EVOLUTIONARY CONSEQUENCES OF FOOD-CHAIN LENGTH IN KELP FOREST COMMUNITIES

Kelp forests are strongly influenced by macroinvertebrate grazing on f leshy macroalgae. In the North Pacific Ocean, sea otter predation on m acroinvertebrates substantially reduces the intensity of herbivory on macroalgae. Temperate Australasia, in contrast, has no known predator of comparable influence, These ecological and biogeographic patterns l ed us to predict that (i) the intensity of herbivory should be greater Tn temperate Australasia than in the North Pacific Ocean; thus (ii) A ustralasian seaweeds have been under stronger selection to evolve chem ical defenses and (iii) Australasian herbivores have been more strongl y selected to tolerate these compounds. We tested these predictions fi rst by measuring rates of algal tissue loss to herbivory at several lo cations in Australasian and North Pacific kelp forests. There were sig nificant differences in grazing rates among sea otter-dominated locati ons in the North Pacific (0-2% day(-1)), Australasia (5-7% day(-1)), a nd a North Pacific location lacking sea otters (80% day(-1)). The expe ctations that chronically high rates of herbivory in Australasia have selected for high concentrations of defensive secondary metabolites (p hlorotannins) in brown algae and increased tolerance of these defenses in the herbivores also were supported. Phlorotannin concentrations in kelps and fucoids from Australasia were, on average, 5-6 times higher than those in a comparable suite of North Pacific algae, confirming e arlier findings, Furthermore, feeding rates of Australasian herbivores were largely unaffected by phlorotannins, regardless of the compounds ' regional source, North Pacific herbivores, in contrast, were consist ently deterred by phlorotannins from both Australasia and the North Pa cific, These findings suggest that top-level consumers, acting through food chains of various lengths, can strongly influence the ecology an d evolution of plant-herbivore interactions.