Mechanisms of Coexistence of Optimal Foragers as Determinants of Local Abundances and Distributions of Desert Granivores

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Abstract
Foraging theory can be applied to the study of ecological communities. We developed and tested several local mechanisms of species coexistence based on foraging theory for a desert granivore community in a heterogeneous landscape in the Negev Desert, Israel. The most common species were Allenby's gerbil (Gerbillus allenbyi), Wagner's gerbil (Gerbillus dasyurus), and crested lark (Galerida cristata). Allenby's gerbils were most abundant on sandy substrates; Wagner's gerbils were most abundant on cobble and loess substrates; crested larks occurred across all three. To test among predictions of the different mechanisms of coexistence, we used live-trapping and counts to census rodents and birds, recorded spoor in sand-tracking plots to determine habitat-specific activity patterns, and measured giving-up densities in depletable food patches to estimate relative foraging efficiencies on different substrates and within different habitats. Allenby's gerbil had an advantage in habitat selection by being the most efficient forager (lowest giving-up densities on seeds in feeding trays) in the sandy habitat. The crested lark was the tolerant forager. Its foraging abilities were affected little by escape substrate, foraging substrate, or food type. As a result, the crested lark had three advantages: it was the most efficient forager in the loess habitat, it was a better insectivore than the gerbils, and its diurnal habit allowed it first access to seed patches that renew in the afternoon in the sandy habitat when winds typically arise. In response to our measures of foraging aptitude, Wagner's gerbil revealed no advantage over the other species, and factors promoting its success eluded our proposed mechanisms of species coexistence. Foraging ecology and community ecology can be integrated to understand local species distributions, abundances, and community structure and organization. Uniform conceptual and methodological techniques can be applied across taxa and communities. We were able to relatively quickly test which mechanisms of coexistence apply. In the community we studied, there were two species whose presence we feel we understood and one species whose presence remained an enigma.

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