Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm

Abstract

Multiscale patterns of spatial and temporal variation in density and population structure were used to evaluate the generality of a three—trophic—level cascade among sea otters (Enhydra lutris), invertebrate herbivores, 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 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian 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 largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread 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 significant 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 urchin jaws (correlated with test diameter, r2 = 0.968) in sea otter scats indicates that sea urchins <15—20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in south...

Introduced rats indirectly change marine rocky intertidal communities from algae- to invertebrate-dominated

“…Thus, the direct and indirect effects of top-level predators on community composition have repeatedly been demonstrated at the population or community level, but landscape-level illustrations of communities transformed by top predators via trophic cascades are still scarce (but see refs. [2][3][4]. Vertebrate predators introduced to oceanic islands throughout an archipelago provide ecologists with opportunities to investigate the presence and extent of such cascades over larger spatial scales.…”

Sea otters, kelp forests, and the extinction of Steller’s sea cow

“…A trophic cascade in which predation by sea otters limits herbivorous sea urchins is essential for the maintenance of kelp forests across much of the North Pacific Ocean's rocky reefs at higher latitudes (20,21,38,39). Kelp density and standing biomass (a reasonable surrogate for sea cow carrying capacity) is >10-fold lower at islands where sea otter populations have been reduced or lost compared with otherwise similar islands where sea otters abound (21,40). Moreover, transitions between kelp-dominated and deforested ecosystem states with varying sea otter densities occur rapidly, as sharply punctuated phase shifts (41,42).…”

“…We have documented the influences of sea otters on coastal ecosystems by measuring changes that occurred with the recovery of local populations from the Pacific maritime fur trade (21), and then with the more recent collapse of recovered sea otter populations because of killer whale predation (47).…”

“…This particular case is intriguing and potentially informative because various aspects of the rise and fall of sea cows are reasonably well known; the final step in the sea cows' demise occurred just several hundred years ago, in the presence of modern human observers; and dynamics of the coastal kelp forest interaction web, to which the sea cow was intimately linked, have been extensively studied and are well understood. Although aboriginal people (and early Russian fur traders in the Commander Islands) exploited sea cows and may have reduced or even exterminated local populations through that process, humans also hunted and reduced sea otter populations to such low levels that the otter's keystone role in maintaining the kelp forest ecosystem (20) was lost (21)(22)(23). Sea cows lived in kelp forests and fed on kelp, thus raising the question of the relative importance in driving the sea cow's demise of the direct effects of human exploitation vs. the indirect effects of reduced food or altered habitat caused by the ecological extinction of sea otters.…”

The High Cost of Motherhood: End-Lactation Syndrome in Southern Sea Otters (Enhydra Lutris Nereis) on the Central California Coast, Usa

“…and to quantify sea otter impacts on prey populations (e.g. Hines and Pearse 1982, Ostfeld 1982, Kvitek and Oliver 1988, Estes and Duggins 1995. Studies of sea otter foraging ecology benefit from the unique properties of sea otter behavior: specifically, sea otters dive to the sea bottom to capture their prey but then return to the surface to handle and consume it while lying on their backs, usually within sight of shore, making it possible for a trained observer with a high powered telescope to directly record all aspects of diet composition and feeding activity from tagged individuals (see Chapter 1, Methods).…”