Dietary Niche Overlap and Size Partitioning in Sympatric Varanid Lizards

Dietary separation is an important means of differentiating ecological niches and avoiding interspecific competition between sympatric species. Congeneric species that overlap in geographic distribution provide an excellent opportunity to explore the mechanisms of coexistence. Two monitor lizards, Varanus gouldii and V. rosenbergi (Varanidae), are sympatric at a local scale in the northern Jarrah Forest of Western Australia. Both species are wide-ranging terrestrial predators of a similar size and may differentiate their ecological niche by utilizing alternative foraging strategies resulting in dietary separation. Because varanid lizards are an important group of terrestrial high-order predators in the Old World, any such separation may have important implications for faunal community structure. In total, 169 scat and stomach samples were analyzed revealing extensive dietary overlap between the species. Dietary intake was not distinguishable between species or related to individual body size. Invertebrates were most important in terms of frequency and volume, although reptiles, mammals and birds were also commonly identified. Dietary partitioning is not the mechanism allowing these congeneric varanid lizards to coexist.