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The Use of Timed Fixed-Area
Plots and a Mark-Recapture
Technique in Assessing
Riparian Garter Snake
Populations1
Abstract.-Wandering garter snake (Thamnophis
elegans vagrans) populations along a thin-leaf alder
(Alnus tenuifolia) riparian community in northern
New Mexico were sampled using timed fixed-area
plots and a mark-recapture method, Both methods
served to determine yearly differences and relative
magnitude of snake density between years. But
population estimates determined by timed fixedarea plots were inconsistent between study plots in
the same year.
Robert C. S~aro,~
Scott C. Belfit,3J. Kevin
Aitkin," and Randall D. BabbS
Research studies often attempt to determine the effects of disturbance or
management regimes on the abundance of wildlife species (Cooperrider et al. 1986, Fi tch 1987, Parker
and Plummer 1987, Ralph and Scott
1980). How well the method of data
collection and analyses reflect actual
populations is critically important for
assessing the validity of these studies. Snakes are difficult subjects for
field studies because of their secretive and cryptic habits (Fitch 1987).
paper presented at symposium, Management of Amphibians. Reptiles, and
Small Mammals in North America. (Flagstaff, AZ,July 19-21, 1988.)
~ o b e rC.
f Szaro is Research Wildlife Biologist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station,
Arizona State University Campus, Tempe, AZ
85287- 1304.
3 ~ c o C.
# Belfit is Wildlife Biologist, Department of the Army, Wildlife Management Section. Fort Huachuca, AZ 856 136000. Belfd's current address is P.0.Box 336,
Fort Belvoir, VA 22OM-0336.
J. Kevin Aitkin is Wildlife Technician,
USDA Forest Service, Rocky Mountain Forest
and Range Experiment Station, Arizona
State University Campus, Tempe,AZ 852871304.
anda all D. Babb, formerly Wildlife Technician, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station,
Arizona State University Campus, Tempe. AZ
85287- 1304 is currently Sportfishing Program
Coordinator, Arizona Game and Fish Department. 2222 West Greenway Road,
Phoenix, AZ 85023.
*
Many attempts to census snakes have
been inaccurate (Turner 1977, Fitch
1987).Population estimates can be
influenced by sex, reproductive condition, and stage of maturity, all of
which are critical determinants of
activity within species (Gibbons and
Semlitsch 1987). Differences among
juveniles and breeding and nonbreeding females, and males of ten
lead to much different risks of capture at various stages of the season
and time of day. Overall population
estimates can be distorted as a result,
requiring separate estimates by sex
and age class (Fitch 1987).
Two methods often used to estimate snake density are direct counts
and mark-recapture analyses. Systematic searches of defined areas (direct counts) yield species occurrence
data, and usually require less time
and effort than mark-recapture methods (Jones 1986). Using direct counts,
Bury and Luckenbach (1977) successfully censused desert tortoise (Gopherus agassizii) populations with a
quartet and grid location system.
Bury (1982) used a removal method
to assess reptile community structure
in the Mohave Desert (Zippin 1956,
1958).Bury and Raphael (1983) refer
to searches conducted per unit effort
of time as time-constraint procedures. Usually it is impossible to find
every snake in an area, making it
necessary to estimate population size
from capture-recapture ratios (Fitch
1987). Yet, when several density estimates become available from the
same area at different times, they often show such drastic discrepancies
that the basic methods have been
thought invalid (Turner 1977).
Turner (1977) had no confidence in
the density estimates for snakes derived from mark-recapture techniques. However, since his critical
review, estimation techniques have
greatly improved with the development of models and computer programs that test model assumptions
and estimate standard errors (Arnason and Baniuk 1980, White et al.
1978,1982, Otis et al. 1978, Brownie
et al. 1985).
