Browsing by Author "Winnie, John A. Jr."

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The many effects of carnivores on their prey and their implications for trophic cascades, and ecosystem structure and function
(2016-09) Winnie, John A. Jr.; Creel, Scott
Despite some controversy, a wide range of research across multiple taxa have established that carnivores strongly influence prey population dynamics both through direct offtake and indirect risk effects. Because of these powerful top-down effects carnivores can influence ecosystems across multiple trophic levels. Here we discuss research addressing carnivore direct- and indirect effects on prey, and how these effects can influence overall ecosystem structure and function.
A survey of the effects of wolf predation risk on pregnancy rates and calf recruitment in elk
(2011-12) Creel, Scott; Christianson, David A.; Winnie, John A. Jr.
We have previously found that the behavioral responses of elk to the risk of predation by wolves are correlated with changes in habitat selection, altered diets, reduced food intake, decreased fecal progesterone concentrations, and decreased calf recruitment. P. J. White et al. recently questioned these results, concluding that “multiple lines of evidence and/or strong validation should be brought to bear before relying on indirect measures of how predators affect pregnancy rates.” Here, we systematically surveyed available data and found that five studies (with data from 10 widely distributed populations) have directly detected decreases of 24–43% in elk pregnancy rates in response to increased predation risk. This survey includes data not discussed by White et al. from their own research, which reveal a 32% decrease in pregnancy rates following wolf recolonization in central Yellowstone. Following the survey of available data, we discuss several methodological and statistical problems in White et al.'s study that would be expected to mask the effects of predation risk. While other factors also clearly affect elk recruitment, multiple lines of evidence using a broad array of methods have detected an association between predation risk and reduced reproduction in elk.
Underestimating the frequency, strength and cost of anti-predator responses with data from GPS collars: an example with wolves and elk.
(2013-11) Creel, Scott; Winnie, John A. Jr.; Christianson, David A.
Field studies that rely on fixes from GPS-collared predators to identify encounters with prey will often underestimate the frequency and strength of antipredator responses. These underestimation biases have several mechanistic causes. (1) Step bias: The distance between successive GPS fixes can be large, and encounters that occur during these intervals go undetected. This bias will generally be strongest for cursorial hunters that can rapidly cover large distances (e.g., wolves and African wild dogs) and when the interval between GPS fixes is long relative to the duration of a hunt. Step bias is amplified as the path travelled between successive GPS fixes deviates from a straight line. (2) Scatter bias: Only a small fraction of the predators in a population typically carry GPS collars, and prey encounters with uncollared predators go undetected unless a collared group-mate is present. This bias will generally be stronger for fission–fusion hunters (e.g., spotted hyenas, wolves, and lions) than for highly cohesive hunters (e.g., African wild dogs), particularly when their group sizes are large. Step bias and scatter bias both cause underestimation of the frequency of antipredator responses. (3) Strength bias: Observations of prey in the absence of GPS fix from a collared predator will generally include a mixture of cases in which predators were truly absent and cases in which predators were present but not detected, which causes underestimation of the strength of antipredator responses. We quantified these biases with data from wolves and African wild dogs and found that fixes from GPS collars at 3-h intervals underestimated the frequency and strength of antipredator responses by a factor >10. We reexamined the results of a recent study of the nonconsumptive effects of wolves on elk in l