Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory

The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied i n an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availabilit y for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape pa tch types (shrub patches and inter-shrub patches) in 1997. Shrub patches we re shown to have higher plant and seed-rain density than inter-shrub patche s. Patch use and seed selection by M. arenarius foragers were monitored thr ough the spring, summer and autumn of 1997. After a pulse of seed productio n ill the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summe r, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two pa tch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increa se in the use of shrub patches by individual foragers. In the autumn, a pul se of seed production by the shrub species Atractylis serratuloides and Noa ea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in t he proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behav iour showed linked changes in seed selection and patch use resulting in imp ortant differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productiv ity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephem eral plants in the landscape.