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12 Community Ecology Two species of gerbils, the 24 g Allenby’s gerbil and the 40 g greater sand gerbil, live together on sand dunes in the Negev Desert. These species are very much alike. They eat mostly seeds (Bar et al. 1984), they are nocturnal, they live in burrows, they are caught by the same predators, and they compete intensively with each other. | Illllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll 12 Community Ecology Burt P. Kotler and Joel S. Brown 12.1 Prologue Two species of gerbils the 24 g Allenby s gerbil and the 40 g greater sand gerbil live together on sand dunes in the Negev Desert. These species are very much alike. They eat mostly seeds Bar et al. 1984 they are nocturnal they live in burrows they are caught by the same predators and they compete intensively with each other e.g. Mitchell et al. 1990 . They invite a central question of community ecology What promotes the coexistence of close competitors How do these two species escape competitive exclusion Perhaps the answer has to do with their use of habitats. The two species use the varied substrata of the sand dunes differently. Allenby s gerbil predominates on sand dunes stabilized by vegetation while the greater sand gerbil predominates on less stable sand dunes Rosenzweig and Abramsky 1986 . Habitat segregation intensifies at higher population densities Abramsky and Pinshow 1989 Abramsky et al. 1990 1991 . Foraging theory suggests that habitat selection is based on the costs and benefits of habitat use Fretwell 1972 Rosenzweig 1981 . For this to explain species coexistence each species must have a habitat that it uses and exploits better than its competitor Brown 1989b . That is Allenby s gerbils should use the stabilized sand more because they forage more efficiently there and greater sand gerbils should forage more efficiently lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll 398 Burt P. Kotler and Joel S. Brown on the looser substratum. Experiments show however that Allenby s gerbils forage more efficiently in both habitats Brown Kotler and Mitchell 1994 . Habitat selection resulting from the costs and benefits of foraging evidently does not provide the necessary conditions for the gerbils coexistence. So did .
