Seed density in monospecific and mixed patches affects individual and collective foraging in ants

摘要

Granivore ant foraging should respond to seed density through changes in individual and collective behaviour. Overall seed availability and the presence of multiple types of seeds in a patch may affect these behaviours. We presented ants with three species of seeds in different mixtures and in two seasons contrasting in seed availability to assess differences in probabilities of seed collection. Removal rates were recorded every 2 h. We fitted a model that uses data on seed removal to disentangle the individual (probability that an ant removes a seed of species i, p(i)) and collective (number of active ants h) components of foraging. We found that removal increased with seed density due to increased p(i) values and recruitment of new ants. This suggests that ants evaluated resource density, collecting seeds only from dense patches. This was confirmed by observations. The presence of additional seeds of other species in the seed mixture elicited recruitment but had nearly no effect on p(i). This may be observed if different ants prefer a seed species and ignore the rest. In the low seed-availability season, h increased, ants travelled longer distances, and foraging was directed to the most profitable seed-patches. This selectivity for some seed-patches during times of scarcity may appear surprising. It is true that, by abandoning low-density patches, individuals may reduce their seed acquisition rates, but also increase collective gains by forming trails only to dense patches. This may compensate for the large collective investments in the form of more scouts that explore longer distances.