch cluster (Johansson et al. 2020, 2022). A review of North American and European butterfly species range expansion ecology indicated that various detrimental impacts could be caused by droughts and extreme precipitation events (Parmesan 2001). Drought can also lead to a decline in abundance of grassland dependent communities such as grasshoppers (Fartmann et al. 2022).

Drought impacts have also been reported on vertebrates. For example, the babbler Turdoides bicolor populations in the Kalahari Desert had more reproductive investment and success during drought events compared to the years after the events (Bourne et al. 2020), whereas drought suppressed breeding in the wren Malurus alboscapulatus in Papua New Guinea (Boersma et al. 2022). Amphibians are particularly sensitive to drought and heat waves, especially during the breeding season (Piha et al. 2007), due to loss of wetted habitats (McDevitt-Galles et al. 2022; Beranek et al. 2022). The importance of refugia within the landscape was highlighted, such as larger more permanent water bodies (McDevitt-Galles et al. 2022) or well vegetated wetlands (Beranek et al. 2022), enabling adults to survive the drought.

Flooding and extreme precipitation events

With respect to invertebrate taxa, Cesarz et al. (2017) observed that extreme flooding had led to a change in the structure of soil nematode food webs, specifically resulting in the loss of K-strategists and a decline in plant-feeding nematodes. Conversely, Nicolai and Ansart’s (2017) review of terrestrial gastropod responses to climate change highlighted the paucity of studies on the impacts of floods. Vertebrates are also at risk - 52–62% of nearly 2,400 amphibian, bird and mammal species are considered to be highly exposed to flood in China’s 32 Priority Areas for Biodiversity Conservation (y Juárez et al. 2016). Floods also directly increased the distance between realised and preferred roosting and feeding sites among English Grus grus cranes, as well as indirectly raising activity and energy expenditure levels (Soriano-Redondo et al. 2016).

In terms of plant responses, productivity decreased in species-rich floodplain plots that were highly inundated (Fischer et al. 2016). In addition, species loss in such plots is also an issue when inundation extends for long periods (Wright et al. 2015). The adverse ecological impacts of flooding on grassland ecosystems include changes in soil properties (De Long et al. 2019; Qu et al. 2023), shifts in microbial and plant community composition (De Long et al. 2019; Dodd et al. 2023), a decrease in plant bio-mass (Qu et al. 2023), an increase in resource competition (Grant et al. 2014), and intensification in invasibility (Kreyling et al. 2008). Similar to findings discussed in the previous section for drought periods, flooding can also lead to alterations in the carbon dynamics of Irish peatlands as shown through impacts on dissolved organic carbon concentration in correlation with stream discharge (Jennings et al. 2020).

Impacts on vertebrate species have been reported in response to both extreme snow and rainfall events. For example, extensive snow cover was linked to declines in Swiss owl Tyto alba populations (Altwegg et al. 2006); to elevation-dependent decreases in the American pika abundance (Johnston et al. 2019); and also to reductions in caribou populations across the Arctic (Vors and Boyce 2009). A severe hailstorm in the U.S. destroyed 17% of previously identified nests for shrubland passerine populations (Hightower et al. 2018). Meanwhile, heavy rainfall was a factor in failed underground nests made by grassland owl Athene cunicularia (Fisher et al. 2015) and a North American snow blizzard killed over 42,000 livestock and captive animals, an approximate 222-fold increase over the background mortality rates (Martin 2022). Studies focusing on extreme precipitation events and extreme temperature anomalies found a body size-dependent correlation with subsequent mortality rates in two Australian wren species (Gardner et al. 2017), and also combined to delay the dates of first egg-laying and reduce the number of renesting attempts in two North American grouse species (Martin and Wiebe 2004).

Heat waves and extreme temperature anomalies

Heat waves have been associated with negative impacts on butterfly populations, e.g. reduced numbers of eggs laid in small populations of grassland butterfly Cupido minimus (Piessens et al. 2009) and negative impacts on larval and pupal survival in Pieris napi butterflies (Bauerfeind and Fisher 2014). Extreme temperature anomalies were also identified as one of the key factors in models explaining population decrease in the Canadian grassland butterfly Parnassius sminthius (Roland and Matter 2016). Some evidence exists that tolerance to heat waves can depend on physical characteristics of butterflies such as the colour, with darker species exhibiting greater tolerance (Ashe-Jepson et al. 2023).

