622 Functional Groups Figure 11 Temporal changes in Jamaica’s coral reef ecosystem in coral to macroalgal abundance relative to herbivorous fish and sea urchin, Diadema antillarum After herbivorous fishes were extirpated by overfishing, grazing remained high due to compensatory increases in Diadema abundance The reef ecosystem collapsed soon after the mass mortality of the urchin (see text) (Adapted from Done TJ (1995) Ecological criteria for evaluating coral reefs and their implications for managers and researchers Coral Reefs 14: 183–192.) Keystone Species to Functional Groups: Some Species Are More Equal Than Others Some species have a disproportionately great impact on ecosystems Hurlbert (1997) noted, ‘‘The general functional importance of a species (can be) defined as the sum, over all species, of the changes (sign ignored) in productivity which would occur on removal of the particular species from the (system)’’ (page 369) Most communities and ecosystems are dominated by a few species that are both abundant and important to that system (Figure 12) The few tree species that dominate given forests, corals that dominate zones of reefs, and specific grasses that comprise specific prairies need not be functionally grouped to be easily understood The loss of community or ecosystem dominants has a large impact on natural systems Diseases can eliminate dominant organism with impacts registering throughout the system American chestnut blight eliminated chestnuts from forests in eastern North America White-band disease killed most of the dominant elkhorn and staghorn corals and a pathogen caused the Diadema decline in shallow Caribbean reefs Rinderpest decimated wildebeest populations in Africa, and paramoebae caused a mass mortality of the green sea urchin in Nova Scotia Each of these dominant or ‘‘foundation’’ (sensu Dayton, 1972) species underwent a radical decrease in abundance, causing their respective ecosystems to change In these cases, the functional importance of these species is unquestioned Keystone species have been redefined by Paine (1995) and Power et al (1996) to include organisms that are functionally important but in relatively low abundance in the system Pathogens aside, this limits keystone species to apex predators Classic keystone predators such as sea otters or the predatory sea star, Pisaster, are the only species in their system and thus define their ecological function However, many systems not Figure 12 Functional importance and abundance of species The relatively low diversity of dominant/foundation and keystone species as well as the relatively low ecosystem importance of rare organisms make these groups poorer candidates for functional groupings (Modified from Hurlbert SH (1997) Functional importance vs keystoneness: Reformulating some questions in theoretical biocenology Austr J Ecol 22: 369–382.) have an obvious apex predator In contrast with the marine systems of the eastern North Pacific in which sea otters and Pisaster roam, the western North Atlantic has no single apex predator Top coastal predators include cod, haddock, hake, and wolffish When all these top predators were extirpated simultaneously by overfishing, the system changed Therefore, whether there is a single apex predator (i.e., a keystone species) or a functional group filling the same role matters little In other words, the ecological function can be carried out by a single species or by a suite of species, but the ecosystem will not change unless the ecological function changes significantly In the end, the ecological function, independent of the diversity, may be most important to the structure of ecosystems