Island populations of the Lake Erie watersnake (Nerodia sipedon insularum) have experienced a dramatic environmental change in the form of a new abundant prey species, the round goby (Neogobius melanostomus). Effects of this biotic change on watersnake reproductive characteristics were tested by comparing offspring size and the relationship between female size and offspring number before and after the round goby invasion. Offspring number, but not offspring size, increased as a consequence of watersnake diet change. In addition, female size explained a greater proportion of the variation in offspring number following the round goby invasion (60% vs. 25%). The combined effects of increased female size and offspring number are projected to result in an increase in watersnake fecundity exceeding 25% and may be stimulating watersnake population growth since the round goby invasion. The rapidity of watersnake diet change and consequent change in fecundity is consistent with the maximization hypothesis of life history theory and suggests that watersnake life history may have been more strongly limited by resource availability prior to the round goby invasion.