The ecology of extinctions in kelp forest communities

Abstract

We recognize three levels of extinction–global, local, and ecological – and provide examples of each. The protection and recovery of sea otters (Enhydra lutris) has provided abundant evidence of the consequences of their local extinction from kelp forest communities in the North Pacifc Ocean. These consequences include release of benthic invertebrate populations from limitation by predation; deforestation of kelp beds due to increased grazing by herbivorous sea urchins, one of the otter's main prey; and various cascading effects resulting from the biological and physical importance of kelp in coastal ecosystems. These interactions probably were important agents of selection for certain species. Two other examples are discussed: Steller's sea cow (Hydrodamalis gigas), a case of global extinction, and spiny lobsters, a possible case of ecological extinction. We speculate that grazing by sea cows was an important disturbance to surface-canopy-forming kelps and other algae in the littoral zones, but also point out that any such interactions probably acted in concert with physical disturbances by ocean waves. The ecological and evolutionary importance of sea cow grazing probably will remain a matter of speculation and conjecture because the species is globally extinct. Predation by spiny lobsters limits a variety of littoral and sublittoral invertebrate populations, particularly mollusks In one remarkable example, the reduction or local extinction of spiny lobsters enabled predutory whelks to increase in size and abundance, ultimately resulting in a predutor-prey role reversal. From these and other case studies we can clued that (1) the extinction of consumers may have brad and sometimes unexpected influences on kelp forest ecosystems; (2) direct or indirect interactions with now-extinct species probably exerted important selective influences on many extant forms; (3) such ecological and evolutionary influences are best understood where local or ecological extinctions, followed by recoveries, have provided comparisons in space or time; and (4) because of various ecological and behavioral barriers, local extinctions and their ecological consequences may not be simply reversed by protecting or reintroducing depleted or locally extinct species.