THE SOUTHWESTERN NATURALIST 58(4): 446–449
DECEMBER 2013
AN EFFECTIVE METHOD FOR INCREASING THE CATCH-RATE OF PITFALL TRAPS DONALD T. MCKNIGHT,* TYLER L. DEAN,
AND
DAY B. LIGON
Department of Biology, Missouri State University, 901 South National Avenue, Springfield, MO 65897 *Correspondent:
[email protected] ABSTRACT—Pitfall traps are widely used by researchers to survey a variety of terrestrial taxa. To test a method for improving the results of pitfall traps, we built a Y-shaped drift fence with six pitfall traps on each arm. We improved the pitfall traps by staking four pieces of aluminum flashing around each of the traps to increase their effective area. For experimental purposes, we used the aluminum flashing wings every other day. Traps without flashing caught a total of 395 reptiles and amphibians representing 15 species, and traps with the flashing caught a total of 624 reptiles and amphibians representing 20 species. RESUMEN—Trampas de hoyo son ampliamente utilizadas por investigadores para muestrear una variedad de taxa terrestre. Para poner a prueba un m´etodo para mejorar los resultados de las trampas de hoyo, construimos una cerca de deriva en forma de Y con seis trampas en cada brazo. Mejoramos las trampas de hoyo al colocar cuatro secciones de l´aminas de aluminio alrededor de cada trampa para aumentar su a´ rea efectiva. Para el experimento, las alas de aluminio fueron utilizadas cada dos d´ıas. Trampas sin las alas capturaron un total de 395 reptiles y anfibios que represent´o a 15 especies, y las trampas con las alas capturaron un total de 624 reptiles y anfibios que represent´o a 20 especies. Pitfall traps are a common means of surveying animals in a wide range of taxa, including amphibians, reptiles, small mammals, and invertebrates (Campbell and Christman, 1982; Bury and Corn, 1987; Mengak and Guynn, 1987; Corn, 1994; Dent and Walton, 1997). For practical reasons, 18.9 L buckets (i.e., the ubiquitous five-gallon buckets) are generally the largest pitfall traps used, but previous research has shown that efficiency of pitfall trap increases with the diameter of the trap (Greenberg et al., 1994; Crosswhite et al., 1999; Brennan et al., 2005; Maritz et al., 2006; Todd et al., 2007). We present a method for inexpensively and efficiently increasing the effective diameter of pitfall traps. MATERIALS AND METHODS—The study area, Boehler Seeps and Sandhills Preserve, is a 196-ha preserve located in Atoka County, Oklahoma. This site contains two general communities, bluejack oak woodlands and acid hillside seeps. The seeps feed two beaverformed lakes (Boehler Lake and Hassel Lake) that are encompassed by sandy hillsides that rapidly transition into the upland oak forests. These habitats harbor a wide variety of flora and fauna, including >50 species of reptiles and amphibians (Patton and Wood, 2009; D. T. McKnight and D. B. Ligon, pers. obser.). To test our method for improving pitfall traps, we constructed a Y-shaped, vinyl fabric drift fence with three arms that intersected at 1208 (Fig. 1a; Jones, 1986; Enge, 2001; Farallo et al., 2010). We built this fence at the top of a hill at the southern end of Boehler Lake (34.165638N, 95.889018W). Each arm of the fence was 30.5 m long and 0.9 m tall (Crosswhite et al., 1999; Enge, 2005). We placed six 18.9 L buckets (0.3 m diameter ·
0.37 m deep) along each arm at 6-m intervals (Greenberg et al., 1994; Crosswhite, 1999). We placed the tops of the buckets flush with the ground and packed soil around the bottom of the drift fence to prevent animals from going under it. We placed wet sponges in the buckets to prevent animals from dehydrating (Enge, 2001; Lannoo et al., 2009). We also placed several funnel traps along this fence (Fig. 1a). Double-ended funnel traps were 2.5 m from the nearest pitfall traps, and single-ended funnel traps were only a few centimeters from the nearest pitfall traps. We used the funnel traps every day of this study. To increase the effective diameter of the pitfall traps, we placed two wings on either side of them (Fig. 1b). These wings were 0.6 