Evolution of defense against depletion of local food resources in a mechanistic foraging model

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Abstract

Models of resource defense are usually based on the assumption that individuals fight over the possession of discrete food items. In many territorial species, however, conflicts occur over access to an area in space that contains resources rather than the resources themselves. We investigate under which conditions defense against depletion of local resources instead of single resource items can evolve from a nonaggressive ancestral population using a spatially explicit mechanistic model with resource dynamics and individual movement. We find that, in general, details of the model assumptions have a great influence on the costs and benefits of different behavior in the model. For patchy resources, defense evolves if fighting costs are very high or if individuals cannot unilaterally opt out of conflicts. If resources are distributed uniformly, defense appears only if individuals can make their behavior dependent on distance to their opponent. Introducing role asymmetries during conflicts in general increases the frequency of contests but reduces the probability that they escalate. If losers of a fight control how far they run, aggressiveness disappears or is greatly reduced for patchy resources but increases significantly for uniform resource distribution. Our results show how defense of space and territoriality could evolve even if resources are neither discrete nor clumped. The fact that seemingly minor differences in how individuals make decisions during encounters lead to huge differences in model outcome highlights the need for more mechanistic models of animal conflicts.

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