Sea otters, kelp forests, and the extinction of Steller’s sea cow James A. Estesa,1, Alexander Burdinb, and Daniel F. Doakc aDepartment of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95060; bKamchatka Branch of Pacific Geographical Institute, Russian Academy of Science, Petropavlovsk-Kamchatsky, 683000, Russia; and cEnvironmental Studies, University of Colorado, Boulder, CO 80309 Edited by Yadvinder Malhi, Oxford University, Oxford, United Kingdom, and accepted by the Editorial Board August 6, 2015 (received for review February 18, 2015) The late Pleistocene extinction of so many large-bodied verte- organized around complex interaction webs in which certain brates has been variously attributed to two general causes: rapid species, variously referred to as keystones (11, 12), foundation climate change and the effects of humans as they spread from the species (13), and ecosystem engineers (14), can have dispropor- Old World to previously uninhabited continents and islands. Many tionately strong influences on population and ecosystem dynamics. large-bodied vertebrates, especially large apex predators, main- Included among these strong interactors are many large-bodied tain their associated ecosystems through top-down forcing pro- vertebrates (15). Given the abilities of such species to shape the cesses, especially trophic cascades, and megaherbivores also exert ecosystems in which they occur, it is possible that loss of one or an array of strong indirect effects on their communities. Thus, a more species by direct human exploitation led to ecosystem third possibility for at least some of the Pleistocene extinctions is changes that in turn caused the demise of multiple other species. that they occurred through habitat changes resulting from the loss Although there has been conjecture that such losses could have of these other keystone species. Here we explore the plausibility significantly contributed to the Pleistocene extinctions (16, 17), of this mechanism, using information on sea otters, kelp forests, and the recent extinction of Steller’s sea cows from the Commander detailed understanding of the ecological roles of now-extinct Islands. Large numbers of sea cows occurred in the Commander species makes direct examination of this idea difficult. None- Islands at the time of their discovery by Europeans in 1741. Although theless, reliance of some species on the direct or indirect effects extinction of these last remaining sea cows during early years of the of others in complex natural communities mean that a loss of one Pacific maritime fur trade is widely thought to be a consequence of species may have led to losses of others (18, 19). direct human overkill, we show that it is also a probable consequence Here we explore an example (that of Steller’sseacow, of the loss of sea otters and the co-occurring loss of kelp, even if not Hydrodamalis gigas) that can help inform this hypothesis for a single sea cow had been killed directly by humans. This example Pleistocene extinctions. This particular case is intriguing and supports the hypothesis that the directly caused extinctions of a few potentially informative because various aspects of the rise and large vertebrates in the late Pleistocene may have resulted in the fall of sea cows are reasonably well known; the final step in the coextinction of numerous other species. sea cows’ demise occurred just several hundred years ago, in the presence of modern human observers; and dynamics of the Steller’s sea cow | extinction | sea otter | Commander Islands | kelp coastal kelp forest interaction web, to which the sea cow was intimately linked, have been extensively studied and are well xplanations for the sudden extinction of more than half of the understood. Although aboriginal people (and early Russian fur ENew World’s megafauna at the Pleistocene/Holocene tran- traders in the Commander Islands) exploited sea cows and may sition is a topic of long-standing interest and debate. Argument have reduced or even exterminated local populations through has focused on the relative importance of two would-be causes: that process, humans also hunted and reduced sea otter pop- rapid environmental change associated with late Pleistocene ulations to such low levels that the otter’s keystone role in glacial recession, and the effect of early humans crossing Beringia maintaining the kelp forest ecosystem (20) was lost (21–23). Sea and spreading into the New World. The absence of extinctions in cows lived in kelp forests and fed on kelp, thus raising the the New World during earlier interglacial periods, together with question of the relative importance in driving the sea cow’s de- discovery of similar large-animal losses after the peopling of mise of the direct effects of human exploitation vs. the indirect various other continents and islands, but at widely differing times, effects of reduced food or altered habitat caused by the eco- ’ has cast doubt on climate change as the extinctions principal logical extinction of sea otters. cause (1, 2). This