Abstract
Collective behaviours contributing to patterns of group formation and coordinated movement are common across many ecosystems and taxa. Their ubiquity is presumably due to altering interactions between individuals and their predators, resources and physical environment in ways that enhance individual fitness. On the other hand, fitness costs are also often associated with group formation. Modifications to these interactions have the potential to dramatically impact population-level processes, such as trophic interactions or patterns of space use in relation to abiotic environmental variation. In a wide variety of empirical systems and models, collective behaviour has been shown to enhance access to ephemeral patches of resources, reduce the risk of predation and reduce vulnerability to environmental fluctuation. Evolution of collective behaviour should accordingly depend on the advantages of collective behaviour weighed against the costs experienced at the individual level. As an illustrative case study, we consider the potential trade-offs on Malthusian fitness associated with patterns of group formation and movement by migratory Thomson's gazelles in the Serengeti ecosystem.This article is part of the theme issue ‘Collective movement ecology’.
Highlights
The past 20 years have witnessed an explosion of interest in the mechanisms contributing to patterns of spatial aggregation and coordinated movement of animal groups [1,2,3,4]
Our goal in this paper is to develop a theoretical framework for understanding the potential dynamical implications of collective herbivore behaviour with respect to trophic interactions
There is considerable potential for individuals to improve their individual fitness at the cost of that of other group members by abandoning small groups to join larger groups or pretending to be vigilant while foraging to more effectively compete with conspecifics [11,12]
Summary
The past 20 years have witnessed an explosion of interest in the mechanisms contributing to patterns of spatial aggregation and coordinated movement of animal groups [1,2,3,4]. Deep understanding of the dynamical properties at the population level will no doubt require highly detailed agent-based models far beyond the capacity of this paper Since this field is far too young to have developed a deep body of observational data or controlled experiments, here we use conventional consumer-resource theory applied to the concept of Malthusian (i.e. per capita) fitness as a lens to identify potentially important linkages that may be worthy candidates for deeper study. We start with mechanistic consideration of how forage abundance and maturational changes in forage nutritional quality serve as fundamental constraints that structure gazelle space use at a fine spatio-temporal scale We apply this local perspective to evaluate the multiple effects of group formation on the probability of encounter by predators, the probability of prey capture once encountered by predators and interference among prey herd members. We integrate across group sizes and levels of forage abundance to consider how local levels of Malthusian fitness might translate into aggregate demographic rates for the entire population
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