Miller, Brian, Barb Dugelby, Dave Foreman, Carlos Martinez del Rio, Reed Noss, Mike Phillips, Rich Reading, Michael E. Soulé, John Terborgh, and Louisa Wilcox. 2001. The importance of large carnivores to healthy ecosystems. Endangered Species Update V. 18, I 5:202(9).

RELEVANT TO: DECLINING SPECIES

WILDLIFE

ECOSYSTEM FUNCTION AND PROCESS

RESTORATION

DESCRIPTION OF DOCUMENT

This document by wildlife and conservation biologists reviews scientific literature on the ecosystem effects of large carnivores and discusses pathways through which carnivores contribute to ecosystem processes and species diversity. It outlines how these interactions have strong implications for management within the habitat of carnivores. It then discusses the long-term changes in ecosystem structure and function that result from carnivore decline, specifically how the loss of carnivores simplifies ecosystems over the long-term. The document concludes that scientific data increasingly indicate that carnivores play important roles in ecological health. As long as management is driven by paradigms that view carnivores as problems rather than relying on new scientific information, ecosystems will continue to decline.

MAJOR FINDINGS

  1. The impact of carnivores extends past their prey. Carnivores impact prey and plants both directly and indirectly: Directly by reducing numbers of prey and indirectly by causing prey to alter their behavior so that they become less vulnerable. If a predator selects from a wide range of prey species, the presence of the predator may cause all prey species to reduce their respective niches and thus reduce competition among those species. If the predator is removed, prey species may compete for limited resources and superior competitors may displace weaker competitors leading to less diversity through competitive exclusion (p. 2).
  2. Because herbivores eat seeds and plants, predation on that group influences the structure of the plant community. The plant community, in turn, influences distribution, abundance, and competitive interaction within groups of birds, mammals, and insects. This is exemplified if the third trophic level is removed as it was with the sea otter in the north Pacific. Over-harvesting caused sea otters to decline. As a result, marine invertebrate herbivores increased in number and devastated the kelp forest. This produced a cascade of indirect effects that reduced diversity in a host of fish, seabirds, invertebrates, and raptors. Gradual recovery of the sea otter in recent years has reduced the invertebrate grazers, and the kelp forests and associated fauna have recovered (pp. 2-3).
  3. Large carnivores directly and indirectly impact smaller predators and therefore the community structure of small prey. In California, Soulé et al. (1988) and Crooks and Soulé (1999) documented more species of scrub-dependent birds in canyons with coyotes that in canyons without coyotes. The absence of coyotes allowed behavioral release of opossums, foxes, and house cats. These species preyed heavily on song birds and native rodents (p. 3).
  4. There is a growing body of macroecological evidence documenting the impact of carnivores on ecosystems. In 75% of the studies analyzed by the authors, predator removal had a significant direct impact on herbivore numbers (positive), resulting in a significant impact on plant damage (positive), plant biomass (negative), and plant-reproductive output (negative) (p. 4).
  5. Drastic changes in ecosystems have been linked to carnivore extirpation or control. For example, managers have reduced carnivore numbers to keep ungulates at artificially high levels for recreational hunting. Yet, overabundance of white-tailed deer has been shown to reduce numbers of native rodent species, cause declines in understory nesting birds, obliterate understory vegetation in some forests, and even eliminate regeneration of the oak (Alverson 1988, and others). Similarly, hyperabundance of moose in areas of the Greater Yellowstone Ecosystem without wolves and grizzly bears has reduced neotropical migrant bird populations (Berger et al. 2001). Ripple and Larson (2000) have reported that aspen overstory recruitment ceased when wolves disappeared from Yellowstone National Park. Wolves are a significant predator of elk and wolves may positively influence aspen overstory through a trophic cascade caused by reducing elk numbers, modifying elk movement, and changing elk browsing patterns on aspen (Ripple and Larson 2000) (p. 4).
  6. The subtleties of interactions can vary significantly under different environmental conditions. Abiotic [i.e., non-living] factors, such as type, frequency, and scale of natural disturbances, can influence the relative importance of top-down or bottom-up forces. Disturbance over large geographic scales shortens food chains (at least temporarily) and thus changes interaction dynamics among trophic levels (Menge and Sutherland 1976). Climatic patterns, such as El Niño or La Niña affect the ability of keystone predators to regulate prey in aquatic and terrestrial systems. Other factors include migration, habitats occupied, and relative abundance of prey species (p. 5).
  7. Scientific data increasingly indicate that carnivores play an important role in ecological health. The authors emphasize that predators impact prey populations in more than a demographic fashion. They change movement and activity patterns, and these behavioral effects can have large ecological significance. As long as we fail to think in terms of an ecosystem rather than individual species only, we will continue to lose diversity despite good intentions, higher budgets, and increasing human effort. In short, management policies based on reducing carnivore numbers have caused, and will continue to cause, severe harm to many other native organisms (p. 6).

QUESTIONS THIS RAISES FOR THE THREE FORESTS

  1. Is there documentation that indicates the impact (positive or negative) of large carnivore decline or extirpation on herbivores and plant health within these forests, specifically wolves. Are current numbers of herbivores reducing native plant biomass or reproductive output, or causing other damage on your Forest? How widespread is the problem? Has your Forest discussed this issue in relationship to habitat management?
  2. What implication does the scientific data indicating the importance of large carnivores in the ecosystem and the resultant impact of their decline or loss have on the future management of the Three Forests?
  3. Are ecologically valuable habitat types, such as aspen or willow, currently negatively impacted by herbivore grazing on any of the Three Forests? What evidence, if any, has been gathered regarding the impact of large carnivore loss/reduction on the Three Forests' herbivores? What consideration is being given to impacts of large carnivore reduction/loss in current Forest planning?
  4. What carnivores are the Three Forests currently managing for? Do the Three Forests plan to increase their protection of carnivores now and into the future? Will the Three Forests be managed for the potential reinhabitation by large carnivores (such as gray wolves naturally migrating from adjacent states, a strong possibility during the lifetime of the new Forest Plans)?

FOREST MANAGEMENT SIGNIFICANCE

  1. Given the importance of large carnivores to ecological processes, the Three Forests must carefully consider the impact of their loss/reduction and consider appropriate reinhabitation and/or reintroduction of large carnivores.
  2. If monitoring of herbivore impacts on aspen and other ecologically valuable habitat types has not taken place on the Three Forests, or has been limited in scope, such monitoring should be expanded so that the Three Forests fully understand the problems, causes, and appropriate means by which to address these in the Forest Plans.
  3. Wolf "restoration" (naturally occurring or by human management) must be addressed in the Forest Plans.