Intrapopulation niche differentiation has important ecological consequences, in part because individual dietary differences can broaden species’ niches and reduce both intraspecific and interspecific competition. Such individual dietary specialization can be a result of diets differing with body size, age class, sex, or some combination of these factors. We focused on dietary specialization in a generalist predator, the northern watersnake (Nerodia sipedon), a species that feeds primarily on fishes and amphibians but also consumes arthropods, annelids, and mollusks. Our study occurred in shallow wetland and palustrine forest habitats at the Sloughs Wildlife Management Area (Henderson County, Kentucky, USA). We used stable isotope analyses to determine differences in dietary resource utilization patterns and intraspecific niche partitioning between sexes and across snake body lengths (snout-vent length). More specifically, we used δ15N stable isotopes to test intrapopulation differences relating to trophic level and δ13C stable isotopes to test differences relating to habitat. We predicted that larger snakes would feed at higher trophic levels. Based on observations, we also predicted that larger snakes would consume more terrestrial prey.
Results/Conclusions
Overall, females had enriched δ15N levels, indicating that they fed at higher trophic levels than males (F1=12.02, p=0.0008). Females also foraged over a larger range of trophic levels (δ15N; F1,109=11.24, p=0.001). Both isotopes became enriched as snake size increased (δ13C, F1 = 30.01, p<0.0001; δ15N, F1=71.59, p<0.0001), with longer snakes taking more prey from terrestrial habitats and feeding at higher trophic levels. However, the effects of size differed by sex for both isotopes. In males, δ13C enriched at a greater rate than in females (F1=8.36, p=0.005), indicating that males’ diets shifted more quickly to terrestrial prey as body size increased. In females, δ15N enriched at a greater rate (F1=7.47, p=0.007), suggesting females shifted more strongly to prey from higher trophic levels with increasing body size. δ13C variance tended to be greater in males (F1,109=3.67, p=0.058), with males possibly feeding across a wider range of terrestrial and aquatic habitats. In sum, sex, body size, and their interaction had complex effects on diet and intraspecific niche partitioning, likely reducing intraspecific competition and resulting in a broad dietary niche for this common generalist predator. This individual diet variation may help explain why this species is often able to co-exist with other Nerodia species.