For other invertebrates, results are more varied; e.g. for soil fauna, a series of simulated winter heat waves was shown to result in lower acarid abundance and biomass, but collembolan biomass and diversity remained unchanged (Bokhorst et al. 2012), whereas a controlled heat wave and extreme precipitation event resulted in reductions to collembola community density, but no changes to species composition (Krab et al. 2013). A global synthesis of research into bumblebee climate change adaptation also called for more research into the effects of extreme weather on bees’ thermal tolerance (Maebe et al. 2021). Interestingly, the protected slug Geomalacus maculosus exposed to simulated extreme temperature anomalies displayed higher tolerance scores than other, previously tested slug species (Carnaghi et al. 2021).

Variability in impact has also been reported for vertebrates. High simulated temperature anomalies had little effect on two Mexican nectivorous bats (Ortega-Garcia et al. 2020), but a natural heat wave caused a mass bat mortality event among individuals roosting in a Cambodian temple (Pruvot et al. 2019). Heat waves also potentially affect amphibian locomotion and reproduction (Narayan, 2017) and increase the rate of body mass loss in vipers (Opera aspis; Dezetter et al. 2022). Meanwhile, for birds developing during heat waves, risks of thermoregulatory system ‘mis-calibration’ were highlighted by Ruuskanen et al. (2021) if periods of high temperatures are viewed as the norm from a physiological perspective. Acclimatisation effects can also be seen on shorter time scales; for example, while a simulated heat wave in lab-housed Australian finches led to an increase in evaporative water loss, the heat exposed finches also acclimatised to the point that their CO₂ production was lower (Cooper et al. 2020).

Two studies feature predicted outcomes, rather than actual observations. Jones et al. (2016) projected that future heat wave and drought combinations could see numbers of adult and juvenile Australian Tiliqua rugosa skinks fall significantly by 2080, while Bateman et al. (2020) reported that 544 bird species across the USA are likely to be at exceptional risk from increased extreme spring heat and fire weather frequency based on a 3°C rise in mean temperature.

In terms of the impact on flora, heat stress can lead to changes in flowering patterns such as a reduction in the number of flowering stems (observed in two Italian orophytic species; Abeli et al. 2012) and extensions of the first flowering date (Fox and Jonsson, 2019). More generally, extreme temperature anomalies documented across a European growing season contributed to significant reductions in vegetative cover, especially in crop-, shrub- and grassland (Baumbach et al. 2017). A larger-scale study focusing on stress ‘memory’ mechanisms in plants highlighted a paucity of research into their responses to extreme heat or cold exposure (Walter et al. 2013).

Episodes of extreme cold have been studied relatively infrequently and when studied it has often been in combination with other extreme events. Among invertebrates, native and non-native Florida bee species increased and decreased flower visitation rates, respectively, up to a year after a cold spell (Downing et al. 2016). Graham et al. (2021), reported a significant reduction in abundance for some bee species after a compound hot and cold extreme temperature event in the USA. An 8-month period on a Russian steppe that featured a significant cold spell followed by a heat wave and drought, was associated with plant species’ phenological events occurring up to 14 days early and mollusc populations declining by up to 33% (Solov’ev et al. 2015). In Finland, dipteran ungulate parasites Lipoptena cervi displayed inhibited immune response to antigens as adults, following exposure as larvae to a simulated ‘winter’ extreme heat episode, but not after a cold spell (Kaunisto et al. 2015).

In the case of vertebrates, ‘cold surge’ events reduced survival rates in sampled populations of US chickadees Poecile atricapillus (Latimer and Zuckerberg 2019), while a winter that featured a cold spell and snow blizzard saw Chersophilus duponti lark populations on Spanish steppes either go extinct or shrink significantly compared to the control period (Perez-Granados et al. 2023).

Other event types

Only six studies reported on extreme weather events other than those already mentioned (Table B.4). In this regard, storms featured most often. Tsunamis and storm surges were reported to have negative effects on the diversity of terrestrial and aquatic invertebrates and the functional redundancy in Chilean wetlands (Coccia et al. 2022). Winter storms severely restricted the northward distribution of seven North American waterfowl species, especially among those with specialist diets or habitat types (Masto et al. 2022) and a review of 607 primate species identified a considerable proportion to be at risk from tropical storms and droughts, including 189 species listed as ‘threatened’, across three global hotspots (Zhang et al. 2019). Wardell-Johnson et al. (2021) suggested that terrestrial biomes in Oceania will see numerous impacts resulting from higher frequencies of events, including storms and fires. In tropical regions, birds (Sekercioglu et al. 2012) and bats (Sherwin et al. 2013) are forecast to be under extensive pressure from events including El Niño/La Niña oscillations, fires, tropical storms and heat waves, with migratory species being perhaps spared some of the consequences.

