SEA OTTERS AND KELP FORESTS IN ALASKA - GENERALITY AND VARIATION IN ACOMMUNITY ECOLOGICAL PARADIGM

Multiscale patterns of spatial and temporal variation in density and p opulation structure were used to evaluate the generality of a three-tr ophic-level cascade among sea otters (Enhydra lutris), invertebrate he rbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 1 53 randomly selected sites distributed among five locations with and f our locations without sea otters. Both sea urchin and kelp abundance d iffered significantly among locations with vs. without sea otters in t he Aleutian Islands and southeast Alaska. There was little (Aleutian I slands) or no (southeast Alaska) overlap between sites with and withou t sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these a nalyses demonstrate that sea otter predation has a predictable and bro adly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages als o differed significantly (although less dramatically) between location s with and without sea otters. Temporal variation in community structu re was assessed over periods of from 3 to 15 yr at sites in the Aleuti an Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained lar gely unchanged at most sites where sea otters were continuously presen t or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abun dance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed sign ificantly, and in the expected directions, at sites that were being re colonized by sea otters. Sea urchin biomass declined by 50% in the Ale utian Islands and by nearly 100% in southeast Alaska following the spr ead of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly signific ant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large-scale differences in echinoid recruitment coupled with size- selective predation by sea otters for larger urchins. The length of ur chin jaws (correlated with test diameter, r(2) = 0.968) in sea otter s eats indicates that sea urchins <15-20 mm test diameter are rarely eat en by foraging sea otters. Sea urchin populations in the Aleutian Isla nds included high densities of small individuals (<20 mm test diameter ) at all sites and during all years sampled, whereas in southeast Alas ka similarly sized urchins were absent from most populations during mo st years. Small (<30-35 mm test diameter) tetracycline-marked urchins in the Aleutian Islands grew at a maximum rate of approximate to 10 mm /yr; thus the population must have significant recruitment annually, o r at least every several years. In contrast, echinoid recruitment in s outheast Alaska was more episodic, with marry years to perhaps decades separating significant events. Our findings help explain regional dif ferences in recovery rates of kelp forests following recolonization by sea otters.