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Light Bait Improves Capture Success of Aquatic Funnel-Trap Sampling for Larval Amphibians
Stephen H. Bennett, Jayme L. Waldron, and Shane M. Welch

Southeastern Naturalist, Volume 11, Issue 1 (2012): 49–58

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2012 SOUTHEASTERN NATURALIST 11(1):49–58 Light Bait Improves Capture Success of Aquatic Funnel-Trap Sampling for Larval Amphibians Stephen H. Bennett1,*, Jayme L. Waldron2, and Shane M. Welch2 Abstract - Aquatic funnel traps are a non-destructive means of surveying amphibians in lentic habitats, particularly as compared to dip-net surveys that disturb aquatic vegetation and the substrate, and affect the water column through increased turbidity. The objective of this study was to examine the utility of glow stick-baited aquatic funnel traps for larval amphibians, with a particular emphasis on ambystomatid larvae. We sampled 12 isolated ponds in the Mid-Atlantic Coastal Plain of South Carolina between April and June 2010 and used detection/non-detection capture data to model the probability of capturing larval amphibians in baited and un-baited funnel traps. Further, we used count data (captures per trap) to examine whether glow stick-baited traps captured more amphibian larvae than un-baited traps. We captured four Ambystoma species (A. mabeei, A. opacum, A. talpoideum, and A. tigrinum) and tadpoles from the families Bufonidae, Ranidae, and Hylidae in light-baited funnel traps. Captures of both Ambystoma larvae and tadpoles were positively associated with light-baited traps, and we were 8.8 times more likely to capture Ambystoma larvae and 5.7 times more likely to capture tadpoles in glow stick-baited traps as compared to un-baited traps. Our results indicate that glow sticks can greatly improve capture success of larval amphibians in funnel traps, and we recommend their use as an active sampling method that is unbiased by surveyor experience and skill-level. Introduction Surveys for new populations of protected Ambystoma species, as well as effective monitoring of known populations and their breeding habitats, are critically important conservation needs (Bevelheimer et al. 2008, Curtis and Paton 2010, Dodd and Dorazio 2004, Pechmann et al. 1991). The fossorial habits of adult Ambystoma make both surveying and monitoring of adult populations difficult (Bevelheimer et al. 2008, Palis 1997a ). Standard ambystomatid survey and monitoring techniques for terrestrial habitats include visual searches, drift fences with pitfall traps, and cover-board arrays; however, these methods are only effective at capturing adults or newly metamorphosed salamanders during migrations (Palis 1997a, Shaffer et al. 1994, Skelly and Richardson 2010). Drift fences with pitfall traps can be costly to construct and require constant monitoring to avoid salamander mortality (Bishop et al. 2006, Palis 1997b). Cover-board arrays do not require constant monitoring, but are typically not as effective at capturing ambystomatids (Houze and Chandler 2002) as other methods. These techniques tend to be better suited for use in long-term monitoring 1South Carolina Department of Natural Resources, 100 Assembly Street, Columbia, SC 29201. 2Biological Sciences, 706 Coker Life Sciences, University of South Carolina, Columbia, SC 29208 *Corresponding author - 50 Southeastern Naturalist Vol. 11, No. 1 of plethodontid salamanders, but neither is well suited for large-scale, shortterm surveys to document Ambystoma distributions. The genus Ambystoma contains 32 species, nine of which occur in, with three limited to, the Southeastern United States: A. bishopi Goin (Reticulated Flatwoods Salamander), A. cingulatum Cope (Frosted Flatwoods Salamander), A. jeffersonianum Green (Jefferson Salamander), A. mabeei Bishop (Mabee’s Salamander), A. maculatumn Shaw (Spotted Salamander), A. opacum Gravenhorst (Marbled Salamander), A. talpoideum Holbrook (Mole Salamander), A. texanum Matthes (Smallmouth Salamander), and A. tigrinum Green (Tiger Salamander). Several of these species are afforded some federal and state protection, or a measure of conservation concern within or throughout their range (Table 1). All Ambystoma species occurring in the Southeast share certain life-history traits to varying degrees. Adults are typically fossorial, inhabiting uplands ranging from mesic mixed hardwood forests to xeric pine-dominated sandhills (Martof 1968, Means 1996, Palis 1996, Petranka 1998, Semlitsch and Wilbur 1988, Shoop 1960). Adult salamanders migrate to breeding sites from October through March, depending upon the species. Breeding sites include a variety of wetlands, ranging from ephemeral ponds to relatively permanent wetlands (Anderson and Williamson 1973, Anderson et al. 1971, Walls and Altig 1986). Typically, these salamanders select breeding sites that are fish-free wetlands, despite the range of wetland types (Gibbons and Semlitsch 1991, Hardy and Anderson 1970, Petranka 1998). Successful short-term surveys for species distributions can be accomplished when temporal and spatial patterns of ambystomatid life history are considered. Ambystomatid larvae are fully aquatic, spending