Compound events

When compound events were studied, these generally included droughts, which were combined with extreme precipitation events (Ahlers et al. 2015; Zou et al. 2023), extreme temperature anomalies (Dreesen et al. 2014; Ewald et al. 2015; Tinsley et al. 2015; Visoiu and Whinam 2015; Ali et al. 2016; Paniw et al. 2021), or fire (Slingsby et al. 2017; Beranek et al. 2022).

Ahlers et al. (2015) reported that droughts combined with extreme precipitation events affected site occupancy rates of two semi-aquatic mammal species, muskrats Ondatra zibethicus and mink Neovison vison. Elsewhere, an analysis of four ecosystem types across Central Asia revealed that resistance and resilience to drought and extreme precipitation varied depending on the ecosystem history with wet and dry events (Zou et al. 2023).

Impacts on fauna from the combination of drought with extreme temperature events vary, from English farmland invertebrate taxa showing increased abundances during hot, dry years compared to cold, wet years (Ewald et al. 2015), to the effective extinction of the invasive frog Xenopus laevis in Wales, at least partially as a result of a combination of a cold spell and drought (Tinsley et al. 2015).

The impact of drought combined with extreme temperature anomalies on plant communities has been shown to lead to increased leaf death in certain treatments, but no effect on senescence or resistance measures among three Belgian perennial species (Dreesen et al. 2014) and changes in diversity and biomass production in a Swedish valley community (Ali et al. 2016). Severe defoliation events (Visoiu and Whinam 2015) and deleterious impacts on canopy cover (Paniw et al. 2021) have also been observed in a Tasmanian alpine plant community and a Spanish shrubland area, respectively. These studies highlight that impacts from these extreme events can be long term, with significant impacts still visible after six months in Tasmania and 14 years in Spain (Visoiu and Whinam 2015; Paniw et al. 2021).

Drought, combined with a ‘mega-fire’, led to a decline in post-event adult abundance of Australian Litoria aurea frog populations (Beranek et al. 2022). A 44-year analysis of the status of a South African Mediterranean-type fire-dependent ecosystem identified a link between consecutive hot and dry days in the first summer post-fire, and declines in species richness and diversity (Slingsby et al. 2017).

An evaluation of the effects of heat waves, cold spells, extreme precipitation events and droughts on three insect orders and reptiles across the Netherlands over 20 years found few correlations with population dynamics (Malinowska et al. 2014).

IMPACTS ON FORESTS

Forests and woodlands form parts of several protected habitats of specific conservation interest in Ireland, such as Alluvial forests with Alnus glutinosa and Fraxinus excelsior (28 SACs), Bog woodlands (12 SACs) and Old sessile oak woods with Ilex and Blechnum in the British Isles (39 SACs). They have also been extensively studied in the literature (Table B.4) and are therefore covered separately from other terrestrial ecosystems.

Drought

Multiple studies have shown negative ecological responses of forests to drought, including tree mortality (Allen et al. 2010; Anderegg et al. 2013; Allen et al. 2015; Lloret et al. 2018; da Rocha et al. 2020; Schuldt et al. 2020; Beloiu et al. 2022; Vatandaşlar et al. 2023) and green biomass (Na-U-Dom et al. 2017), reduction in radial growth (D’Andrea et al. 2020), reduction in net ecosystem production (Lee et al. 2021), decline in forest canopy and tree saplings (Beloiu et al. 2022), canopy dieback (Losso et al. 2022), insect outbreaks (Lloret et al. 2018), and decline in the overall forest ecosystem services (Lindner et al. 2010).

Forests are key components of the terrestrial carbon cycle, sequestering and storing carbon as biomass. Drought events are strongly linked to carbon dynamics through tree growth and tree mortality (Lloret et al. 2018; Nolet et al. 2018; da Rocha et al. 2020; Vatandaşlar et al. 2023). For example the 2018 extreme drought negatively impacted the health, structure and functioning of European forests, causing unprecedented tree mortality (Schuldt et al. 2020; Beloiu et al .2022). Similarly, an extreme drought coupled with high temperatures in 2019 in south-eastern Australia led to extensive canopy dieback (Losso et al. 2022). Vatandaşlar et al. (2023) also found that drought in Turkey led to mortality of Taurus fir species in the Hadim Forest Enterprise.

