Limited effects of a keystone species:trends of sea otters and kelp forests at the Semichi Islands, Alaska

Abstract

Sea otters are well known as a keystone species because of their ability to transform sea urchin-dominated communities into kelp-dominated communities by preying on sea urchins and thus reducing the intensity of herbivory. After being locally extinct for more than a century, sea otters re-colonized the Sernichi Islands in the Aleutian Archipelago, Alaska in the early 1990s. Here, otter populations increased to about 400 individuals by 1994, but rapidly declined to about 100 by 1997. Roughly 7 yr after initial otter re-colonization, there were only marginal changes in sea urchm biomass, mean maximum test size, and kelp density. These small changes may be the first steps in the cascahng effects on community structure typically found with the invasion of a keystone species. However, no wholesale change in community structure occurred following re-colonization and growth of the sea otter population. Instead, this study describes a transition state and identifies factors such as keystone species density and residence time that can be important in dictating the degree to which otter effects are manifested.

Changes in sea urchins and kelp following a reduction in sea otter density as a result of the Exxon Valdez oil spill

“…An approximate 90% reduction in sea otters in the western Aleutian Islands was followed by a 9-fold increase in sea urchin Strongylocentrotus polyacanthus biomass and a nearly 90% reduction in kelp (Estes et al 1998). In contrast, in both the Knight Island area of Prince William Sound (our study) and the Semichi Islands (Konar 2000), a reduction in sea otter abundance on the order of 50% resulted in no apparent effect on sea urchin biomass, and little or no change in kelp abundance.…”

“…In some cases, sea urchin densities have dramatically declined and kelp abundance has increased within a few months of sea otter reoccupation (Laur et al 1988, Watson 1993. However, little response was observed in the 10 yr following reoccupation by sea otters in the Sernichi Islands, Alaska (Konar 2000). An approximate 90% reduction in sea otters in the western Aleutian Islands was followed by a 9-fold increase in sea urchin Strongylocentrotus polyacanthus biomass and a nearly 90% reduction in kelp (Estes et al 1998).…”

“…Both types of evidence have led to similar conclusions regarding the generalized effect of the sea otter foraging on kelp forest community structure. Fewer studies have examined the transitions during recolonization by sea otters (Laur et al 1988, Watson 1993, Estes & Duggins 1995, Konar 2000, in this issue), and only 2 recent studies (Estes et al 1998, Konar 2000 have examined community response to a reduction in the abundance of sea otters. Yet it is these sorts of observations of dynamic systems that may provide greater understanding of the role of sea otters, and keystone predators in general, in determining community structure.…”

Community-wide distribution of predator–prey interaction strength in kelp forests

“…A problem with such definitions is that species shown to have the potential for strong effects (sensu Paine; ref. 1) do not always impart such effects because of variability in their density (5,19), and species considered weak interactors might impart stronger than expected ecological effects in cases of high density (11). Furthermore, theoretical work suggests that the removal of a supposedly weak interactor can sometimes drive other species to extinction (7).…”

Seasonal Zonation Patterns of Benthic Amphipods in a Sandy Shore Surf Zone of Korea

“…However, numerous nearshore dive studies have been conducted at other locations in the Aleutians, primarily focusing on the flora and fauna of hard substrates (Estes et al, 1989(Estes et al, , 2004Vicknair, 1997;Konar, 2000;Reisewitz et al, 2006). The recent decline of sea otters (Enhydra lutris) throughout the Aleutian archipelago has caused a shift in the rocky nearshore community, from one dominated by kelps to one dominated by sea urchins (Strongylocentrotus polyacanthus) (Estes et al, 2010).…”