Journal of Theoretical Biology 245 (2007) 763–774 Population dynamical consequences of gregariousness in a size-structured consumer–resource interaction Tobias van Kooten a,Ã , Lennart Persson a , Andre´ M. de Roos b a Department of Ecology and Environmental Science, Umea˚ University, S–901 87 Umea˚, Sweden b Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94084, 1090 GB Amsterdam, The Netherlands Received 4 September 2006; received in revised form 27 November 2006; accepted 5 December 2006 Available online 13 December 2006 Abstract Many animal species live in groups. Group living may increase exploitation competition within the group, and variation among groups in intra-group competition intensity could induce life-history variability among groups. Models of physiologically structured populations generally predict single generation cycles, driven by exploitation competition within and between generations. We expect that life-history variability and habitat heterogeneity induced by group living may affect such competition-driven population dynamics. In this study, we vary the gregariousness (the tendency to aggregate in groups) of a size-structured consumer population in a spatially explicit environment. The consumer has limited mobility, and moves according to a probabilistic movement process. We study the effects on the population dynamics, as mediated through the resource and the life-history of the consumer. We find that high gregariousness leads to large spatial resource variation, and highly variable individual life-history, resulting in highly stochastic population dynamics. At reduced gregariousness, life-history of consumers synchronizes, habitat heterogeneity is reduced, and single generation cycles appear. We expect this pattern to occur for any group living organism with limited mobility. Our results indicate that constraints set by population dynamical feedback may be an important aspect in understanding group living in nature. r 2006 Elsevier Ltd. All rights reserved. Keywords: Physiologically structured population models; Life-history variability; Exploitation competition; Gregariousness; Aggregation 1. Introduction In many species, individuals grow in size during a significant part of their life (Werner, 1988; Ebenman and Persson, 1988). This growth is usually associated with changes in morphology and metabolics, which in turn affect important ecological properties such as foraging rate, competitive ability and predation risk (for references see de Roos et al., 2003). Hence, in many species, ecological performance or even function is size-dependent (Persson, 1985; Werner, 1988; Persson, 1988; Lundberg and Persson, 1993). The dependence of foraging and competitive ability on size has important consequences for population dynamics. In a size-structured consumer population, size- dependence in foraging or metabolic properties tends to induce population dynamical cycles driven by the dom- inance of a single cohort or generation of similar-sized individuals (de Roos et al., 2002; Murdoch et al., 2002; de Roos and Persson, 2003; Murdoch et al., 2003). In this type of population dynamics, which has been coined single generation cycles, a single, large yearclass is able to reduce the resource to such low levels that they themselves barely survive, and all less efficient foragers starve. The growth of individuals of the dominant yearclass is severely slowed down due to within-generation exploitation competition, and the period of these cycles is generally determined by the mortality in the dominant cohort. Single generation cycles have been observed in a variety of natural and lab populations (Murdoch et al., 2002). The characteristics of such cycles depend on the exact relation between size and competitive ability (Persson et al., 1998; de Roos and Persson, 2003). ARTICLE IN PRESS www.elsevier.com/locate/yjtbi 0022-5193/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtbi.2006.12.011 Ã Corresponding author. E-mail addresses: tobias.v.kooten@emg.umu.se (T. van Kooten), lennart.persson@emg.umu.se (L. Persson), aroos@science.uva.nl (A.M. de Roos).