Prolonged droughts can decrease the resistance and resilience of forest ecosystems, depending on factors such as structural composition of the forest, functional diversity, and the presence or absence of indigenous species (Domec et al. 2015; Wiesner et al. 2020; Marcotti et al. 2021; Bartha et al. 2022). For instance, forests with heterogeneous perennial species exhibit greater resilience in extreme drought conditions compared to those dominated by annual species (Wiesner et al. 2020; Bartha et al. 2022). Domec et al. (2015) found that intensively managed loblolly pine forest plantations were more susceptible to extended periods of drought than wild hard-wood forests in North Carolina, USA, and Marcotti et al. (2021) found that resistance to drought exhibited higher values for Austrocedrus chilensis trees in mixed forest in Argentina compared to monospecific forests. Similarly, Wiesner et al. (2020) found that longleaf pine savannas with a higher plant functional diversity had improved energy storage and responded more rapidly to drought conditions in Southern Georgia, USA. These studies highlight the importance of diverse forest ecosystems to enhance the resistance and resilience of trees to extreme weather conditions.

The negative ecological responses in terms of tree growth and survival to extreme drought have significant implications for the terrestrial carbon cycle and pose further threats to the ecological services that forests provide, such as reducing the carbon storage capacity, loss of sequestered forest carbon stocks, and associated atmospheric feedbacks (Lindner et al. 2010; da Rocha et al. 2020; Marcotti et al. 2021). This can be seen through direct impacts on CO₂ emissions. An extreme drought led to a transient increase in CO₂ emissions due to increase in respiration and decrease in ecosystem productivity from both an evergreen Korean pine stand and a deciduous oak stand (Lee et al. 2021). However, the pine stand showed a stronger rise in ecosystem respiration and corresponding decline in net ecosystem production compared to the oak stand (Lee et al. 2021), which could be of relevance to Irish SACs with sessile oak woods. This is supported by findings for Irish conditions by Zou et al. (2025), which showed that simulated drought led to an increase in soil respiration in an evergreen coniferous forest, but actually decreased soil respiration in a deciduous broadleaf forest. The authors emphasised the importance of site characteristics such as soil type, prevailing soil moisture and soil carbon, to the impact of extreme drought and precipitation events on soil greenhouse gas emissions (Zou et al. 2025).

Storms

The forest ecosystem experiences various negative ecological impacts due to storm related natural disasters such as hurricanes, cyclones, and tsunamis. Hurricane events can affect forest structures and functions by damaging the canopy, mainly by reducing leaf area, reducing biomass, accelerating senescence, declining tree growth, and ultimately leading to tree mortality (Rypkema et al. 2019; Gong et al. 2021). Negron-Juarez et al. (2014) observed higher tree mortality in tropical rainforests than in temperate forests as a result of severe cyclones. However, the extent of damage varied depending on factors such as stem density, collateral damage, and root system depth (Negron-Juarez et al. 2014). Porwal et al. (2012) revealed significant devastation and irreversible ecological and biological losses, including a decline in biodiversity, in the forest ecosystems and coastal regions of the Nicobar Islands, India, after the December 26, 2004, tsunami.

Flooding and extreme precipitation events

Flooding has not been studied as extensively in forest and woodland as it has in other ecosystems. The main impacts of flooding events on forest ecosystems include tree mortality (Resende et al. 2020; Oliveira et al. 2021), decay of tree stands and dieback of forests (Lofgren et al. 2014; Durło et al. 2015), and reduction in above-ground biomass (Oliveira et al. 2021). In two separate studies investigating the impact of flooding events due to dam construction on forests in the Amazon basin, flooding events were shown to increase tree mortality, decrease above-ground biomass, and reduce species richness (Resende et al. 2020; Oliveira et al. 2021).

Heat waves and extreme temperature anomalies

Extreme temperature events, such as high temperatures and heat waves, can lead to increased insect infestations and pathogen outbreaks (Berec et al. 2013; Fettig et al. 2013), decreased plant growth rates (Nezval et al. 2021), and increased mortality (Fettig et al. 2013). Research in the Bohemian Forest in Austria found a positive correlation between rising air temperatures and bark beetle attacks, as the bark temperatures contribute to the development of the beetle (Berec et al. 2013). Higher air temperatures, severe drought conditions, and altitude can affect tree growth and potentially accelerate tree mortality (Fettig et al. 2013; Nezval et al. 2021). For example, Fettig et al. (2013) noted that individual trees or tree populations experiencing extreme climate conditions outside their climatic niches exhibit maladaptation, leading to reduced productivity and increased susceptibility to insect infestations and pathogens. Nezval et al. (2021) also observed that Norway spruce growth seasons were longer at higher altitudes, and the external weather variables greatly impacted radial and meristem growth. However, previous studies have shown that mature and intricate forest ecosystems have the potential to more efficiently mitigate temperature deterioration and reduce the negative ecological impacts associated with extreme events, unlike young or modified forest stands (Norris et al. 2012).