Although time consuming, determining accurate population estimates
is necessary to develop management
policies not only for abundant species, such as the wandering garter
snake (Thamnophis elegans vagmns),
but also for aquatic or semi-aquatic
endangered snake species such as the
Concho water snake (Nerodia harteri
paucimaculata) (Scott and Fitzgerald
1985) and the narrow-headed garter
snake (Thamnophis rufipunctatus)
(Lowe 1985). However, because the
wandering garter snake, is less secretive than most kinds of snakes, and is
concentrated in riparian habitats, it is
probably one of the best adapted to
this sort of investigation (Fitch, per-
sonal communication). The results of
this work should be directly applicable to other snake species normally
concentrated in riparian ecosystems
and may be especially useful for censusing endangered species where
large samples to detemine the accuracy of sampling techniques are not
available. Our previous work
showed the inadequacy of simple
transects and depletion sampling in
determining garter snake populations along the Rio de las Vacas, New
Mexico (Szaro et al. 1985).The objective of this study was to compare
timed fixed-area plots and a markrecapture technique in assessing the
impacts of management regimes on
riparian ecosystems in the arid
Southwest by sampling wandering
garter snake populations along the
Rio de las Vacas.
Figure I .-Grazed section of the Wo
i de las Vacas, New Mexico. Notice the lack of shrub
growth and the unstable stream banks.
Methods and Study Areas
The Rio de las Vacas, is a montane
stream draining the San Pedro Parks
Wilderness Area, Santa Fe National
Forest, New Mexico. Under low flow
conditions, stream width ranges
from 2.8 to 10.5 m and averages 7.6
m. The study area is 17 km sEutheast
of Cuba, in Sandoval County, at 2600
m. Two cattle exclosures enclosing
stream reaches (each about 1 km long
by 50 m wide) were installed in the
early 1970's (Szaro et al. 1985). Contiguous, downstream areas, privately
owned and grazed by livestock, were
used for comparison. The most apparent difference between the grazed
and exclosed stream segments was
the band of small riparian trees and
shrubs in the exclosures (figs. 1 and
2). Thin-leafed alder (Alnus tenuifolia)
and a mixture of willow species
(Salix spp.) edged the exclosure
streambanks but were widely scattered where the streambanks were
grazed (9.5 + 1.16,7.5 + 1.23, and 0.3
+ 0.14 trees/250 m2in exclosures 1,2,
and grazed areas, respectively).
Snake populations were estimated
by timed fixed-area plot sampling,
and mark-recapture sampling in both
grazed and ungrazed areas. For the
former, 16 plots (10 x 25 m), with the
long edge being defined by the
stream bznk, were intensively
sampled for 20 minutes in each of the
two ungrazed exclosures and one
grazed stream segment along the Rio
de las Vacas, for a total of 48 plots
(fig. 3). During sample periods we
turned rocks, logs, debris piles, and
generally searched the area. All plots
Figure 2.-Shrubby growth in Exclosure 2 along the Rio de las Vacas, New Mexico.
were sampled once between 0900
and 1300 hours (MST) within a 3-day
period each month. Sampling times
were determined from preliminary
activity period sampling that showed
two distinct periods of activity
(morning and late afternoon). All
snakes captured were placed in a
cloth sack at their point of capture,
until the end of the sampling period.
Plot sampling began in June 1984 and
was replicated in July, August, and
September of that year and in the
same months in 1985. Total time
spent sampling was approximately
64 hours per year, excluding time between samples to process snakes.
For mark-recapture estimates, we
searched the entire extent of both exclosures and a similarly sized downstream grazed stream area. The plots
used for the timed-fixed plot sampling were a subset of the area used
for the mark-recapture sampling. All
captured snakes were marked by
clipping three subcaudal scales (Blanchard and Finster 1933, Woodbury
1956).Mark-recapture sampling periods occurred in the same months as
the plot sampling; but snakes were
captured, marked, and released durRio de las Vacas
Edomrs1
Figure 3.-Study areas and sample plot layout along the Rio de las Vacas, New Mexico.
ing intensive searches for 6 consecutive days by 3 to 4 collectors. All
snakes were released where captured. Approximately equal time and
effort was spent searching for snakes
in each of the three areas. Time of
day bias was minimized by alternating starting areas daily. Sampling
began at 0900 hours (MST) and continued until dusk. Only captures
within 10 m of the stream were used
in the mark-recapture analyses to allow a direct comparison to plot sampling estimates. Thus, the plot sampling represents a sample within the
exclosures and the grazed stream
area, whereas the mark-recapture
sampling represents an "open"
population estimate of each study
area. Total time spent sampling and
marking snakes was approximately
450 hours per year including time to
process snakes.
The approach to mark-recapture
analysis was to analyze each year
separately using closed population
models calculated by program CAPTURE, which allows unequal catchability (Otis et al. 1978, White et al.
