Intra- and interspecific competition among the water snakes Nerodia sipedon and Nerodia rhombifer

Abstract

I investigated the potential role of intra- and interspecific competition for food in Midland Watersnakes (Nerodia sipedon pleuralis) and Diamond-Backed Watersnakes (Nerodia rhombifer rhombifer) during the summers of 1999 and 2000. Snakes offered prey (juvenile spotted bass, Micropterus punctulatus) at low densities (five fish/enclosure/week) had significantly lower proportional changes in mass than snakes offered prey at high densities (10 fish/enclosure/week). However, differences in mass changes of N. sipedon were not significant at constant relative densities of prey to snakes. Nerodia sipedon and N. rhombifer that were offered 10 fish per week and tested individually had significantly higher mass changes than snakes tested in intraspecific pairs. For snakes maintained in interspecific pairs and offered 10 fish per week, N. sipedon had higher mass changes than N. rhombifer, although the difference was not significant. I lab-tested the potential for exploitation as a mechanism of competition between N. sipedon and N. rhombifer by comparing rates of gastric breakdown after feeding snakes a goldfish (Carassius auratus), forcing regurgitation at six, 12, and 18 h after ingestion, and comparing ash-free dry masses (AFDMs) of the digested fish. Relatively higher AFDMs of digested fish were obtained from N. sipedon than from N. rhombifer at six and 12 hours after ingestion. Thus, the former species exhibited a faster rate of gastric breakdown than the latter. These differences in competitive abilities may potentially contribute to the higher abundance of N. sipedon than N. rhombifer in the upper Pascagoula River system of southeastern Mississippi.