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

Abstract

The late Pleistocene extinction of so many large-bodied vertebrates has been variously attributed to two general causes: rapid climate change and the effects of humans as they spread from the Old World to previously uninhabited continents and islands. Many large-bodied vertebrates, especially large apex predators, maintain their associated ecosystems through top-down forcing processes, especially trophic cascades, and megaherbivores also exert an array of strong indirect effects on their communities. Thus, a third possibility for at least some of the Pleistocene extinctions is that they occurred through habitat changes resulting from the loss of these other keystone species. Here we explore the plausibility of this mechanism, using information on sea otters, kelp forests, and the recent extinction of Steller's sea cows from the Commander Islands. Large numbers of sea cows occurred in the Commander Islands at the time of their discovery by Europeans in 1741. Although extinction of these last remaining sea cows during early years of the Pacific maritime fur trade is widely thought to be a consequence of direct human overkill, we show that it is also a probable consequence of the loss of sea otters and the co-occurring loss of kelp, even if not a single sea cow had been killed directly by humans. This example supports the hypothesis that the directly caused extinctions of a few large vertebrates in the late Pleistocene may have resulted in the coextinction of numerous other species.

Keywords: Commander Islands; Steller’s sea cow; extinction; kelp; sea otter.

Fig. 1.

Trajectories of sea otter population declines in the Aleutian Islands during the 1990s and early 2000s (Upper) and the Commander Islands after the onset of the Pacific maritime fur trade in 1743 (Lower). (Upper) Data points are from skiff surveys of Adak Island. (Lower) Line assumes that sea otters were at maximum density in 1743 and extinct by 1753, and that the decline was exponential. Open red boxes indicate time window of kelp forest phase shift at Adak Island and the corresponding estimated time of kelp forest phase shift in the Commander Islands.

Fig. 2.

Declines in kelp density after sea otter population collapse at Adak and Amchitka Islands. Predecline data were obtained in 1987. Postdecline data from Adak and Amchitka were obtained in 1997 and 1999, respectively. Error bars (SEs) are too small to show on the graphs.

Fig. 3.

Some known food web pathways by which changes in abundance of predators and other large vertebrates influence the abundance of other species through indirect interactions involving top-down forcing. (A) Classic trophic cascades. (B) Apparent competition. (C) Mesopredator release. (D) Trophic cascade beginning with megaherbivore and with megaherbivores sustaining predators via scavenging. Arrow line weights correspond with interaction strengths. Extinctions might occur as a result of increased interaction strength at any of the negative linkages. See text for examples.

Fig. 4.

Modeled sea cow population trajectories in response to kelp forest phase shift after the ecological extinction of sea otters in the Commander Islands. Main figure shows boxplots for the number of living sea cows each year after the onset of starvation, including variation across both estimated starvation-caused mortality rates and life history parameters. (Inset) Boxplot for the number of living animals predicted to remain in 1768, the year of the historically recorded extinction, segregated by the assumed starvation-caused annual mortality rate. Only simulations using the lowest of the eight mortality rates predict more than one survivor by 1768, and even this most optimistic scenario always predicts fewer than seven survivors by that year. Note that these predictions do not account for emigration, which would likely reduce local numbers far below those caused just by elevated mortality.