The future of coral reefs

Coral reefs, with their millions of species, have changed profoundly becaus e of the effects of people, and will continue to do so for the foreseeable future. Reefs are subject to many of the same processes that affect other h uman-dominated ecosystems, but some special features merit emphasis: (i) Ma ny dominant reef builders spawn eggs and sperm into the water column, where fertilization occurs. They are thus particularly vulnerable to Allee effec ts, including potential extinction associated with chronic reproductive fai lure. (ii) The corals likely to be most resistant to the effects of habitat degradation are small, short-lived "weedy" corals that have limited disper sal capabilities at the larval stage. Habitat degradation, together with ha bitat fragmentation, will therefore lead to the establishment of geneticall y isolated clusters of inbreeding corals. (iii) Increases in average sea te mperatures by as little as 1 degreesC, a likely result of global climate ch ange, can cause coral "bleaching" (the breakdown of coral-algal symbiosis), changes in symbiont communities, and coral death. (iv) The activities of p eople near reefs increase both fishing pressure and nutrient inputs. In gen eral, these processes favor more rapidly growing competitors, often fleshy seaweeds, and may also result in explosions of predator populations. (v) Co mbinations of stress appear to be associated with threshold responses and E cological surprises, including devastating pathogen outbreaks. (vi) The fos sil record suggests that corals as a group are more likely to suffer extinc tions than some of the groups that associate with them, whose habitat requi rements may be less stringent.