Responses of two sympatric lizards to biotic and abiotic constraints along an altitudinal gradient

Abstract

Abstract: Understanding the mechanisms that underlie the patterns of diversity and relative abundance in ecological communities is a major goal in ecology. In a tropical dry shrubland from southern Ecuador, we investigated the relative densities of two species of sympatric lizards (Medopheos edracanthus and Stenocercus ornatus) at various altitudes between 1450 and 1750 m a.s.l. We were interested to evaluate to what extent the abiotic factors (i.e., altitude gradient) or biotic factors (i.e., competition and parasitism) are involved in determining the distribution and abundance of the two coexisting species. In the case of the low-land species (M. edracanthus), abundance was best predicted by both altitude and density of the sympatric lizard, while for the high-land species (S. ornatus), the density of its competitor had the main effect. Ectoparasites had differentiated effects on the two species, mediated by altitude, with a general negative impact on the body condition. Our results indicate that the ratio between abundances of the two species at any given site is probably due to altitude indirect effects on competition and parasitism.