Is the Peninsular Effect a Red Herring?

Abstract

Since Simpson (1964) recognized that mammalian species density decreased from base to tip on North American peninsulas, many authors have debated the presence or absence of such a trend in other animal groups and other peninsulas (MacArthur and Wilson 1967; Cook 1969; Kiester 1971; Taylor and Regal 1978; Wamer 1978; Lee 1980; Seib 1980, 1981; Gilpin 1981). Simpson explained this as a result of the higher rate of extinction and lower rate of immigration on peninsulas when compared to the adjacent mainland. Taylor and Regal (1978) prepared a model of Simpson's hypothesis and used this model to explain the diversity gradient in heteromyid rodents inhabiting the Baja California peninsula. Gilpin (1981) objected to the assumptions of Taylor and Regal's model and demonstrated how the molecular theory of island biogeography could be applied to peninsular diversity patterns. Seib (1980) could not find a peninsular effect in lizard and snake species density on the Baja California peninsula and preferred to explain the distributions of these groups in terms of historical events which produced a gradual interdigitation of northern and southern faunas. Recently, however, Seib (1981) has acknowledged that a peninsular effect may be present. Because of the scarcity of data for most major peninsulas of the world, the general applicability of the peninsular effect as a biogeographic phenomenon is uncertain. The lizard and snake faunas of Baja California, Florida, Yucatan, and Iberia are well known; we have critically examined the species density patterns of these vertebrates on these four peninsulas and demonstrate (1) that the peninsular effect (sensu Simpson 1964) is not a concomitant feature of peninsulas and may not be a general biogeographic pattern and (2) that historical events probably exert a greater influence on peninsular diversity than the shape of the landmass.