Cold events have negative impacts on the forest ecosystem, including damage to trees (Abbas et al. 2017; Menzel et al. 2015; Jarzyna et al. 2021), tree mortality and loss (Foran et al. 2015), diminished litter and nutrient return (Liu et al. 2022), and a decline in seed dispersal (Zhou et al. 2013). Several studies identified negative impacts from late spring frost on European beech (Fagus sylvatica L.; Menzel et al. 2015; D’Andrea et al. 2020; Jarzyna et al. 2021). Extreme snow and ice interferences have been observed to decrease litter accumulation and the subsequent nutrient cycling in a subtropical evergreen broad-leaved forest in China (Liu et al. 2022). However, extreme dry weather events had a more significant impact compared to extreme ice and snow events (Liu et al. 2022).

Wildfire

While wildfires are an integral part of the earth system (Bowman et al. 2009) and some species of terrestrial fauna have developed to adapt to—or even rely on—wildfire conditions for reproduction and survival, they have increased in frequency and severity with climate change (IPCC 2021). Wildfires are predicted to occur more frequently in locations where they would have been relatively rare in the past, such as Irish woodlands (de Rigo et al. 2017; Hawthorne et al. 2018). Wildfires result in tree mortality, forest loss, and the subsequent decline in ecological values in forest ecosystems (Coop et al. 2016; Parks et al. 2019; Blumroeder et al. 2022). They can lead to conversion of forests to non-forest and transitions from densely forested areas to more open landscapes such as savannas, meadows and oak scrub communities (Coop et al. 2016; Parks et al. 2019). Coop et al. (2016) found that the non-forested vegetation types showed a notable ability to withstand future fire events and recover from reburning. Furthermore, wildfires change land cover and contribute to soil degradation and the loss of organic carbon, further impacting the overall health and functioning of the forest ecosystem (Parks et al. 2019; Blumroeder et al. 2022).

Compound events

Droughts have been found to have negative effects on forests due to their compounding nature with other extreme events (Nolet et al. 2018; D’Andrea et al. 2020; 2021; Serra-Maluquer et al. 2021; Beloiu et al. 2022; Losso et al. 2022). Recent studies found that compound drought and heat wave events reduced forest carbon uptake (Gazol and Camarero 2022; van der Woude et al. 2023; Yin et al. 2023) and increased tree mortality (Neumann et al. 2017; Gazol and Camarero 2022; van der Woude et al. 2023).

Studies on future climate projections show that the occurrence of compound drought and heat wave or heat stress extreme events will likely increase globally. For example, Gazol and Camarero (2022) predicted that over the next decades, the occur-rence of compound events of hot summers and dry conditions will likely increase in most European regions and may trigger severe tree mortality. Yin et al. (2023) also demonstrated that the frequency of extreme drought and heat wave events globally is projected to increase tenfold under the highest emissions scenario by the late twenty-first century.

In addition to compound drought and heat wave or heat stress extreme events, coupled flooding and drought events can also affect forests. For example, Serra-Maluquer et al. (2021) investigated how recurrent periods of severe drought and flooding reduced tree growth of six prominent tree species in Iberia over the short and long term.

Extreme weather events such as droughts and cold spells can interact with other disruptive agents, such as insect infestations through weakening of tree defence systems and increasing susceptibility to other threats (Foran et al. 2015; Lloret et al. 2018; Nolet et al. 2018). For example, Lloret et al. (2018) found that the combined effects of drought and bark beetle infestation in a historical forest (Pinus edulis) in the southwestern region of North America resulted in extensive tree mortality and die-off. Nolet et al. (2018) also found that repeated droughts, thaw-freeze cycles, and insect infestations slowed radial growth in sugar maple and American beech trees in the Papineau-Labelle Wildlife Reserve in Quebec, Canada. Foran et al. (2015) studied the effects of extreme weather events on tree mortality in Cambridge, Massachusetts. They considered various scenarios, including hurricanes, heat stress, snow or ice loading, Asian longhorn beetle infestations, and the cumulative effects of these scenarios, and found up to 57% of trees in the urban forest could die as a result of these events (Foran et al. 2015). The cumulative impacts of compound extreme events on forests is expected to increase and can potentially accelerate ecosystem misadaptation to changing climatic conditions.

