The Use of Timed Fixed-Area Plots and a Mark-Recapture Technique in Assessing Riparian Garter Snake Populations1

Robert C. S~aro,~ Scott C. Belfit,3 J. Kevin Aitkin," and Randall D. BabbS

Research studies often attempt to de- termine the effects of disturbance or management regimes on the abun- dance of wildlife species (Cooper- rider 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 stud- ies. Snakes are difficult subjects for field studies because of their secre- tive and cryptic habits (Fitch 1987).

paper presented at symposium, Man- agement of Amphibians. Reptiles, and Small Mammals in North America. (Flag- staff, AZ, July 19-2 1 , 1988.)

* ~ober f C. Szaro is Research Wildlife Bi- ologist, USDA Forest Service, Rocky Moun- tain Forest and Range Experiment Station, Arizona State University Campus, Tempe, AZ 85287- 1304.

3 ~ c o # C. Belfit is Wildlife Biologist, De- partment of the Army, Wildlife Manage- ment Section. Fort Huachuca, AZ 856 13- 6000. 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 85287- 1304.

anda all D. Babb, formerly Wildlife Tech- nician, USDA Forest Service, Rocky Moun- tain Forest and Range Experiment Station, Arizona State University Campus, Tempe. AZ 85287- 1304 is currently Sportfishing Program Coordinator, Arizona Game and Fish De- partment. 2222 West Green way Road, Phoenix, AZ 85023.

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 fixed- area plots were inconsistent between study plots in the same year.

Many attempts to census snakes have been inaccurate (Turner 1977, Fitch 1987). Population estimates can be influenced by sex, reproductive con- dition, and stage of maturity, all of which are critical determinants of activity within species (Gibbons and Semlitsch 1987). Differences among juveniles and breeding and non- breeding females, and males of ten lead to much different risks of cap- ture 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 esti- mate snake density are direct counts and mark-recapture analyses. Sys- tematic searches of defined areas (di- rect counts) yield species occurrence data, and usually require less time and effort than mark-recapture meth- ods (Jones 1986). Using direct counts, Bury and Luckenbach (1977) success- fully censused desert tortoise (Go- pherus 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 proce- dures. 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 esti- mates become available from the same area at different times, they of- ten 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 de- rived from mark-recapture tech- niques. However, since his critical review, estimation techniques have greatly improved with the develop- ment of models and computer pro- grams that test model assumptions and estimate standard errors (Ar- nason and Baniuk 1980, White et al. 1978,1982, Otis et al. 1978, Brownie et al. 1985).

Although time consuming, deter- mining accurate population estimates is necessary to develop management policies not only for abundant spe- cies, 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 secre- tive 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 appli- cable to other snake species normally concentrated in riparian ecosystems and may be especially useful for cen- susing endangered species where large samples to detemine the accu- racy of sampling techniques are not available. Our previous work showed the inadequacy of simple transects and depletion sampling in determining garter snake popula- tions along the Rio de las Vacas, New Mexico (Szaro et al. 1985). The objec- tive of this study was to compare timed fixed-area plots and a mark- recapture 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.

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). Con- tiguous, downstream areas, privately owned and grazed by livestock, were used for comparison. The most ap- parent 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 scat- tered where the streambanks were grazed (9.5 + 1.16,7.5 + 1.23, and 0.3 + 0.14 trees/250 m2 in exclosures 1,2, - and grazed areas, respectively).

Snake populations were estimated by timed fixed-area plot sampling,

Figure I .-Grazed section of the Wio de las Vacas, New Mexico. Notice the lack of shrub growth and the unstable stream banks.

and mark-recapture sampling in both two ungrazed exclosures and one grazed and ungrazed areas. For the grazed stream segment along the Rio former, 16 plots (10 x 25 m), with the de las Vacas, for a total of 48 plots long edge being defined by the (fig. 3). During sample periods we stream bznk, were intensively turned rocks, logs, debris piles, and sampled for 20 minutes in each of the 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 be- tween samples to process snakes.

For mark-recapture estimates, we searched the entire extent of both ex- closures and a similarly sized down- stream grazed stream area. The plots used for the timed-fixed plot sam- pling were a subset of the area used for the mark-recapture sampling. All captured snakes were marked by clipping three subcaudal scales (Blan- chard and Finster 1933, Woodbury 1956). Mark-recapture sampling peri- ods occurred in the same months as the plot sampling; but snakes were captured, marked, and released dur-

ing intensive searches for 6 consecu- tive days by 3 to 4 collectors. All snakes were released where cap- tured. Approximately equal time and effort was spent searching for snakes in each of the three areas. Time of day bias was minimized by alternat- ing starting areas daily. Sampling began at 0900 hours (MST) and con- tinued until dusk. Only captures within 10 m of the stream were used in the mark-recapture analyses to al- low a direct comparison to plot sam- pling estimates. Thus, the plot sam- pling 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 CAP- TURE, which allows unequal catch- ability (Otis et al. 1978, White et al. 1978,1982) as recommended by Pol- lock (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 be- tween 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 compari- sonwise error rate is equal to P/n, where n is the number of compari- sons. For example, with 3 compari- sons the actual P value per compari- son would be 0.05/3 or 0.017.

