Abstract
Predators alter prey dynamics by direct killing and through the costs of antipredator responses or risk effects. Antipredator behavior includes proactive responses to long-term variation in risk (e.g., grouping patterns) and reactive responses to short-term variation in risk (e.g., intense vigilance). In a 3-year field study, we measured variation in antipredator responses and the foraging costs of these responses for 5 ungulates (zebra, wildebeest, Grant’s gazelle, impala, and giraffe) that comprised more than 90% of the prey community available to the 2 locally dominant predators, lions and spotted hyenas. Using a model-selection approach, we examined how vigilance and group size responded to attributes of the predator, prey, and environment. We found that 1) the strength of antipredator responses was affected by attributes of the predator, prey, and environment in which they met; 2) grouping and vigilance were complementary responses; 3) grouping was a proactive response to the use of dangerous habitats, whereas vigilance was a reactive response to finer cues about predation risk; 4) increased vigilance caused a large reduction in foraging for some species (but not all); and 5) there was no clear relationship between direct predation rates and the foraging costs of antipredator responses. Broadly, our results show that antipredator responses and their costs vary in a complex manner among prey species, the predators they face, and the environment in which they meet.
Highlights
In the results reported here, we used this single threshold of 400 m for each species in analyses of the responses of group size, vigilance, and foraging, but the results changed little when we used species-specific thresholds identified by inspection of group size plotted as a function of distance
Because we were interested in responses of prey to immediate predation risk, we considered 2 functional forms for the relationship of antipredator responses to the distance to predators and considered interactions between prey species and predator presence
For variables that prey might use to assess predation risk, group size was affected by predator presence, the species of predator, an interaction between predator presence and the species of prey, and habitat type (Tables 1 and 2)
Summary
Even the simplest prey species alter their behavior in response to predation risk (Turchin and Kareiva 1989) by retreating to safe habitats (Kotler et al 1991; Sih 1997), altering group size (Caro 2005; Creel and Winnie 2005), or increasing vigilance (Brown and Kotler 2004) In many cases, these antipredator responses are known to carry costs by decreasing feeding rates (Kotler et al 1991; Brown and Kotler 2004), altering diets (Christianson and Creel 2010), or provoking physiological stress responses (Clinchy et al 2004, 2013). Similar experiments with many invertebrates and small vertebrates show that antipredator responses can carry foraging costs that affect fitness (Peckarsky et al 1993; Relyea and Werner 1999; Brown and Kotler 2004)
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