The role of the midland water snake, Nerodia sipedon, as a competitor, predator, and prey in freshwater communities

John G. Himes


I investigated the role of the midland water snake (Nerodia sipedon ) as a competitor, predator, and prey in freshwater communities associated with the Pascagoula River system of southeastern Mississippi. Nerodia sipedon is very common and, as an upper-level predator, may play an important ecological role in these communities. Nerodia sipedon and the co-occurring diamondback water snake (Nerodia rhombifer ) were tested in a series of enclosures set at the edges of outdoor ponds and in the laboratory for evidence of intra- and interspecific competition. Nerodia sipedon and the co-occurring cottonmouth (Agkistrodon piscivorus ) were tested in the laboratory for evidence of predator-prey interactions. To determine dietary compositions (and thus obtain evidence on competition and predation), data on stomach contents were collected from museum specimens of N. sipedon and A. piscivorus . The results of the field experiments indicated that (1) intraspecific competition in adult N. sipedon may occur at high snake densities or low prey (fish) densities, (2) adult N. sipedon may be superior competitors to N. rhombifer at high snake densities, and (3) preferential cannibalism of non-kin, neonatal N. sipedon by maternal snakes occurs in the absence of alternative prey. The laboratory experiments indicated that (1) adult N. sipedon and N. rhombifer ultimately do not exhibit differences in digestion rates of fish, (2) the frequency of utilization of different foraging modes (sit-and-wait versus active) does not differ between related and unrelated neonatal N. sipedon , (3) capture efficiencies and handling times of frogs and fishes differ between neonatal and adult N. sipedon , (4) fishes do not alter their patterns of microhabitat selection or activity in the presence of adult N. sipedon , and (5) neonatal and adult N. sipedon avoid areas containing odor trails of A. piscivorus . The study on dietary compositions of snakes from museum specimens indicated that niche breadth was greater in small snakes than in large snakes and that dietary overlap was relatively high between species (N. sipedon and A. piscivorus ) and between size classes (<50 cm and>≥50 cm snout-vent length) within each species. Overall, the results of this project indicate that snakes, through their integrated roles as competitors, predators, and prey, may be major players in freshwater communities. Therefore, researchers that are interested in studying freshwater communities and the various biological interactions that take place within them would be well advised to consider the potential roles of aquatic snakes.