Population Consequences of Aggregative Movement

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Abstract
(1) Gregarious behaviour is an important factor influencing survival and reproduction of animals, as well as population interactions. In this paper I develop a model of movement with attraction or repulsion between conspecifics. To facilitate its use in empirical studies, the model is based on experimentally measurable features of individual behaviour. (2) Attraction among individuals leads to movement which is biased towards areas of high population density. However, aggregations can arise only if mutual attraction is strong enough to dominate the dispersive effect due to random motion. I use the model to define the conditions for formation of an aggregated spatial pattern in homogeneous space. The model is applied to the aggregative movement in Aphis varians. (3) To investigate how spatial inhomogeneity can interact with density-dependent movement I consider a population of animals foraging for a patchy resource, e.g. insect herbivores moving among patches of their host-plants. The results of the model indicate that gregarious behaviour of herbivores strongly amplifies the effects of patch size on herbivore density. Field studies of the Mexican bean beetle-garden bean system provide experimental support for this theoretical prediction. (4) The modelling framework developed in this paper provides a tool for studying effect of aggregation on predator-prey and competitive interactions, since demographic and population interaction terms can be readily added to the model.