IMPACTS ON COASTAL AND MARINE ECOSYSTEMS

Coastal environments such as saltmarshes, dunes, estuaries and lagoons feature prominently in Irish SACs and SPAs. Seagrass meadows occur frequently across Irish SACs with mudflats and sandflats not covered by seawater at low tide (Habitat Code 1140; Aquatic Services Unit, 2007). Extreme weather events such as storms and heat waves may impact profoundly on these coastal ecosystems and nearshore communities. The recent increase in the frequency and intensity of these extreme weather events will inevitably bring additional challenges to the recovery of coastal ecosystems following an extreme event (Wang et al. 2008; Triki and Bshary, 2019; Weiskopf et al. 2020; McCarthy et al. 2023). There are no SACs or SPAs in Irish open water marine environments but they have clear relevance to the SACs and SPAs in coastal ecosystems. Overall the impact of extreme events on open water marine environments is highly understudied and only poorly understood. The majority of papers in marine environments focus on coastal ecosystems with few studies looking beyond the intertidal and nearshore environment.

Storms

Open water marine environments and coastal ecosystems such as reefs or seagrass meadows, or intertidal ecosystems such as rocky shores, salt-marshes and estuaries, experience similar impacts from extreme storm events. Most of the impacts are linked to the disturbance by wave and wind action as well as increased turbidity of the water column and elevated deposition of sediment, though sustained high winds have also been observed to impact water temperatures due to shifting water currents (Chang et al. 2013).

Storm events cause immediate behavioural changes in macrofauna (Lea et al. 2009; Bacheler et al. 2019; Matley et al. 2019). In response to storm events, fish and sea turtles exhibit increased movement rates and emigrate to deeper habitats (Bacheler et al. 2019; Matley et al. 2019). Most macrofauna return to the original habitat post storm, but this is species dependent and long-term impacts on species’ behaviour and habitat occupation have been observed in stingrays (Matley et al. 2019). Storm events also lead to an increased dispersal of Northern fur seal pups (Lea et al. 2009). Outside of active behavioural responses, the distribution of species can also be impacted by storm events through changes in water flow and currents, which can aid the spread of invasive species, such as the hurricane-induced acceleration of the spread of invasive lionfish to South Florida (Johnston and Purkis 2015). Apart from species distribution, macrofauna abundance (Chang et al. 2013; Triki et al. 2018) and body condition (Matich et al.2020) can also be impacted negatively by storm events. In contrast, no short-term impact was observed on seagrass-associated fish assemblage in seagrass meadows following a storm event, albeit the event caused a loss in seagrass cover (Côté-Laurin et al. 2017). This study also observed that seagrass beds closer to villages were more affected by the storm event, highlighting that anthropogenic pressures can lead to synergistic effects when combined with extreme events (Côté-Laurin et al. 2017). Short and Neckles (1999) reported that storm effects on seagrasses are species specific and can cause shifts in the composition of seagrass communities. More recently, Sachithanandam et al. (2022) reported that cyclone Lehar had a negative impact on the floristic composition of seagrass species such as Thalassia hemperichii, Halodule pinnifolia, and Halodule uninervis. They found that seagrass meadows were uprooted owing to the strong currents with large amplitude waves and storm surge across the coastal region. In a modelling study, Szewczyk et al. (2024) also reported a decrease of canopy biomass of North Atlantic kelp forests with increasing winter storm intensity. However, Schoenrock et al. (2020) noted the lack of consistent historic monitoring of Irish kelp ecosystems and emphasised that further monitoring is necessary to improve our understanding of their resilience. This lack of consistent historic monitoring was equally noted for Irish coastal dune ecosystems and the vulnerability of them to increasing storminess requires further research (Farrell and Connolly 2019).

A storm event had negative impacts on population density and/or species diversity of flora, sessile fauna and mobile fauna in a rocky shore ecosystem on the Atlantic western Coast of Portugal (Ferreira et al. 2017). The fact that the rocky shore ecosystem was located in a Marine Protected Area did not improve the recovery capability of the ecosystem after the storm event (Ferreira et al. 2017).

Storm events also have a significant impact on plankton communities. Chung et al. (2012) observed an increase in diatom abundance for approximately 24 hours post storm due to nutrient entrainment in the Northwest Pacific, while Atkinson et al. (2021) observed changes in planktonic size spectra and reduced plankton biomass. Plankton size spectra recovered within months demonstrating an inbuilt resilience of the system (Atkinson et al. 2021).