several months—or longer for paedomorphic salamanders—in breeding ponds prior to metamorphosis. Thus, breeding ponds contain high relative abundances of these species, making this aquatic stage of their life cycle well suited for surveys that attempt to establish species presence. This consideration is important given that adults are largely Table 1. Federal and state protection status of Ambystoma species that occur in the southeastern Coastal Plain. States offering conservation status are indicated in parentheses. R = species is listed throughout its geographic range, T = threatened, E = endangered, NR = not ranked, NSR = Not ranked in southeastern United States, and SPC = species of special concern or in need of management. Protection status Species Federal State Source A. bishopi E(R) E (FL, GA) FFWCC 2010, Jenson and Stevenson 2009, USFWS 2009 A. cingulatum T(R) T (AL, FL, GA) FFWCC 2010, Jenson and Stevenson 2009, USFWS 2009 A. mabeei NR T (VA) Mitchell and Reay 1999 A. maculatum NR NSR A. opacum NR NSR Levell 1997 A. talpoideum NR SPC (NC, TN) Lannoo 2005 A. tigrinum NR E (DE, MD); SPC (NC, SC) Lannoo 2005 2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 51 fossorial and difficult to detect. Common aquatic sampling methods include dipnet sweep samples, pull-seine nets, and aquatic funnel traps (Adams et al.1998, Bishop et al. 2006, Ghioca and Smith 2007, Wilson and Pearman 2000). In recent years, considerable effort has been directed toward surveying and monitoring of the federally threatened Flatwoods Salamander (A. cingulatum) and the federally endangered Reticulated Flatwoods Salamander (A. bishopii). Aquatic sampling of Flatwoods Salamander breeding sites is an accepted method to document presence (Bishop et al. 2006, Palis 1996). Because Ambystoma larvae are nocturnal feeders and retreat to dense vegetation during the day, active diurnal surveys that use nets and seines may have limited success. This issue is further compounded when larvae occur at low densities (Bishop et al. 2006, Palis 1997a, Sekerak et al. 1996), requiring intensive dip-net sampling to maximize detection probability. Bevelheimer et al. (2008) determined that dip-net sampling was more effective at capturing Flatwoods Salamander larvae than un-baited aquatic funnel traps. When standardized by time, dip-net sampling was 5–10 times more effective than passive traps (Bevelheimer et al. 2008). Despite the relative success of the technique, intensive dip netting can disturb aquatic vegetation, substrate, and affect the water column through increased turbidity (S.H. Bennett, pers. observ.). Bishop et al. (2006) determined that sampling efforts for Flatwoods Salamander larvae required forty-five 1-m net sweeps for each larva captured. Thus, surveys using 0.5-m-wide nets could be expected to disturb approximately 28 m2 of pond habitat per larva collected. The actual amount and severity of disturbance depends on an individual’s sampling technique and the degree to which sampling is temporally replicated. At best, dip-net surveys will result in minimal damage along sampling transects, but more severe, localized damage may result from multiple sweeps in areas with dense vegetation. The negative effects of localized habitat damage are likely two-fold. First, damage to habitats used for foraging and cover could affect survival. Secondly, increased turbidity potentially affects detection probability, making it problematic to assess occupancy and abundance from monitoring data. Thus, it is important that biologists use effective sampling techniques that maximize capture probability while minimizing habitat damage due to surveys. Grayson and Roe (2007) suggested that photochemical glow sticks (light sticks) provide an effective bait for salamander larvae. These authors compared adult and larval amphibian capture success between glow-stick baited and unbaited aquatic funnel traps at one pond and determined that glow sticks were effective at increasing the capture rate of aquatic amphibians, primarily adult Notophthalmus viridescens Rafinesque (Eastern Newt) and larval Rana catesbeiana Shaw (Bullfrog). The objective of this study was to further examine the utility of glow-stick baited aquatic funnel traps for amphibian surveys, with particular emphasis on aquatic ambystomatid larvae. This information is important given the conservation needs of several ambystomatid salamanders. 