Evidence for micropatch partitioning and effects of boundaries on patch use in two species of gerbils

1. Spatial heterogeneity in resource abundance creates opportunities to exp loit patchiness at different scales. The ability to subdivide patches into smaller micropatches is termed micropatch partitioning. At various scales, patches can be recognized by distinct boundaries or by vague boundaries del ineated by changes in resource abundances. The ability to micropatch partit ion was tested for and the effect on foraging of vague vs distinct boundari es was assessed for two species of coexisting desert rodents, the gerbils G erbillus allenbyi and G. pyramidum. To do so, the distribution of seeds in experimental resource patches in a large enclosure were manipulated. 2. Both G. allenbyi and G. pyramidum are imperfect micropatch assessors. Wh en seeds were concentrated into micropatches, there were lower patch-wide g iving-up densities (GUDs) than if seeds were randomly distributed throughou t the patch. Both species could micropatch partition at scales of 0.14 m(2) . 3. GUDs in experimental micropatches were greater than expected for perfect assessors. 4. With G. allenbyi, a micropatch with fewer vague boundaries was foraged t o a lower GUD. In contrast, GUDs for G. pyramidum were similar regardless o f the types of patch boundaries. 5. Gerbillus allenbyi showed greater error in the assessment of micropatch size. 6. These results have implications for diet selection and coexistence. Thes e species coexist via temporal partitioning, with G. pyramidum biasing its activity to early in the night when resource patches are richer and G. alle nbyi biasing its activity to later in the night. Gerbillus pyramidum encoun ters a richer and more variable environment where errors in patch assessmen t are costlier, and micropatch partitioning is more valuable. Gerbillus all enbyi encounters a poorer environment where resources vary at larger spatia l scales.