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Aim We studied the relationship between the size and isolation of islands and bat species richness in a near-shore archipelago to determine whether communities of vagile mammals conform to predictions of island biogeography theory. We compared patterns of species richness in two subarchipelagos to determine whether area per se or differences in habitat diversity explain variations in bat species richness.

Location Islands in the Gulf of California and adjacent coastal habitats on the Baja California peninsula in northwest Mexico.

Methods Presence-absence surveys for bats were conducted on 32 islands in the Gulf of California using acoustic and mist-net surveys. We sampled for bats in coastal habitats of four regions of the Baja peninsula to characterize the source pool of potential colonizing species. We fitted a semi-log model of species richness and multiple linear regression and used Akaike information criterion model selection to assess the possible influence of log(10) area, isolation, and island group (two subarchipelagos) on the species richness of bats. We compared the species richness of bats on islands with greater vegetation densities in the southern gulf (n = 20) with that on drier islands with less vegetation in the northern gulf (n = 12) to investigate the relationship between habitat diversity and the species richness of bats.

Results Twelve species of bats were detected on islands in the Gulf of California, and 15 species were detected in coastal habitats on the Baja peninsula. Bat species richness was related to both area and isolation of islands, and was higher in the southern subarchipelago, which has denser vegetation. Log(10) area was positively related to bat species richness, which increased by one species for every 5.4-fold increase in island area. On average, richness declined by one species per 6.25 km increase in isolation from the Baja peninsula.

Main conclusions Our results demonstrate that patterns of bat species richness in a near-shore archipelago are consistent with patterns predicted by the equilibrium theory of island biogeography. Despite their vagility, bats may be more sensitive to moderate levels of isolation than previously expected in near-shore archipelagos. Differences in vegetation and habitat xericity appear to be associated with richness of bat communities in this desert ecosystem. Although observed patterns of species richness were consistent with those predicted by the equilibrium theory, similar relationships between species richness and size and isolation of islands may arise from patch-use decision making by individuals (optimal foraging strategies).