This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. 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. 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