Density-dependent dispersal and habitat use in size-structured populations: An experiment in wild Trinidadian guppies

Individual responses to density perturbations often depend on individual size or stage, and can include demographic changes (survival, reproduction, growth) and spatial responses (dispersal, habitat shift). While these responses are well characterized, their interaction is seldom considered. Size-specific effects of density can result from size- or stage-specific spatial responses mediated by dominance interactions: subordinate individuals might suffer disproportionately from overcrowding conditions if displaced from high-quality habitats by dominant individuals. To investigate this, we performed an experiment in wild guppies where we observed demographic and spatial responses to density manipulations and tested for an interaction between them. We found recruitment, growth, and female survival to be decreased at high density. Dispersal was costly, causing a reduction of body condition and growth. Shifts in microhabitat use with density were size-dependent: at increased density, large individuals were more likely to remain in a microhabitat, while small individuals were likely to move and suffer reduced growth. At decreased density, growth improved when remaining in the same microhabitat (for large individuals) or moving to a different one (for small individuals). Our results show that regulation under density perturbation can occur through asymmetric interactions that disproportionately affect smaller individuals.