jueves, 27 de septiembre de 2012

Trophic cascades linking wolves (Canis lupus), coyotes (Canis latrans), and small mammals: Brian J. Miller et al. (2012)

When large carnivores are extirpated from ecosystems that evolved with apex predators, these systems can change at the herbivore and plant trophic levels. Such changes across trophic levels are called cascading effects and they are very important to conservation. Studies on the effects of reintroduced wolves in Yellowstone National Park have examined the interaction pathway of wolves (Canis lupus L., 1758) to ungulates to plants. This study examines the interaction effects of wolves to coyotes to rodents (reversing mesopredator release in the absence of wolves). Coyotes (Canis latrans Say, 1823) generally avoided areas near a wolf den. However, when in the proximity of a den, they used woody habitats (pine or sage) compared with herbaceous habitats (grass or forb or sedge)– when they were away from the wolf den. Our data suggested a significant increase in rodent numbers, particularly voles (genus Microtus Schrank, 1798), during the 3-year study on plots that were within 3 km of the wolf den, but we did not detect a significant change in rodent numbers over time for more distant plots. Predation by coyotes may have depressed numbers of small mammals in areas away from the wolf den. These factors indicate a top–down effect by wolves on coyotes and subsequently on the rodents of the area. Restoration of wolves could be a powerful tool for regulating predation at lower trophic levels.

After the Green World Hypothesis (Hairston et al. 1960) and experiments by Paine (1966), scientists began to take more interest in how top predators affect ecosystem form and function. The removal of apex predators initiates a cascade of indirect effects that trickle downward across trophic levels, and that cascade changes the structure and function of an entire system (see reviews by Terborgh et al. 1999; Miller et al. 2001, Soulé et al. 2003, 2005; Ray et al. 2005; Terborgh and Estes 2010). The cascade caused by removing a top predator has dramatic effects on the conservation of flora and fauna. [...]
[...] In general, small mammals are important in many systems, providing an abundant source of food for small predators (mammalian, avian, and reptilian) and affecting seed dispersal, plant biomass, and plant nutrient content. Voles (genus Microtus Schrank, 1798) were the most important food source for coyotes (Canis latrans Say, 1823) in western Montana, and the relative use of habitats by coyotes was identical to the ranking of densities of vole population in those habitats (Reichel 1991). We hypothesized a similar relationship for our study site in Grand Teton National Park in Wyoming. In 1999, we began an ongoing study of coyote and small mammal population structure in areas away  from the Teton pack wolf den, with the hope of eventually testing the effects of wolves on these factors. In 2002, the wolf pack became secure, and we began to investigate whether wolf activity around their den changed coyote behavior and thus levels of exploitation on small mammals. [...]

Study area:
Grand Teton National Park lies on an active fault line in northwestern Wyoming. The Park covers 123 998 ha between the elevations of 1 951 and 4 198 m. It includes the Teton Range to the west and is bordered by the Gros Ventre Mountains to the southeast, the Washakie range to the northeast, the Yellowstone Volcanic Plateau to the north, and the Snake River drainage to the south (Love and Reed 1971). [...]

Coyote en el Grand Teton National Park
Is it possible that the extirpation of wolves in the early 20th century initiated trophic and competitive adjustments across broad ecosystems in the American West as coyotes were elevated from the status of mesopredator to that of apex predator? We suggest that this scenario is plausible. After wolf extirpation, coyote densities increased and distributions generally expanded (Bekoff 1977). Because coyotes could only partially mimic the role of wolves in driving ecosystem function, ecological and evolutionary relationships were distorted over large geographic and temporal scales (Paquet et al. 2010). Finally, coyotes are effective generalist predators, and they can drive down densities of prey and smaller predators (Henke and Bryant 1999; Biggins 2000; Berger et al. 2008; Prugh et al. 2009). This adversely affects biodiversity and demography of prey, small predators, and other mesopredators (see Smith and Quin 1996; Moseby et al. 2006; Letnic et al. 2009; Ritchie and Johnson 2009).
To our knowledge, the study reported herein is the first to link wolf presence to small-mammal densities as mediated by coyotes. The findings of these linkages are consistent with the mesopredator release hypothesis in that wolves likely affect trophic interactions through both known and unknown pathways. Since being reintroduced, it appears that interference competition from wolves may be limiting coyote distribution and densities in parts of the Greater Yellowstone Area (Berger and Gese 2007). Additional research on these two sympatric predators will be needed in the future for a more complete understanding of any cascading effects owing to these interactions. Density and  behaviorally mediated cascades associated with large predators appear to represent potent ecological forces potentially capable of affecting food webs through interactions of numerous species (Terborgh and Estes 2010). Repatriation of apex predators could be a powerful tool for regulating predation at lower trophic levels.

Trophic cascades linking wolves (Canis lupus), coyotes (Canis latrans), and small mammals: Brian J. Miller, Henry J. Harlow, Tyler S. Harlow, Dean Biggins, and William J. Ripple. Canadian Journal of Zoology, 2012, 90(1): 70-78, 10.1139/z11-115 http://bit.ly/ORlwVe (Publicado en la web por http://www.cof.orst.edu/ )

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