Spatial patterns of seed predation by harvester ants (Messor Forel) in Mediterranean grassland and scrubland

Seed predation by harvester ants is one of the main processes involved in the seed bank dynamics of Mediterranean grassland and scrubland. We investigated the spatial patterns of seed predation by Messor spp. in these systems from a multi-level approach (nest-site selection, foraging patch selection and seed predation risk), using ten experimental plots (20 m × 40 m) set in central Spain. All habitats in grasslands are dominated by M. barbarus, while in scrubland this species shares seed exploitation with M. bouvieri. In this type of system, the nest-hole and foraging territory distribution of the two species show a certain degree of segregation. Messor bouvieri focuses on the exploitation of dry scrub, while M. barbarus also forages on subhumid and humid patches. We interpret this as a consequence of the different foraging behaviour and nest site requirements of the two species. In general, the spatial distribution of seed predation risk reflects the patterns detected for the nest-holes and foraging areas, although scrubland has a more heterogeneous predation pattern than grassland.