IV.8 Seascape Patterns and Dynamics of Coral Reefs
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IV.8 Seascape Patterns and Dynamics of Coral Reefs Terry P. Hughes OUTLINE 1. Human use and abuse of coral reefs at multiple scales 2. Biogeography, hot spots, and conservation priorities 3. Population dynamics and dispersal 4. Habitat fragmentation in the sea 5. No-take areas, dispersal, and seascape dynamics Coral reef ecosystems exhibit complex dynamics driven by multiple, interacting processes that operate across a range of scales, from local to global and from days to millions of years. Many reefs have been degraded by human action in recent decades, reducing their capacity to absorb recurrent natural and unnatural disturbances. Rebuilding and sustaining the resilience of coral reefs will depend on interventions that are based on an improved understanding of multiscale processes. The current emphasis on conservation of biodiversity hot spots and on establishing networks of no-take areas does not adequately recognize the functional role of key species groups and the critical seascape connections between protected and unprotected reefs. GLOSSARY biodiversity hot spots. Regions with exceptionally high species richness, often selected as priority targets for the protection of marine ecosystems. endemics. Species with small geographic ranges. functional group. A group of species that share a common ecological function, regardless of their taxonomic affinities. An example is the herbivores found on coral reefs, a diverse assemblage that includes many species of fish, sea urchins, and threatened species such as green turtles and dugongs. pandemics. Species with very large geographic ranges. planula. The free-swimming larva of corals. Planulae are released directly by brooded corals following internal fertilization. Spawning corals release both eggs and sperm, and fertilization is external. spatial refuge. A location where a species or local population is less likely to be affected by its predators, competitors, or pathogens or other processes impacting on its survival, growth, and reproduction. 1. HUMAN USE AND ABUSE OF CORAL REEFS AT MULTIPLE SCALES Coral reefs are iconic high-diversity ecosystems that are important for coastal human societies, primarily in developing countries. They support the livelihoods of well over 250 million people, primarily through subsistence fisheries and international tourism. Despite their intrinsic aesthetic, cultural, and social value, many coral reefs worldwide have been degraded, especially in the past 20–30 years, reducing their capacity to regenerate from natural and human disturbances . The primary causes of these declines are coastal runoff resulting from land clearing and increased urbanization , overfishing, and climate change. Through time, the scale of human impacts has grown, with even the most remote reefs being increasingly vulnerable to global warming and ocean acidification. Coral reefs are structured by spatial processes that range in scale from global to local, and their capacity to regenerate following disturbance depends on sources of resilience that operate at multiple scales. However, the scales of management of marine ecosystems are usually mismatched to the scales of important processes and to a growing array of human impacts. Interventions are often fragmented and too small in scale to be effective. An emerging approach to management highlights the importance of key multiscale processes undertaken by critical functional groups of species (including the role of humans) that sustain ecosystem resilience across temporal and spatial scales ranging from global to local. 2. BIOGEOGRAPHY, HOT SPOTS, AND CONSERVATION PRIORITIES Many conservation groups and governments focus on the preservation of biodiversity hot spots as a priority. However, there are several new lines of evidence to suggest that regions with low species richness are more vulnerable and are of no less priority for intervention (Hughes et al., 2002). The primary coral reef biodiversity hot spot is located in the central Indo-Pacific, a large triangular region that straddles the equator, centered on the Philippines, Indonesia, Malaysia, and Papua New Guinea. The diversity of corals and other reef-associated species declines northward and southward away from the central Indo-Pacific hot spot as well to the east across the Pacific and westward across the Indian Ocean. Two secondary coral reef hot spots occur in the Red Sea and in the Caribbean. The similarity in regional-scale biodiversity patterns among major groups such as corals, reef fish, molluscs, and crustaceans reflects their shared history and a common set of mechanisms (e.g., barriers to dispersal) that exert a similar influence on many taxonomic groups. On land, biodiversity hot spots generally contain large numbers of endemic species that are potentially vulnerable to extinction because of their restricted distribution , especiallyifthey arealsouncommonorhighly specialized. However, the central Indo-Pacific hot spot is...