m long and 0.5 m tall. They were made of aluminum flashing with a wooden stake attached to the middle. Our prediction was that animals that were walking parallel to the fence, but not immediately adjacent to it, would encounter the wings and be directed into the pitfall trap. We only used wings on the four pitfall traps in the center of each arm. It was impossible to get four wings on the pitfall traps in the center of the array because the traps were almost touching each other. We did not place wings on the outermost pitfall traps because those traps were adjacent to the single-ended funnel traps, and we could not install wings without interfering with the funnel traps. Because catch-rates of drift fences can be affected by environmental factors such as temperature, humidity, and rainfall, we used a second drift fence as a control for daily variation in movement of reptiles and amphibians (Gibbons and Bennett, 1974; Read and Moseby, 2001). The second drift fence was originally built for a survey in 2008 (Patton and Wood, 2009). It was constructed of aluminum flashing and was 15.2 m long and 0.6 m tall. It had four 18.9 L pitfall traps (one every 5
December 2013
McKnight et al.—Catch-rate of pitfall traps
447
senting 20 species, and pitfall traps without wings caught a total of 395 reptiles and amphibians (mean = 1.00/ trap/day) representing 15 species (Table 1). As predicted, rates of capture for pitfall traps with wings were significantly greater than rates of capture for pitfall traps without wings (P = 0.0221). On days when the experimental fence had wings, the control fence caught a mean of 2.01 reptiles and amphibians per trap, and on days when the experimental fence did not have wings, it caught a mean of 1.14 reptiles and amphibians per trap (Table 1). There was no significant difference between the mean ranks of the control fence on days when the experimental fence did and did not have wings (P = 0.1759), but there was a significant positive correlation between the rates of capture of the two drift fences (P < 0.0001).
FIG. 1—a) Top-view diagram of the experimental drift fence array. Double-ended funnel traps were 2.5 m from the nearest pitfall traps, and single-ended funnel traps were only a few centimeters from the nearest pitfall traps. b) Top-view diagram of four aluminum wings installed on a pitfall trap. Image is not to scale. m) and was located on the northern end of Boehler Lake (34.169368N, 95.888898W). Wings were never used on this drift fence. Because both fences would experience the same weather, we expected the catch-rates of the two fences to be closely correlated. We checked the traps from 23 May–13 July and 2–15 August 2012. During both trapping periods, we alternated nights with and without wings. This resulted in 33 nights with wings and 33 nights without wings. We checked both drift fences every morning between 0800 and 1100 h. The data from both drift fences deviated from a normal distribution; therefore, we used a one-way Mann-Whitney U test to compare the number of reptiles and amphibians captured by traps with and without wings (Zar, 1996). We only used the data from the 12 experimental pitfall traps in the analysis. We did not include the other pitfall traps or the funnel traps as controls because of concerns about the independence of their rates of capture. We used a second one-way Mann-Whitney U test to compare the rates of capture of the control drift fence on days when the experimental fence did and did not have wings. We used the mean daily rates of capture in the analysis rather than the actual number of amphibians and reptiles captured because, on three days, some buckets on the control fence flooded with water, allowing animals to escape. We used a sequential Bonferroni correction to control the family-wise error rate for the two Mann-Whitney U tests (initial a = 0.05; Holmes, 1979). Finally, we calculated a Spearman rank correlation coefficient (a = 0.05) to confirm that the catch-rates of the two drift fences were correlated (Zar, 1996). We performed all tests in Minitab, release 16.
