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

  • Himes J
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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

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  • J G Himes

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