Population Size Estimates of Eckerd College water snakes (Nerodia)

1
Population Size Estimates of Eckerd College water
snakes (Nerodia)
Jeffrey Ackley COLLEGIUM OF NATURAL SCIENCES,
ECKERD COLLEGE ST. PETERSBURG FLORIDA 33711
INTRODUCTION The mark-recapture technique of
population size estimation is an indispensable
tool for monitoring the current status of
important animals such as water snakes, while
additionally providing data on the productivity
and health of ecosystems as a whole. The high
food conversion efficiency of cold blooded
ectotherms granted by low metabolic upkeep allows
for dense communities as high as 1289
snakes/hectare (King et al., 2006). Populations
such as these are rare and ecologically critical.
Like much of the biota of Florida, Nerodia is
under threat from various sources including
deliberate killing upon being confused with
cottonmouths, chemical runoff, and habitat
fragmentation. Mans influence on these animals is
not uniformly destructive thankfully. The size
and direction of anthropocentric impacts are not
constant, therefore the net productivity of
modified habitat can be higher than it was
previously (Lack, 2004).
CONCLUSIONS The total density of XXX/ha is very
high for all snakes including aquatics. A
previous study of 40 snake species found almost
half were less than 1/ha. The total median 5
snakes/ha was lower than the median of 9 water
snakes (22/ha) which ranged from 1-1289/ha. King
et al. (2006) High abundances were surprising due
to the ponds being man made, one as recently as
2004. Despite being small, new, and artificial,
Zeta and Omega ponds are very productive
ecosystems. Nerodias presence as a top tier
predator is no doubt aided by ectothermy, as the
energy requirements of a similarly sized warm
blooded animal would likely prevent it from
reaching a similar density. An understanding of
why certain ponds attract and maintain snakes,
and how can this can be applied to other natural
and modified habitats, could lessen the total
impacts of continued development in Florida and
everywhere else.
FIG.1 Passive Integrated Transponder (PIT) Tag
injected into lateral muscles (retrieved from
www.bnl.gov )

• METHODS
• Nerodia were collected several times in 2007.
• Recaptures were identified by PIT tags (FIG. 1).
• Open and closed models were used to estimate
  adult populations and density.
• Assumptions of closed populations no migration,
  birth, or mortality during the study. Equal
  capture probability, and no observer impact.

POPULATION MODELS
Schumacher-Eschmeyer
(Closed) Baileys
Triple Catch (Open) Schumacher-Eschmeyer N
estimated number of snakes in population. Cd
number of snakes (marked and unmarked) caught on
day d. Md total number snakes marked to date d.
Rd recaptures on day d. s2 variance. m number
of sampling days. Baileys Triple Catch N
estimated number of snakes in population at tx
(1st sampling session t0, 2nd t1, 3rdt2).
Mx total number snakes marked at tx. nxtotal
snakes caught at time x. m0x recaptures from M0
at time x. m1x recaptures from M0 at tx. S.E.
standard error. t was obtained from Students t
table, using m-1 degrees of freedom.
STUDY SITE Omega pond (XXX ha) was made in 1996
and is slightly brackish. Zeta (XXX ha 2004) is
entirely freshwater. A 10 m wide strip of land
separates them. The edges are composed of cut
grass, Spartina, broom grass and cattails. Other
residents include shorebirds, invertebrates and
several fish species.
.
RESULTS3 One hectare (ha) is 100 meters X 100
meters
Schumacher-Eschmeyer 1 Baileys1
Mean2 Density (snakes/ha) Adults 93
(60-127) 51 (-24-126) 72
XXX Total4 148
81 115
XXX Total Biomass (kg of snakes/ha)
XXX 1 Parentheses represent 95 confidence
interval range. 2 Mean was the average of both
methods and was used to calculate density. 3
Combined estimates treated data from Omega and
Zeta as one population due to inter-pond
migration. 4Juveniles were too small to tag. The
total estimate was based on their proportion of
the captured snakes. TABLE 1. Water snake
population and density estimates given by open
(Baileys) and closed (Schumacher- Eschmeyer)
models. (Baileys and Schumacher-Eschmeyer were
based on Caughley (1977) and Schineider (2000)
respectively.
Literature Cited CAUGHLEY, G. 1977. Analysis of
vertebrate populations. John Wiley and Sons, New
York, New York, U.S.A. KING, R.B., A.
QUERAL-REGIL, AND K.M. STANFORD. 2006. Population
size and recovery criteria for the threatened
Lake Erie watersnake integrating multiple
methods of population estimation. Herpetological
Monographs 2083-104. LACK, M.J. 2004.
Population patterns of Copperbelly Water Snakes
(Nerodia erythrogaster neglecta) in a riparian
corridor impacted by mining and reclamation.
American Midland Naturalist. 153357-369.
SCHNEIDER, J.C. (ed.) 2000. Manual of fisheries
survey methods II with periodic updates.
Michigan Department of Natural Resources,
Fisheries Special Report 25, An Arbor.
FIG. 2 Aerial view of the study site within
Eckerd College Campus Saint Petersburg, Florida
FIG. 3 Nerodia fasciata
Retrieved from coloquinte.spaces.msn.com
ACKNOWLEDGMENTS
This study has been generously supported by
Eckerd College through the Natural Sciences
Collegium, and through ECOS funding of the Eckerd
Herpetology Club. Additionally I would like to
thank the numerous students and faculty of Eckerd
College who have assisted with animal capture,
data recording, and experimental advice.