1978,1982)as recommended by Pollock (1981,1982). Because we were
unable to estimate survival using the
timed fixed-area plots, we do not
present these estimates here for the
mark-recapture analysis. However,
all sampling periods were pooled
and survival estimators between
years estimated using the Jolly-Seber
Model (Seber 1986, Szaro et al., in
preparation).
Inferences about differences between years and exclosures were
based on Bonferroni's method for
multiple comparisons by fixing the
experimentwise error rate at 0.05
(Milliken and Johnson 1984).Thus,
the overall experimentwise error rate
is less than P (in this case 0.05); but
for each comparison, the comparisonwise error rate is equal to P/n,
where n is the number of comparisons. For example, with 3 comparisons the actual P value per comparison would be 0.05/3 or 0.017.
Results
We are confident the mark-recapture
estimates accurately reflect population densities on the three study areas and use these as the basis for
comparison for the timed fixed-area
plot results. Mark-recapture estimates were based on 118 individuals
and 35 recaptures (118/35) in exclosure 1in 1984,72/28 in 1985,127/30
in exclosure 2 in 1984,74126 in 1985,
12/ 2 in the grazed area in 1984, and
1011 in 1985.
We asked two questions of the
sarnpling methods. First, were there
any differences in population estimates between years? Both methods
indicated decreases in population
size on all three areas between 1984
and 1985. However, yearly differences were significant only for markrecapture estimates and for the timed
fixed-area plot estimates in exclosure
2 (P 5 0.05) (table 1). Mark-recapture
estimates revealed that snake populations decreased by 41% to 54%
from 1984 to 1985 in all study areas.
Decreases in mean number of snakes
per fixed-area plot were not as uniform, varying from 31% on exclosure
1 to 65% on exclosure 2 and the
grazed stream segment.
Second, were there differences between the study areas? Population
estimates between exclosures and the
grazed stream segment within a
given year were significantly different by both census methods and for
both years (P 5 0.05) (table 1). Population estimates by both methods
were not significantly different between exclosures, except in 1985
when the estimate determined by
timed fixed-area plots for exclosure 2
was 50% of that on exclosure 1 (P 5
0.05) (table 1).
Estimating population size by restricting the mark-recapture estimates to a 10 m band on either side
of the stream served a twofold purpose. First, it allowed us to estimate
the number of snakes per unit area.
Second, it made estimates by both
techniques more readily comparable,
because all plot sampling was confined to the 10-m band next to the
stream where most of the available
down litter, grass clumps, and
shrubby vegetation was concentrated. In exclosure 1, there were 3.86
and 2.28 snakes per 250 m2in 1984
and 1985, respectively. In exclosure
2, there were 4.53 and 2.23 snakes
per 250 m2in 1984 and 1985, respectively. Along the grazed stream reach
there were 1.00 and 0.38 snakes per
250 m2in 1984 and 1985, respectively.
Based on these estimates, we caught
between 20.2% (exclosure 2,1985)
and 38.6% (exclosure 1,1985) of the
snakes present in the exclosures. On
the grazed area we caught 28% of the
snakes in both 1984 and 1985.
Discussion
Apparent short-term downward
population fluctuations averaging
about 50% have been found in several mark-recapture studies (Fukada
1969, Platt 1969, Fitch 1975, Feaver
1977, Gregory 1977). Many studies of
snakes have related population
changes over several years to successional changes (Clark 1970, Fitch
1982) or to environmental factors,
such as decreases in annual precipitation (Clark 1974, Clark and Fleet
1976). Another possibility, is that a
study like this actually destroys hiding places (turning rocks, logs, etc.);
and even if each piece is put back
carefully, the site has opened up and
changed (Clark, personal communication).
We undoubtedly had some impact
on the quality of the available habitat
by our intensive searching tactics;
but we did try to be as careful as possible to return moved objects back
into their original positions. Parker
and Plummer (1987) suggest that
these apparent fluctuations in density result from changes in activity
level (which affect recapture probabilities) rather than from actual
changes in density (Lillywhite 1982,
Pough 1983).There are three possible
explanations for these results: (1)
snakes simply moved out of the plot
and exclosure areas; (2) snakes became inactive in burrows or cover
sites because of environmental conditions; or (3) snakes died.
