We studied changes in a population of red-footed Amazonian tortoises, Geochelone carbonaria, consequent to isolation in an insular forest fragment. Altered age structure, population density, and body growth rate are shown here for the first time to be associated responses. Age structure was strongly biased toward juveniles and growth rates were reduced compared to the mainland. Our data suggest that density-dependent processes induced by habitat fragmentation changed demography and life history parameters in a scant 16 years.