Empirical Tests of Biased Body Size Distributions in Aquatic Snake Captures
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Copeia 2008, No. 2, 401–408 Empirical Tests of Biased Body Size Distributions in Aquatic Snake Captures John D. Willson1, Christopher T. Winne1, and Michael B. Keck2 Ecologists often rely on a suite of demographic parameters—such as age structure, body size distributions, population density, and sex ratios—to understand life history patterns, population dynamics, and community structure of snakes. Unfortunately, in many cases little consideration is given to how sampling techniques may influence the outcome of demographic studies. Herein, we use a combination of field capture techniques, an extensive database of field-captured snakes, and laboratory and field experiments to evaluate how capture methods may influence demographic assessments of several North American semi-aquatic snake species, including Agkistrodon piscivorus, Farancia abacura, Nerodia fasciata, N. floridana, N. rhombifer, N. taxispilota, Regina rigida, Seminatrix pygaea, and Thamnophis sauritus. We found that commercially available aquatic funnel traps (i.e., minnow traps) generally yielded biased assessments of population demography, but that the nature and magnitude of these biases varied predictably by species and trap type. Experimental manipulations of funnel opening diameter in aquatic funnel traps demonstrated that such modifications allowed for capture of larger snakes but that the size of funnel opening necessary to capture the largest individuals varied between species. Additionally, we found differences between snake species in their ability to escape from different types of traps at birth, suggesting that escape of neonates through trap mesh can lead to the lack of small snakes often observed in field samples. Overall, our results demonstrate that capture methods may bias assessments of snake population demography, but that careful design of sampling methodology, with consideration of potential biases, can yield meaningful data on snake biology. EMOGRAPHY, the statistical study of populations, 2001; Willson et al., 2005). Population studies that employ especially with reference to age, size, density, and standardized snake collection techniques have been con- D distribution, provides a key to understanding life ducted mostly in habitats with unusually high snake density history patterns, population dynamics, and community (Godley, 1980; Sun et al., 2001; Luiselli, 2006). However, composition, all of which have obvious applications to even in these situations, sampling biases seldom have been conservation biology. Indeed, snake population demogra- critically examined (but see Rodda, 1993; Sun et al., 2001; phy has been the foundation for many studies of snake Rodda et al., 2005). Inherent biases exist with all techniques ecology and conservation (Plummer, 1985; Brown and and sound experimental design requires knowledge of Weatherhead, 1999; Blouin-Demers et al., 2002). Moreover, sampling biases before appropriate conclusions can be because snakes exhibit cryptic behavior and low or sporadic made. activity patterns, demography is often used as a proxy for The demography of semi-aquatic snakes has received direct monitoring of population parameters (Madsen and considerable attention in the literature (Shine, 1986a, Shine, 2000; Lacki et al., 2005; Willson et al., 2006). 1986b; Houston and Shine, 1994; King et al., 1999), and Unfortunately, knowledge of how sampling methodology has been a major area of investigation in our own ecological influences the assessment of snake population demography research (Winne et al., 2005, 2006a; Willson et al., 2006). is limited (but see Prior et al., 2001). For example, newborn Commonly used techniques for sampling aquatic snakes and juvenile snakes are often under-represented in demo- include visual encounter surveys (Rodda, 1993; Sun et al., graphic studies, a phenomenon generally interpreted as 2001), flipping natural or artificial cover objects (i.e., evidence for high juvenile mortality (Parker and Plummer, ‘‘coverboards;’’ Grant et al., 1992; Parmelee and Fitch, 1987). However, sampling biases are seldom assessed with 1995; Webb and Shine, 2000), road cruising (Fitch, 1987), sufficient rigor to reject the alternative hypothesis that a terrestrial pitfall or funnel trapping (often in conjunction paucity of small snakes may be due to ineffective sampling with drift-fences; Fitch, 1951; Gibbons and Semlitsch, 1981; techniques for juveniles. Enge, 1997), and aquatic funnel trapping (Keck, 1994). Adequate assessment of population demography requires Among these methods, passive capture techniques (i.e., standardized sampling techniques with a thorough under- trapping) are particularly useful because they are less prone