Activity periods of wandering garter snakes varied between individuals from our preliminary sample of
wandering garter snake populations
along the Rio de las Vacas in July
1983. We failed to decrease significantly the total numbers of animals
caught per plot even after 3 days of
removal sampling (Szaro et al. 1985);
but at other times snakes were difficult to find. However, we feel the intensive sampling effort of at least 1
week each month minimized the effect of changes in snake behavior on
population estimates.
The almost 50% difference in 1985
between exclosures in mean number
of snakes caught while plot sampling
was probably a result of a shift in areas used by the snakes and not differences in mortality between the
two exclosures. Monthly trends in
total number of snakes caught also
showed a dramatic difference in the
number of snakes caught per month
while plot sampling in both exclosures. However, this difference was
not reflected in the overall number of
snakes caught during mark-recapture
sampling (fig. 4). In fact, overall we
caught more snakes in exclosure 2
than in exclosure 1in all months in
1985.
The difference in plot sampling estimates between exclosures in 1985
was not a result of changes in daily
activity patterns, because equal proportions of snake captures in both
exclosures were before 1300 (63%in
exclosure 1 and 59% in exclosure 2,
chi-square, P > 0.05). Furthermore,
differences in captures between years
and methods were not sex-based, because there were no significant differences in sex ratios between years
or method in a given study section
(chi-square, P > 0.05) (fig. 5). However, there were distributional differences in snake captures between
years and exclosures.
In 1984,34.6% and 34.7% of all
captures on exclosures 1 and 2, respectively were made on the plot areas. In contrast, 42.1% and 20.6% of
all captures on exclosures 1 and 2, re-
Plot Sampling
Exclosure 1
Exclosure 2
a
June
July
Aug.
Sept.
June
Grazed Reach
July
Aug.
Sept.
1985
1984
Mark-Recapture Sampling
Exclosure 1
Exclosure 2
1
Grazed Reach
El
June
July
Aug.
Sept.
June
1984
July
Aug.
Sept.
1985
Sampling Period
Figure 4.-Total numbers of wandering garter snakes caught in June, July, August, and September 1984 and 1985 along the Rio de las Vacas, New Mexico during timed fixed-area plot
and mark-recapture sampling.
243
spectively, were made on the plot
areas in 1985.
We cannot explain this distributional shift in exclosure 2. Although
we did not plot sample in 1986 and
1987, mark-recapture efforts in those
years showed a similar distributional
pattern (Szaro et al., unpublished). In
exclosure 1,33.0 % and 37.3% of all
captures in 1986 and 1987, respectively were on the old plot areas,
whereas in exclosure 2, these values
were 10.0% and 9.8%.
We feel that the distributional
changes in exclosure 2 were not an
artifact of plot sampling, because
snakes in exclosuse 2 did not return
to plot areas after plot sampling had
stopped. In any case, our sampling
potentially would have been more
destructive in exclosure 1 than in exclosure 2 because of the higher incidence of turnable rocks in that exclostare.
Whatever the cause, these changes
in distribution indicate that initial
randomized selection of plots did influence density estimates for exclosure 2. Although it would increase
substantially the amount of time necessary to adequately sample vegetation, a better approach would be to
randomly select plots within exclosures each sampling period rather
than repeatedly sampling the same
plots.
In conclusion, the use of timed
fixed-area plots enabled us to quantify dramatic differences in snake
abundance between exclosires and
the grazed area. However, this Sampling method is of questionable merit
because of the significant difference
in exclosure population estimates for
1985. Further study incorporating
newly randomized plots for each
sampling period may solve this problem. Care should be taken to determine if snakes are distributing themselves in a nonrandom pattern. At
this time, we recommend the more
labor-intensive mark-recapture estimators for assessing the impacts of
riparian management regimes on
snake populations.
Acknowledgments
We thank D. R. Clark, H. S. Fitch, K.
B. Jones, and N. J. Scott, Jr. for their
constructive reviews of this paper. H.
Berna, M. Cady, C. Engel-Wilson, X.
Hernandez, D. Johnson, M. Lane, W.
Legarde, L. Simon, and D. Smith
aided in the collection of the field
data. Special thanks to Jim and Mary
Bedeaux for their gracious hospitality and allowing us to sample on
their property.
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