Extreme events such as high storm waves are a major concern for coastal wetlands (Lorrain-Soligon, et al. 2021), which serve as a natural barrier against natural hazards including coastal floods, storm surges, and coastal erosion (Barbier et al.2011; Arkema et al.2013; Bhargava and Friess, 2022). For instance, Servino et al. (2018) noted that an extreme weather event (hailstorm) caused the abrupt loss of almost 24% of mangroves in Eastern Brazil, and one year later, there was little sign that it had recovered. Similarly, Mai et al. (2020) reported that cyclones, lightning, typhoons, and other severe weather events caused stem and branch breakage, tree mortality, and indirectly decreased resistance to pest and disease assault, leading to widespread deterioration of man-groves. The loss of coastal wetlands will directly or indirectly result in a reduction in valuable ecosystem services, such as reduced carbon sequestration supplied by such natural ecosystems, aside from effects on habitat and biological populations (Davis et al. 2015; Weiskopf et al. 2020).

Hurricanes and storms bring wind and rain, which may profoundly impact coastal habitats, communities, and animals such as sea turtles (Dewald and Pike 2014), spiders (Hataway and Reed 2020), sharks (Udyawer et al. 2013), topshell snails (Zamir et al. 2018), and microbial mats (Lingappa et al. 2022). Dewald and Pike (2014) reported that over a span of four decades, hurricanes adversely impacted 97% of sea turtle breeding sites in the north-western Atlantic and north-eastern Pacific Oceans as their seasonal occurrence closely coincided with the times of sea turtle breeding and egg incubation.

Furthermore, seabird breeding could be also negatively affected by severe weather such as storms (Newell et al. 2015) and severe wind (Frederiksen et al. 2008). For instance, Newell et al. (2015) found a rise in breeding failure of four seabird species at a colony in the North Sea due to the impact of a summer storm and concluded that seabird populations are affected by more frequent storms. The authors demonstrated that, due to differences in exposure and sensitivity, the total impact of extreme weather differs both across and within species (Newell et al. 2015).

Additionally, previous studies suggest that the survival of benthic species could be challenging when exposed to extreme storm events including hurricanes. Teixidó et al. (2013) reported that between 22% and 58% of the benthic species’ cover was lost when they were exposed to a coastal storm in the NW Mediterranean Sea. They found that sheltered locations, on the other hand, displayed no significant changes. Furthermore, Corte et al. (2017) reported that as storm induced wave power rose, invertebrate assemblages’ species diversity, abundance, and biomass all decreased on a Brazilian tidal flat (Corte et al. 2017). The study also noted that species extinctions sparked changes towards increased β-diversity, although the fauna appeared to rebound within a few weeks (Corte et al. 2017). More recently, Hutchings et al. (2023) concluded that hurricanes can have considerable impacts on fouling communities inhabiting harbours, demonstrating that the pre-hurricane levels of local species were not able to be reached after the storm.

Heat waves

Coastal and marine ecosystems are also susceptible to the effects of heat waves such as those observed off the west coast of Ireland in 2023 (McCarthy et al. 2023). While some species of seagrasses show an adaptive transgenerational response to historic heat exposure (DuBois et al. 2020), a strong negative direct impact of heat waves on seagrass meadows has generally been observed in most studies, such as an increase in mortality and community decline as well as shifts in reproductive mechanisms post die-back (Guerrero-Mesguer et al. 2017; Kendrick et al. 2019; Johnson et al. 2021). Similarly, a significant loss in genetic diversity of forest-forming seaweeds was observed post heat wave (Gurgel et al. 2020). Heat wave-induced seagrass community decline can lead to further indirect impacts on other species like turtles and dugongs, with consequences for ecosystem services such as fishery and tourism (Kendrick et al. 2019).

Negative impacts of heat waves have been observed on fouling assemblages in intertidal zones (Castro et al. 2021) and other sessile fauna (Ciona Intestinalis; Clutton et al. 2021). Marine macrofauna can show an ability to cope with acute extreme temperatures in the short term through adjusted metabolic responses (Grimmelpont et al. 2023). However, negative impacts on fish populations from extreme temperatures have also been observed (Alexander et al. 2017; Crozier et al. 2020; Anastasiadi et al. 2021). The impact of heat waves can be exacerbated by existing anthropogenic pressures on fish species in coastal ecosystems and affect already vulnerable species to a higher degree (Crozier et al. 2020). On the other hand, human responses to extreme events, such as changes in fishing patterns, can also lead to unintended long-term impacts on populations, highlighting the complex inter-dependencies of anthropogenic pressures and extreme events (Alexander et al. 2017).

Paalme et al. (2020) identified increases in zoobenthic biodiversity but negative effects on macrovegetation species composition as a result of a marine heat wave. They observed that concurrent coastal upwelling events in the NE Baltic Sea caused by the same weather patterns that caused the heat wave resulted in a cooling effect which counteracted some of the heat wave effects (Paalme et al. 2020). They also note that abundance of invasive species specifically benefited from the impacts of the combined heat wave and coastal upwelling events (Paalme et al. 2020). Marine heat waves can also have secondary effects on coastal ecosystems. Diamond et al. (2020) reported that a maritime heat wave that affected the northwest Atlantic during the winter of 2012/13 reduced plankton availability, leading to the starvation of sea birds.