standing of inherent methodological biases. Nonetheless, to observer biases than active survey methods (e.g., visual many studies abandon systematic or randomized sam- surveys; Rodda, 1993) and reduce biases in capture proba- pling in favor of opportunistic or haphazard methods to bility caused by short-term shifts in environmental condi- maximize captures and avoid the logistical difficulties tions because captures are integrated over time (Willson and associated with statistically rigorous sampling of rare or Dorcas, 2003). First described by Keck (1994), aquatic funnel secretive snakes (Parker and Plummer, 1987; Prior et al., trapping using commercially available ‘‘minnow,’’ ‘‘craw- 1 Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802; E-mail: (JDW) [email protected]; and (CTW) [email protected]. Send reprint requests to JDW. 2 Grayson County College, 6101 Grayson Drive, Denison, Texas 75020; E-mail: [email protected]. Submitted: 7 February 2007. Accepted: 24 September 2007. Associate Editor: M. J. Lannoo. F 2008 by the American Society of Ichthyologists and Herpetologists DOI: 10.1643/CH-07-035 402 Copeia 2008, No. 2 Table 1. Characteristics of Cylindrical Funnel Traps Used to Capture Snakes in Aquatic Habitats. Length Funnel extension Funnel opening Max mesh size Trap type (cm) Diameter (cm) (cm) diameter (cm) (cm) Price (U.S.) Plastic minnow 43 16 11 2.5 0.4 3 0.6a $7.25 Galvanized steel minnow 42 19 11 2.5b 0.6 3 0.6 $8.96 Galvanized steel eelpot 80 20 11c 2.5b 0.6 3 0.6 $17.25 a Mesh size ranges from 0.1 3 0.3–0.4 3 0.6 cm. b Funnel openings of steel and eelpot traps may be widened. Funnel openings of eelpot traps used in Texas were widened to 3.8 or 5.1 cm to investigate how much they must be widened to allow for capture of the largest snakes (see Methods). Some steel minnow traps used in South Carolina were widened to ca. 3.0–3.5 cm. c Funnels of eelpot traps used in this study were extended to approximately 20 cm by attaching an additional 9-cm piece of steel mesh to the widened funnel opening. fish,’’ or ‘‘eelpot’’ traps has recently emerged as an effective Snakes were captured using aquatic funnel traps and method for sampling semi-aquatic snake species (Seigel et ‘‘other methods.’’ We used two aquatic funnel trap types al., 1995; Willson et al., 2005; Winne, 2005). Several types of (Table 1), frequently marketed to capture live minnows and aquatic traps are commercially available under various crayfish for fishing bait, for trap comparisons of body size regional names; however, for the purposes of this study we distributions on the SRS: galvanized steel (‘‘1/4-inch hard- refer to standard ‘‘gee’’ traps as ‘‘minnow traps’’ and ‘‘gee’’ ware cloth’’) cylindrical minnow traps (model G-40; Cuba traps with a central extension as ‘‘eelpot traps’’ (Table 1). Specialty Manufacturing Company, Fillmore, NY) and Although previous studies have suggested that aquatic plastic cylindrical minnow traps (model 700; N.A.S. Incor- funnel trap captures may be biased towards actively foraging porated, Marblehead, OH). We used plastic traps as pur- snakes (Keck, 1994; Winne, 2005), the potential for chased from the manufacturer, but we widened funnel sampling methodology to bias demographic data has openings (i.e., the entrances into the trap) to ca. 3–3.5 cm in seldom been evaluated. some steel traps by forcing a wooden dowel through the Herein we use a combination of laboratory experiments funnel opening. We set funnel traps in shallow water with and an extensive database of field-captured snakes to approximately 3–5 cm of the trap remaining above water evaluate size biases of several North American semi-aquatic level and checked traps daily for snakes. We did not snake species captured in commercially-available aquatic intentionally bait traps, although incidental captures of fish funnel traps. Specifically, we (1) compare body size distri- and amphibians often resulted in ‘‘natural baiting’’ (Keck, butions of field-captured snakes among sampling tech- 1994; Winne, 2005). ‘‘Other’’ capture methods included niques to determine if aquatic traps capture all sizes of searching for snakes under aquatic and terrestrial artificial snakes present within the population, (2) manipulate the coverboards (including wood and metal; Grant et al., 1992), size of funnel trap openings to determine the role of funnel terrestrial drift fences with pitfall or funnel traps (Fitch, opening diameter in limiting the upper body size of snakes 1951; Gibbons and Semlitsch, 1981), road collecting (Fitch, captured in traps, and (3) experimentally release neonates of 1987), and incidental hand captures. For comparisons with seven snake species into aquatic funnel traps at birth and aquatic trap captures, we combined captures made with all after a period of growth, to test the hypothesis