Under low-resource conditions, the outcome of competition between a generalist omnivore and an herbivore may depend on whether one species can more efficiently use a limiting resource. I evaluated the levels of omnivory and competitive interactions between two abundant sympatric emydid turtles, Trachemys scripta and Pseudemys floridana, that coexist across a productivity gradient in lakes in northern Florida. A foraging experiment indicated that both species were omnivorous in the juvenile stage, but P. floridana was more herbivorous and T. scripta consumed more insects and carrion. This low level of resource overlap was consistent with previous studies of gut contents and stable isotope analyses. In a 13-mo enclosure experiment in a natural pond under low-resource conditions, the generalist T. scripta grew more slowly with conspecifics than with P. floridana, and the more specialized P. floridana grew at the same rate with both T. scripta and conspecifics. Trachemys scripta with P. floridana exhibited the highest growth rate (3.5 mm/yr), slightly faster than P. floridana with conspecifics (2.3 mm/yr). Trachemys scripta was apparently unable to process macroalgae as well as P. floridana and the low abundance of invertebrate prey in the pond may have limited growth rates with conspecifics. Where only low-quality, basal resources are available, gut specializations in P. floridana allow for more efficient digestion and assimilation of plant nutrients and maintenance of equal growth rates with either conspecifics or T. scripta. Where intraspecific competition in one species is stronger than interspecific effects, this may influence the distribution of trophic generalists across a productivity gradient. A possible mechanism for competition among conspecifics may be aggressive interactions in habitats with limited resources.
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Vol. 66 • No. 3