reasoning led to the now widely held belief that human impacts figured prominently in megafaunal extinctions Significance worldwide (3, 4). The intriguing question is just how aboriginal peoples did this. Was the sudden disappearance of so many large animals entirely Human-caused extinctions are normally thought to result from overexploitation or habitat alteration. A third possible cause of a consequence of hunting and overexploitation (5, 6), a pan- extinction is the loss of interactions associated with keystone demic from diseases introduced by early humans or domesticated species. Using recent and historical information on sea otters animals they may have brought with them (7), human-induced and kelp forests, we show that the extinction of Steller’s sea environmental changes (e.g., as might have accompanied defor- cow from the Commander Islands in the mid-1700s would have estation or burning) (8), or some combination of these processes? been a nearly inevitable consequence of the overhunting of Another possibility, less discussed in the literature on mega- sea otters, which occurred a decade earlier. faunal extinctions, is that reductions and complete extinctions of directly hunted species may have led to the loss of other species Author contributions: J.A.E. designed research; J.A.E. performed research; J.A.E., A.B., and through cascading indirect interactions. Most speculation about D.F.D. analyzed data; and J.A.E. and D.F.D. wrote the paper. such effects has focused on the loss or decline of predators or The authors declare no conflict of interest. scavengers that relied on human-depleted prey [e.g., California This article is a PNAS Direct Submission. Y.M. is a guest editor invited by the Editorial condors (9) and Haast’s eagle (10)]. Although that is one pos- Board. sible pathway to coextinction, there are others. Ecosystems are 1To whom correspondence should be addressed. Email: [email protected]. 880–885 | PNAS | January 26, 2016 | vol. 113 | no. 4 www.pnas.org/cgi/doi/10.1073/pnas.1502552112 Downloaded by guest on September 23, 2021 In this article, we use historical records of sea otter harvest from extinctions of past megafauna as a result of indirect community the early maritime fur trade, the functional relationship between sea effects really is. Turvey and Risely (33) concluded from a pop- SPECIAL FEATURE otter density and kelp abundance, and data on the demographic and ulation viability analysis that the extinction of Steller’s sea cows behavioral responses to food shortages of dugongs (the sea cow’s in the Commander Islands was an inevitable consequence of closest living relative) to argue that the sea cow’s extinction in the human exploitation; the pre-exploitation size of this remnant Commander Islands would have almost inevitably occurred without population was larger than Stejneger’s (29) often-cited estimate asingledirecthumantakeofthespecies.Wediscussthebroader of 1,500 individuals, and contrary to proposals by Anderson (30) ramifications of this finding to the extinction of sea cows elsewhere and others, “environmental changes caused by sea otter declines in the North Pacific and to extinctions of other large vertebrates are unlikely to have contributed to this extinction event.” We after the peopling of the world. agree that direct human hunting of sea cows occurred and may well have figured prominently in the sea cow’s extinction. Results and Discussion However, it is not possible to establish a link between sea cow Steller’s Sea Cow. Hydrodamaline sirenians, which included mortality and human hunting. The mere presence of over- Steller’s sea cow as the last surviving species, radiated from the wintering fur hunters in the Commanders does not demonstrate tropics to coastal waters of the North Pacific Ocean with the that these people were substantially exploiting sea cows [al- onset of late Cenozoic polar cooling (24). On the basis of their though they surely did some of this (29)], much less the number cranial morphology (24) and Steller’s direct observations (25), of sea cows they may have killed and eaten. Direct human effects sea cows are thought to have been obligate algivores, feeding largely are thus likely, but not inevitable. Conversely, Anderson’s(30) on the diverse and highly productive kelps (order Laminariales) that proposal is backed up by compelling evidence: kelp forest collapse appear also to have originated or at least diversified in the North after the loss of sea otters is a virtual certainty. The timing of sea Pacific with late Cenozoic polar cooling (26). Steller’s sea cow otter exploitation and the sea cow’slossontheCommanderIs- ranged across the Pacific Rim from the northern Japanese ar- lands has led us to believe that the sea cow’s extinction from these chipelago to the Pacific coast of Baja California, Mexico (24), a islands would have occurred as an indirect effect of removing sea distribution that was roughly coincident with those of both sea otters, even if no direct hunting of sea cows had occurred or it was otters and kelps.