Site fidelity in foraging wood-ants Formica aquilonia yarrow and its influence on the distribution of foragers in a regenerating environment.

The proportions of the population of red wood-ants Formica aquilonia Yarrow, foraging for sucrose at two artificial feeding sites in the laboratory, approximated to those predicted by the ideal free distribution (Lamb and Ollason, 1993). It was assumed that, for the foraging population to attain the ideal free distribution, each individual would visit and assess the availability of food at both feeding sites and distribute its foraging effort in proportion to the rate of regeneration of food. In order to demonstrate this, large samples of foragers at each of the feeding sites were colour-marked according to site. It was expected that the marked sample of foragers would in time become distributed as predicted by the ideal free distribution; instead, the distribution of marked ants was biased toward the site where they had been marked. Hence some individuals are not continually visiting and feeding at both patches, and such individuals will not be able directly to assess their feeding rates at each patch. There is, in principle, no way that such individuals can distribute themselves as predicted by the ideal free distribution, and as the site-faithful ants form a large proportion of the population, it appears that there is no way that the members of such a population can become so distributed. One resolution of this apparent paradox is provided by the behaviour of the mobile ants, the existence of which is demonstrated by observations of marked ants foraging at the site other than that at which they were marked. If the mobile members of the population assess the quality of both patches and distribute themselves to maximise their rates of feeding, the whole population will become distributed as predicted by the ideal free distribution. A modification of Ollason's (1987) model shows how this distribution could develop. It emerges that there is a minimum proportion of mobile ants in a population, below which the ideal free distribution will not develop, and that the distribution of foragers will depart from the ideal free distribution increasingly as the discrepancy between the regeneration rates of the patches of food increases.