Other events (including compound events)

In comparison to studies investigating the impacts of storms on coastal ecosystems, effects of severe salinity changes, cold spells, floods, and droughts on such ecosystems are poorly covered in the literature. Only three studies so far focussed on the impacts of cold spells. Mollenhauer et al. (2022) found that only a small fraction of young green turtles survived a severe cold shock in the St. Joseph Bay, Florida. Tinsley et al. (2015) reported the extinction of an invasive, non-native frog species, Xenopus laevis, at the Western Cape, South Africa, due to severe cold spells. Firth et al. (2011) observed mortality of an invasive mussel species in the intertidal zone after exposure to air temperatures below 2°C in the south-eastern US. They specifically highlight the co-occurrence of low air temperature with low water tables as an important factor in significantly increasing the impact of the event, and making intertidal species particularly susceptible to extreme cold spells (Firth et al. 2011).

Five studies investigated the impact of flood and/or drought on coastal ecosystems. Olds et al. (2014) showed that coral reefs in marine protected areas were able to resist the impact of extreme floods whereas fished reefs showed impacts. A modelling study in estuarine environments also highlighted the importance of refuge areas to aid recovery after an extreme flood or drought event (Gamito et al. 2010). Both of these studies emphasise the importance of creating protected areas to improve ecosystem resistance and resilience to drought and flood. The impact of severe drought stress under future climate scenarios is expected to limit the spread of invasive Acacia longifolia in coastal dune ecosystems in Portugal (Morais and Freitas 2012). Drought has also been shown to increase the concentration of toxic metals in mangrove sediments which could potentially have severe impacts on fauna and flora as well as ecosystem services such as food provisioning (Costa et al. 2020). Kelly et al. (2020) reported a net loss of salt from an estuarine basin and increased stratification of the water column as a result of an extreme precipitation event in the upstream catchment. Regardless of the type of initial extreme weather events, the disturbance from the extreme event can also cause shifts in food web interactions in coastal ecosystems as secondary impacts (Ciglenecki et al.2015; Harada et al.2020). Despite evidence from terrestrial environments showing a strong response of microbial communities to extreme weather events such as drought and flooding, no studies were identified which investigated the impact of extreme events on coastal microbial communities such as microbial mats and stromatolites, highlighting that this is an area which is understudied and poorly understood (Reinold et al.2019).

In terms of compound events, in a recent study by Missionário et al. (2023), ditch shrimps were shown to be more susceptible to mortality increases at high temperatures and low salinities, with relatively poorer survival seen under heat and hyposaline stress. Similarly, lithium pollution induced a higher stress response in clams when combined with low salinities (Barbosa et al.2023).

CONCLUSIONS

The review firstly highlighted a dearth of studies on the response of Irish ecosystems and in particular SACs and SPAs to extreme weather events. However, from the existing literature from other regions it is clear that the impact of extreme weather events on freshwater, terrestrial, and marine ecosystems are highly context specific. The impact of flooding has been intensively studied in freshwater ecosystems whereas research in terrestrial biomes (including forests and grasslands) mostly focused on heat waves and droughts. Research on marine and coastal ecosystems, on the other hand, mostly centred around the impact of extreme storm events. Many studies also investigated compound events, making it difficult to attribute impacts to a specific event type. Additionally, the impact of extreme weather events is species-dependent with different species even within the same taxon showing divergent responses to the same event. Similarly, resilience and resistance to, and recovery from, extreme events of species and communities is variable and depends on the species, species composition, availability of refuge area, and existence of additional (long term) pressures among other things. For these reasons, drawing generalised conclusions from the existing literature and extrapolating these to an Irish context is difficult.

However, several studies across different ecosystem types show that increased diversity helps improve resilience and resistance to, and/or recovery from extreme events. Additionally, the impact of additional pressures such as anthropogenic impacts generally leads to more negative outcomes. Compound events tend to have a synergistic effect, causing more negative outcomes than single events, except in the instance where the same type of event recurs, which can lead to adaptations and increase resistance and/or resilience. The review also highlighted several areas that are highly understudied such as open water marine ecosystems and marine/coastal microbial communities.

Overall extreme weather events will occur with increasing frequency and severity, and more research is needed to address the dearth of studies on how this will affect Irish ecosystems specifically.