restricted access III.14 Biodiversity and Ecosystem Functioning
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III.14 Biodiversity and Ecosystem Functioning Andrew Hector and Andy Wilby OUTLINE 1. Background and history 2. Biodiversity and ecosystem functioning relationships 3. Mechanisms 4. Multitrophic systems 5. Diversity and stability 6. Ecosystem multifunctionality 7. Ecosystem service provision 8. The next phase of research Forecasts of ongoing biodiversity loss prompted ecologists in the early 1990s to question whether this loss of species could have a negative impact on the functioning of ecosystems . Ecosystem functioning is an umbrella term for the processes operating in an ecosystem, that is, the biogeochemical flows of energy and matter within and between ecosystems (e.g., primary production and nutrient cycling). The first general phase of research on this topic addressed this question by assembling model communities of varying diversity to measure the effects on ecosystem processes. The results of the meta-analyses of this first wave of studies show that biodiversity generally has a positive but saturating effect on ecosystem processes that is remarkably consistent across trophic groups and ecosystem types. These relationships are driven by a combination of complementarity and selection effects with complementarity effects nearly twice as strong as selection effects overall. However, diverse communities rarely function significantly better than the best single species, at least in the short term. In the longer term, biodiversity can provide an insurance value similar to the risk-spreading benefits of diverse portfolios of financial investments. The effects of biodiversity on ecosystem functioning may have been underestimated by the first phase of research because of the short duration of many studies and the focus on singleecosystem processes in isolation rather than a consideration of all important ecosystem functions simultaneously. The next phase of research will focus, in part, on whether the benefits of biodiversity seen in experiments translate to real-world settings. GLOSSARY biodiversity. A contraction of biological diversity that encompasses all biological variation from the level of genes through populations, species, and functional groups (and sometimes higher levels such as landscape units) complementarity effect. The influence that combinations of species have on ecosystem functioning as a consequence of their interactions (e.g., resource partitioning, facilitation, reduced natural enemy impacts in diverse communities) ecosystem functioning. An umbrella term for the processes operating in an ecosystem ecosystem processes. The biogeochemical flows of energy and matter within and between ecosystems, e.g., primary production and nutrient cycling ecosystem service. An ecosystem process or property that is beneficial for human beings, e.g., the provision of foods and materials or sequestration of carbon dioxide selection effects. The influence that species have on ecosystem functioning simply through their speciesspeci fic traits and their relative abundance in a community (positive selection effects occur when species with higher-than-average monoculture performance dominate communities) 1. BACKGROUND AND HISTORY Darwin, in On the Origin of Species, initially proposed that changes in biodiversity could affect ecosystem functioning if niche space is more fully occupied in more diverse communities than depauperate ones. We use ecosystem functioning as an umbrella term to embrace all the biogeochemical processes that operate within ecosystems, primary production for example. This early work was apparently forgotten until the early 1990s, but the same reasoning was around in the mid-twentieth century, when it was proposed that more diverse mixtures of fish species should lead to greater productivity: ‘‘Presumably fish production will increase as the number of niches increases . . . [and] probably the proportion of occupied niches increases as the number of species of fishes increases.’’ Indeed, both of these early studies even presented data in support of this relationship (figure 1). General concern about the impact of anthropogenic biodiversity was voiced at the Rio Earth Summit in 1992, at which time the Convention on Biological Diversity (CBD) was launched with the signatures of 150 heads of government. This international treaty designed to promote sustainable development and the protection of biodiversity was evidence of political acceptance that anthropogenic biodiversity loss may have serious detrimental effects on humankind. Concerns highlighted at Rio and in the Convention also led to renewed scientific interest and a concerted effort by ecologists to understand the effects of changes in biodiversity on ecosystem functioning and the likely signi ficance of such changes for humankind. More than a decade’s worth of research has now been published, accompanied by a debate that focused in large part on the mechanisms underlying the relationship between biodiversity and functioning. Synthesis of the first decade of results through meta-analysis is helping to reveal both pattern and mechanism...