Tuesday, 4 May 2010

Abstracts: Effects of a Snowshoe Hare decline on Survival of Dall's Sheep in Alaska

I can't believe I didn't post this. Steve Arthur is a good biologist, and a good person, and the study is wonderfully clever. In his talk he dissected the relation between hare abundance and predation on Dall's sheep lambs. Coyotes increase in abundance after hare highs, and when hares subsequently plummet, predation shifts to the lambs. But that didn't become apparent until the data was analyzed as a function of survival, and not census, as abundance was generally positively correlated with hare abundance.

Very clever.

Arthur, Stephen M., and Laura R. Prugh

We estimated survival of Dall's sheep (Ovis dalli) in the central Alaska Range during years of differing snowshoe hare (Lepus americanus) abundance to test whether indirect interactions with a cyclic hare population affect Dall's sheep either negatively, by subsidizing predators (apparent competition), or positively, by diverting predation (apparent commensalism). Annual survival of adult ewes was consistently high ( = 0.85); whereas, lamb survival was low and ranged from 0.15-0.63. The main predators of lambs were coyotes (Canis latrans) and golden eagles (Aquila chrysaetos), which rely on hares as their primary food and prey on lambs secondarily. Coyotes and eagles killed 78% of 65 radiocollared lambs for which cause of death was known. Lamb survival was negatively related to hare abundance during the previous year, and lamb survival rates more than doubled when hare abundance declined, supporting the hypothesis of predator-mediated apparent competition between hares and sheep. However, stage-specific predation and delays in predator responses to changes in hare numbers led to a positive relationship between abundance indices of adult Dall's sheep and hares. Lacking reliable estimates of survival, a manager might erroneously conclude that the relationship was apparent commensalism. Thus, support for different indirect effects can be obtained from differing types of data, demonstrating the need to determine the mechanisms that create indirect interactions. Long-term survey data suggest that predation by coyotes is limiting this sheep population below levels typical when coyotes were rare or absent. Understanding the nature of indirect interactions is necessary to effectively manage complex predator-prey communities.

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