A MODEL FOR SPECIES DISTRIBUTIONS ALONG A GRADIENT - LIZARDS AS STUDYSYSTEMS

Terborgh (1971) proposed a model for zonation patterns of animal speci es along environmental gradients, which was subsequently applied to al titudinal gradients of species replacements of birds and mammals in tr opical biomes. Three major factors were invoked to account for the zon ation patterns detected: autecological responses, habitat discontinuit ies, and interspecific interactions (mainly competition). Hen. we expa nd and generalize Terborgh's model to include ectothermic vertebrates. whose thermal requirements may impose severe constraints to physiolog ical performance (including reproduction). Based on our experience, we propose thai our modified model may be applied to understand distribu tional patterns of lizards along the environmental gradient provided b y the tall Andean ranges, by formalizing explicit hypothesis testing.