RESULTS—Pitfall traps with wings caught a total of 624 reptiles and amphibians (mean = 1.58/trap/day) repre-
DISCUSSION—The correlation between the two drift fences validates our use of the smaller drift fence as a control for the daily variation in catch-rate. Also, the fact that the difference between days with and without wings was significant for the experimental fence but not significant for the control fence suggests that our method did improve the rate of capture of the pitfall traps. Further, traps with wings caught six species that were not captured in traps without wings, whereas traps without wings only captured one species that was not in traps with wings (Table 1). Despite the low rates of capture of these species, their presence further supports the conclusion that our method is an improvement over traditional pitfall traps. It seems unlikely that the wings had species-specific effects; rather, it is probable that by virtue of having an increased rate of capture, pitfall traps with wings did a better job of capturing species with low population densities. Therefore, this method also may be helpful in surveying rare species. While this conclusion cannot be tested statistically, it is supported by the fact that the control fence caught the same number of species on days when the experimental fence did and did not have wings. While we only tested the effectiveness of this method at capturing reptiles and amphibians, it is probable that this method also would increase the rate of capture of small mammals and invertebrates. In our study, we rarely caught mammals in pitfall traps with or without wings, and we released invertebrates without counting or identifying them. Also, placing a single wing beside the open end (or ends) of a funnel trap would likely increase its rate of capture as well. Ideally, the wings should be driven slightly into the ground to prevent animals from going underneath them, and they should slightly overhang the lip of the bucket. Most of the wings can be nailed or screwed to a single stake and only need to be installed once per trapping period. When all four wings are in place around a pitfall trap, however, it is difficult to remove animals, debris, and water from the trap. Therefore, we recommend that one of the
448
vol. 58, no. 4
The Southwestern Naturalist
TABLE 1—Total number of individuals captured in pitfall traps on experimental and control drift fences on days when the experimental fence did and did not have wings. Experimental fence (EF)
Control fence
Taxon
With wings
Without wings
When EF had wings
When EF did not have wings
Amphibians Acris blanchardi Ambystoma opacum Ambystoma texanum Anaxyrus americanus charlesmithi Anaxyrus woodhousii Gastrophryne carolinensis Hyla cinerea Hyla versicolor Lithobates areolatus areolatus Lithobates clamitans Lithobates palustris Lithobates sphenocephalus utricularius Notophthalmus viridescens louisianensis Pseudacris streckeri Scaphiopus hurterii Total no. of amphibians Total no. of amphibian species
6 5 0 7 1 113 0 1 1 39 90 146 83 2 115 609 13
5 0 0 3 1 96 0 0 0 6 56 69 73 2 75 386 10
1 0 1 12 0 3 0 0 0 0 25 128 2 2 48 222 9
0 5 0 4 0 2 1 4 0 0 15 75 4 0 32 142 9
1 4 1 1 3 4 0 1 15 7
0 4 2 0 1 1 1 0 9 5
0 0 0 0 0 1 1 0 2 2
0 0 0 0 2 0 1 0 3 2
624 20
395 15
224 11
145 11
Reptiles Anolis carolinensis carolinensis Aspidoscelis sexlineata viridis Carphophis vermis Opheodrys aestivus aestivus Plestiodon fasciatus Sceloporus consobrinus Scincella lateralis Storeria dekayi texana Total no. of reptiles Total no. of reptilian species Total No. of individuals No. of species
wings on each trap be installed such that it can easily be removed and reinstalled. If properly installed, these wings should increase the number and diversity of animals that are captured in pitfall traps without greatly increasing the amount of time that it takes to check the traps. We are indebted to J. Tucker for assistance and advice throughout this research. We also would like to thank E. Ligon for translating the abstract into Spanish. Finally, we would like to thank the Oklahoma Department of Wildlife Conservation and the Delta Foundation for funding this study. We conducted this research under Oklahoma Department of Wildlife Conservation scientific collecting permit #5269 and with the approval of the Missouri State University Institutional Animal Care and Use Committee (IACUC protocol no. 10014). LITERATURE CITED BRENNAN, K. E. C., J. D. MAJER, AND M. L. MOIR. 2005. Refining sampling protocols for inventorying invertebrate biodiversity:
influence of drift-fence length and pitfall trap diameter on spiders. Journal of Arachnology 33:681–702. BURY, R. B., AND P. S. CORN. 1987. Evaluation of pitfall trapping in northwestern forests: traps arrays with drift fences. Journal of Wildlife Management 51:112–119. CAMPBELL, H. W., AND S. P. CHRISTMAN. 1982. Field techniques for herpetofaunal community analysis. Pages 193–200 in Herpetological communities (N. J. Scott, Jr., editor). United States Fish and Wildlife Service, Wildlife Research Report 13:1–239. CORN, P. S. 1994. Straight-line drift fences and pitfall traps. Pages 109–117 in Measuring and monitoring biological diversity: standard methods for amphibians (W. R. Heyer, M. A. Donnelly, R. W. McDiarmid, L. A. C. Hayek, and M. S. Foster, editors). Smithsonian Institution Press, Washington, D.C. CROSSWHITE, D. L., S. F. FOX, AND R. E. THILL. 1999. Comparison of methods for monitoring reptiles and amphibians in upland forests of the Ouachita Mountains. Proceedings of the Oklahoma Academy of Science 79:45–50. DENT, D. R., AND M. P. WALTON (editors). 1997. Methods in ecological and agricultural entomology. CAB International, Wallingford, United Kingdom.
