Parasite-mediated competition in Anolis lizards.

On many small Caribbean islands, two species of Anolis lizard coexist, but the two are typically very different in body size. The two Anolis of St. Maarten, however, are exceptional because they are similar in size and are known to be strongly competitive. One species, A. gingivinus, appears the stronger competitor and occurs throughout the island; the other, A. wattsi, is found only in the central hills. The malarial parasite Plasmodium azurophilum very rarely infects A. wattsi, but in some locations is very common in A. gingivinus. Wherever malaria infects A. gingivinus, A. wattsi is present, but wherever malaria is absent, only A. gingivinus occurs. This pattern of coincidence of malaria and coexistence of both Anolis is observed over distances of only a few hundred meters. The parasite infects both red and white blood cells of A. gingivinus and causes important pathology: immature erthrocytes increase in abundance, blood hemoglobin decreases, monocytes and neutrophils increase, and infected white cells are less likely to produce acid phosphatase. These results argue that malaria mediates competition between the two species and determines the present distribution of the lizards on St. Maarten. This kind of parasite-mediated competition could be common if susceptibility to parasitic infection varies among competitors. The distribution of malaria in the Anolis of Caribbean islands suggests this parasite can play an important role in Anolis community ecology.