The peninsula effect on species diversity: a reassessment of the avifauna of Baja California

Abstract

The peninsula effect, a decrease in species diversity from the base to the tip of peninsulas, has been proposed to explain the relatively poor species diversity of mammals on North American peninsulas. Subsequent work has questioned both the existence of peninsular declines in diversity, as well as the proposed cause (immigration‐extinction dynamics). Previous studies of the Baja California avifauna have shown a gradual decrease in the diversity of breeding birds from the base to the tip of the peninsula. Using newly published data on the breeding land birds, I found a decrease only from the base to the middle of the peninsula, with a slight increase in diversity from the middle to the tip. This result is similar to that for other highly vagile taxa (e.g., Chiroptera. Lepidoptera) and is largely due to the coneave diversity gradient of montane species along the peninsula. Habitat associations of the Baja avifauna and the location of potential source populations suggest that: 1) local habitat heterogeneity is likely the single most important factor influencing the avian diversity gradient along the peninsula; and 2) limited immigration of Neotropical species from mainland areas, and of Nearctic species from the base of the peninsula to the montane southern tip is partly responsible for the form of the diversity gradient along the southern half of the peninsula. My results along with those from previous studies, suggest that rather than colonization/extinction dynamics, habitat heterogeneity and the vagility of the taxa considered have the greatest impact on the observed patterns of species diversity along peninsulas,