Results

We are confident the mark-recapture estimates accurately reflect popula- tion densities on the three study ar- eas and use these as the basis for

Rio de las Vacas

Edomrs1

Figure 3.-Study areas and sample plot lay- out along the Rio de las Vacas, New Mex- ico.

comparison for the timed fixed-area plot results. Mark-recapture esti- mates were based on 118 individuals and 35 recaptures (118/35) in exclo- sure 1 in 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 esti- mates between years? Both methods indicated decreases in population size on all three areas between 1984 and 1985. However, yearly differ- ences were significant only for mark- recapture estimates and for the timed fixed-area plot estimates in exclosure 2 (P 5 0.05) (table 1 ). Mark-recapture estimates revealed that snake popu- lations 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 uni- form, varying from 31% on exclosure 1 to 65% on exclosure 2 and the grazed stream segment.

Second, were there differences be- tween the study areas? Population estimates between exclosures and the grazed stream segment within a given year were significantly differ- ent by both census methods and for both years (P 5 0.05) (table 1 ). Popu- lation estimates by both methods were not significantly different be- tween 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 re- stricting the mark-recapture esti- mates to a 10 m band on either side of the stream served a twofold pur- pose. 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 con- fined to the 10-m band next to the stream where most of the available down litter, grass clumps, and shrubby vegetation was concen- trated. In exclosure 1, there were 3.86

and 2.28 snakes per 250 m2 in 1984 and 1985, respectively. In exclosure 2, there were 4.53 and 2.23 snakes per 250 m2 in 1984 and 1985, respec- tively. Along the grazed stream reach there were 1.00 and 0.38 snakes per 250 m2 in 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 sev- eral 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 succes- sional changes (Clark 1970, Fitch 1982) or to environmental factors, such as decreases in annual precipi- tation (Clark 1974, Clark and Fleet 1976). Another possibility, is that a study like this actually destroys hid- ing places (turning rocks, logs, etc.); and even if each piece is put back carefully, the site has opened up and changed (Clark, personal communi- cation).

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 pos- sible to return moved objects back into their original positions. Parker and Plummer (1987) suggest that these apparent fluctuations in den- sity result from changes in activity level (which affect recapture proba- bilities) 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 be- came inactive in burrows or cover sites because of environmental condi- tions; or (3) snakes died.

Activity periods of wandering gar- ter snakes varied between individu- als from our preliminary sample of wandering garter snake populations along the Rio de las Vacas in July 1983. We failed to decrease signifi- cantly 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 diffi- cult to find. However, we feel the in- tensive sampling effort of at least 1 week each month minimized the ef- fect 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 ar- eas used by the snakes and not dif- ferences 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 exclo- sures. 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 1 in all months in 1985.

The difference in plot sampling es- timates between exclosures in 1985 was not a result of changes in daily activity patterns, because equal pro- portions 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, be- cause there were no significant dif- ferences in sex ratios between years or method in a given study section (chi-square, P > 0.05) (fig. 5). How- ever, there were distributional differ- ences in snake captures between years and exclosures.

In 1984,34.6% and 34.7% of all captures on exclosures 1 and 2, re- spectively were made on the plot ar- eas. In contrast, 42.1 % and 20.6% of all captures on exclosures 1 and 2, re-

Plot Sampling

Exclosure 1

Exclosure 2

a Grazed Reach

June July Aug. Sept. June July Aug. Sept.

1984 1985

Mark-Recapture Sampling

Exclosure 1

Exclosure 2

1 Grazed Reach

E l

June July Aug. Sept. June July Aug. Sept.

1984 1985

Sampling Period

Figure 4.-Total numbers of wandering garter snakes caught in June, July, August, and Sep- tember 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 distribu- tional 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, respec- tively 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 ex- closure 2 because of the higher inci- dence of turnable rocks in that exclo- stare.

Whatever the cause, these changes in distribution indicate that initial randomized selection of plots did in- fluence density estimates for exclo- sure 2. Although it would increase substantially the amount of time nec- essary to adequately sample vegeta- tion, a better approach would be to randomly select plots within exclo- sures each sampling period rather than repeatedly sampling the same plots.

In conclusion, the use of timed fixed-area plots enabled us to quan- tify dramatic differences in snake abundance between exclosires and the grazed area. However, this Sam- pling 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 prob- lem. Care should be taken to deter- mine if snakes are distributing them- selves in a nonrandom pattern. At this time, we recommend the more labor-intensive mark-recap ture esti- mators 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 hospital- ity and allowing us to sample on their property.

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Hark

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