December 2013
McKnight et al.—Catch-rate of pitfall traps
ENGE, K. M. 2001. The pitfalls of pitfall traps. Journal of Herpetology 35:467–478. ENGE, K. M. 2005. Herpetofaunal drift-fence surveys of steephead ravines in the Florida Panhandle. Southeastern Naturalist 4:657–678. FARALLO, W. R., D. J. BROWN, AND M. R. J. FORSTNER. 2010. An improved funnel trap for drift-fence surveys. Southwestern Naturalist 55:457–460. GIBBONS, J. W., AND D. H. BENNETT. 1974. Determination of anuran terrestrial activity patterns by a drift fence method. Copeia 1974:236–243. GREENBERG, C. H., D. G. NEARY, AND L. D. HARRIS. 1994. A comparison of herpetofaunal sampling effectiveness of pitfall, single-ended, and double-ended funnel traps used with drift fences. Journal of Herpetology 28:319–324. HOLM, S. 1979. A simple sequentially rejective multiple test procedure. Scandinavian Journal of Statistics 6:65–70. JONES, K. B. 1986. Amphibians and reptiles. Pages 267–290 in Inventory and monitoring of wildlife habitat (A. Y. Cooperrider, R. J. Boyd, and H. R. Stuart, editors). United States Department of the Interior, Bureau of Land Management Service Center. LANNOO, M. J., V. C. KINNEY, J. L. HEEYMEYER, N. J. ENGBRECHT, A. L. GALLANT, AND R. W. KLAVER. 2009. Mine spoil prairies expand critical habitat for endangered and threatened amphibian and reptile species. Diversity 1:118–132.
449
MARITZ, B., G. MASTERSON, D. MACKAY, AND G. ALEXANDER. 2006. The effect of funnel trap type and size of pitfall trap on trap success: implications for ecological field studies. AmphibiaReptilia 28:321–328. MENGAK, M. T., AND D. C. GUYNN. 1987. Pitfalls and snap traps for sampling small mammals and herpetofauna. American Midland Naturalist 118:284–288. PATTON, T., AND J. WOOD. 2009. A herpetofaunal survey of the Boehler Seeps Preserve, with reports of new county records and recommendations for conservation efforts. Proceedings of the Oklahoma Academy of Science 89:67–78. READ, J. L., AND K. E. MOSEBY. 2001. Factors affecting pitfall capture rates of small ground vertebrates in arid South Australia. I. The influence of weather and moon phase on capture rates of reptiles. Wildlife Research 28:53–60. TODD, B. D., C. T. WINNE, J. D. WILLSON, AND J. W. GIBBONS. 2007. Getting the drift: examining the effects of timing, trap type and taxon on herpetofaunal drift fence surveys. American Midland Naturalist 158:292–305. ZAR, J. H. 1996. Biostatistical analysis. Prentice-Hall, Inc., Upper Saddle River, New Jersey. Submitted 6 October 2012. Accepted 3 February 2014. Associate Editor was Neil B. Ford.
