Evol Ecol Res 5: 329-343 (2003)     Full PDF if your library subscribes.

Search biases, frequency-dependent predation

and species co-existence

Christopher J. Whelan,1* Joel S. Brown2 and Gitogo Maina3

1Illinois Natural History Survey, 30239 South State Route 53, Wilmington, IL 60481, 2Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60647 and 3Department of Biology, Trinity Christian College, 6601 W. College Drive, Palos Heights, IL 60463, USA

Author to whom all correspondence should be addressed.
e-mail: virens@attglobal.net


Search biases – behaviours that intentionally or unintentionally affect a forager’s encounter probability with its resources – can have profound ecological and evolutionary effects. Courchamp et al. (2000) introduced a model of asymmetric apparent competition (hyperpredation) that incorporated a search bias influenced by the frequency of prey. We extend their model and assess the effects of a frequency-dependent search bias on the direct and indirect effects among a predator and two prey. We derive lines of equal harvest for the predator. These lines give all combinations of the abundances of the two prey such that the predator has the same harvest rate. (The predator’s zero net growth isocline is the equal harvest rate line that just meets the predator’s subsistence demand for energy.) All equal harvest rate lines have similar shapes, and all have regions of both positive and negative slope within the state space of prey densities. When they have a positive slope, one prey decreases the predator’s harvest rate and total fitness. This leads to an indirect interaction between the prey types that mimics exploitation (+, −). Under this ‘apparent exploitation’, one prey indirectly harms the second, while the second indirectly benefits the first. At other prey densities when the predator’s line of equal harvest rate has a negative slope, the prey interact indirectly through apparent competition (Holt, 1977, 1983). Our model provides an extension of Martin’s (1988a,b) verbal model, in which nest predators, through density-dependent foraging and search images, select for nest site diversification among co-occurring bird species. Our model shows when Martin’s hypothesis may and may not apply.

Keywords: direct and indirect interactions, frequency-dependent predation, nest predation, search bias, songbird communities, species co-existence, species interactions.

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