Keystone species, top-down trophic control, and conservation

Background/Question/Methods

Food webs—networks of consumer/prey interactions—are integral to living nature. Fundamental to understanding food web dynamics is the relative importance direct vs. indirect pathways on the one hand, and bottom-up vs. top-down control on the other. Keystones—rare species with disproportionately strong effects on food web dynamics—are often apex predators that exert their ecological influences via trophic cascades--top-down forcing effects that progress downward through food webs across multiple trophic levels. Keystone species and trophic cascades occur widely across global ecosystems, or once did. By definition, keystone species exert strong effects on the distribution and abundance of many other species, in turn often influencing the nature and strength of ecosystem services. Almost by definition therefore, keystone species are important to the conservation and management of living resources. In this talk I will provide an overview of the theory of keystone species, illustrate this theory with two case studies (sea otters and wolves), and discuss how resulting scientific knowledge has been and should be used in conservation and management.

Results/Conclusions

Sea otter predation limits sea urchins and other herbivorous invertebrates, in turn enhancing kelp forests and thereby influencing various other species and ecological processes that are linked to kelp. Direct effects of sea otters (shellfish depredation) are competitive with and therefore detrimental to human interests. The diverse indirect effects of this direct interaction, however, are often beneficial to human interests and human welfare. One example of an indirect beneficial effect is enhanced NPP; another is increased carbon sequestration. Wolves, like sea otters, limit the distribution and abundance of their prey, which include large ungulates (elk, deer and moose), livestock, and smaller mammalian carnivores (e.g., coyotes). The direct effects of wolves are sometimes beneficial (reduced coyotes) and sometimes detrimental (reduced livestock and large ungulates) to human interests, while the indirect effects of these interactions are often beneficial. Other species of large, apex predators likely function in similar ways. Despite our reasonably well-founded understanding of diverse keystone functions, management of keystone species has focused largely on direct effects that are detrimental to human welfare. A broader view is needed, regardless of one’s values or philosophical perspective. For biocentrists, keystone species affect biodiversity. For anthropocentrists, the challenge is to properly balance the costs and benefits of various and sundry keystone influences to people.