Evol Ecol Res 1: 285-301 (1999) Full PDF if your library subscribes.
The effect of risk of mortality on the foraging behaviour of animals faced with time and digestive capacity constraints
Peter A. Abrams1
and Oswald J. Schmitz2
1Department of Zoology, University of Maryland, College Park, MD 20742 and 2School of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, CT 06511, USA
Author to whom all correspondence should be addressed.
This article examines the effect of risk of mortality on the optimal diet of an animal foraging for two food types. The foods are characterized by different nutritional or energetic values per unit volume, and the forager has constraints on both the amount of time it has available for foraging and on the volume of food that can be processed per unit time. Such a situation characterizes the diet choice problem faced by many herbivores. The two food types may occur in the same habitat, or they may occur in different habitat patches; in the latter case, they cannot be encountered simultaneously. Unlike earlier analyses of these diet problems, here we consider the risk of mortality when foraging and allow risk to differ between habitats. Optimal time allocation strategies and the resultant functional responses are calculated for both one- and two-habitat situations. Mortality risk can substantially change the forager’s time allocations and, consequently, its functional responses. Increasing mortality risk in both habitats proportionally can increase use of the habitat that has the greater risk. Time allocation often responds in a non-monotonic fashion to changes in the density of a particular food. As a result, functional responses may decrease with increasing food abundance over one or more ranges of abundance. Experimental findings on the response of grasshopper foraging to the risk of spider predation are compared with the theory. Finally, the possible indirect interactions between the food species in this simple food web are discussed.
Keywords: constrained diets, foraging behaviour, functional response, habitat selection, optimal diet, predation risk, time allocation.
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