52 Southeastern Naturalist Vol. 11, No. 1 Field-Site Description We sampled 12 isolated ponds in the South Carolina Coastal Plain in Jasper, Hampton, Bamberg, Orangeburg, and Dorchester counties. All ponds were isolated temporary wetlands, but encompassed a range of morphologies, origins, and past disturbances. Ponds ranged from approximately 0.1 to 18.6 ha and supported aquatic habitats ranging from open-canopied, grass-sedge dominated ponds to closed-canopied ponds characterized by Taxodium ascendens L. (Pond Cypress) and Nyssa sylvatica Marshall (Black Gum). Study ponds were surrounded by various terrestrial habitats, including pine plantations, pine-savanna woodlands, and agricultural areas (e.g., food plots maintained on wildlife management areas). Methods We used aquatic funnel traps (commercially available as minnow traps) with 0.48-cm2 white-plastic mesh to assess the effectiveness of glow-stick baiting for nocturnal sampling of amphibian larvae in ponds. Our sampling protocol was intended to 1) replicate expert-biased sampling used to survey ambystomatid larvae by focusing on aquatic habitats likely to be targeted by dip-net surveys, 2) provide for the safety of trapped animals and their habitats, and 3) ensure equivalent application of glow stick-baited and un-baited traps among pond habitats. Specifically, we deployed an even number of traps per pond (range = 10–24) to ensure a balanced sampling design, i.e., each pond was sampled with an equal number of baited and un-baited traps. Traps were deployed in a linear manner and placed at approximate 5-m intervals. We deployed more traps in larger ponds and in ponds with more diverse habitat structure (e.g., open water, grass-sedge, shrub-thicket). We alternately baited traps with 15.2-cm green glow sticks (premium glow stick; Windy City Novelties, Vernon Hills, IL), which ensured independence among baited and un-baited traps in each pond, i.e., baited traps were separated by approximately 10 m. We placed traps parallel to the pond edge in water depths ranging from 15–23 cm, which ensured that trap entrances were submerged while allowing a portion of the trap to protrude above the water surface. This trap position provided captured animals with access to air. Glow sticks were estimated to function for eight hours, but we commonly observed illumination twelve hours after activation. We deployed traps for one night at each pond between April and June 2010; all traps were deployed in the late afternoon and were retrieved the following morning. We recorded the number of amphibian larvae captured per trap and identified all Ambystoma larvae to species. We retained up to two individuals of each species from each pond for vouchers and released the remaining larvae at the point of capture. We used traps as sampling units and trap type (i.e., glow stick versus no glow stick) as the predictor variable in two separate analyses of capture data. First, we examined capture success using conditional logistic regression. We 2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 53 used a binary response (i.e., 1 = capture, 0 = no capture) to model the probability of capturing larval amphibians in baited and un-baited funnel traps. We stratified data by pond to account for lack of independence among observations from the same pond and to control for differences among ponds, e.g., species composition and amphibian diversity. Thus, conditional logistic regression allowed us to model capture probability while accounting for variation in larval amphibian assemblages. Secondly, we used negative binomial regression to analyze count data of captures per trap. This analysis was included in addition to logistic regression to examine whether glow stick-baited traps captured more amphibian larvae than un-baited traps. We used the generalized estimating equations (Liang and Zeger 1986) to analyze the pond-correlated data. In both analyses, we ran models separately for captures of tadpoles and Ambystoma larvae. All analyses were performed using SAS 9.2. Results Our trapping efforts yielded captures of four Ambystoma species: Mabee’s Salamander, Marbled Salamander, Mole Salamander, and Tiger Salamander. We captured Ambystoma larvae in 57 of 82 (69%) baited traps versus 29 of 82 (35%) un-baited traps. Similarly, we captured tadpoles, including species from the families Bufonidae, Ranidae, and Hylidae, in 72% of baited traps and 56% of un-baited traps. Conditional logistic regression models indicated that we were more likely to capture Ambystoma larvae (β = 1.09 ± 0.23, χ2 = 22.01, df = 1, P < 0.0001) and tadpoles (β = 0.88 ± 0.29, χ2 = 8.96, df = 1, P < 0.01) in traps that were baited with glow sticks than in un-baited traps. Odds ratios indicated that we were 8.83 times more likely to capture Ambystoma larvae and 5.78 times more likely to capture tadpoles in glow-stick baited traps as compared to un-baited traps. Negative binomial regression (df = 1, χ2 = 7.45, P < 0.01) coefficients indicated that counts of Ambystoma larvae (β = 0.83 ± 0.19, Z = 4.41, P < 0.0001) and tadpoles (β = 0.60 ± 0.18, Z = 3.39, P < 0.001) were positively associated with glow stick-baited traps. On average, we captured 2.35 (SD = 3.56, range = 0–17) Ambystoma larvae and 10.66 (SD = 18.14, range = 0–120) tadpoles in traps that were baited with glow sticks (n = 82) versus 1.02 (SD = 2.39, range = 0–14) and 5.85 (SD = 11.55, range = 0–58) in un-baited traps (n = 82), respectively (Fig. 1). Discussion Aquatic funnel traps are a non-destructive means of surveying amphibians in lentic habitats. A major advantage of funnel trapping is that capture success is unbiased by surveyor skill level and experience (Adams et al. 1998); however, funnel traps can provide unpredictable results that necessitate the use of many traps to effectively survey habitats, which incurs additional time and monetary costs (Adams et al. 1998). Because glow sticks improve funnel-trap sampling 54 Southeastern Naturalist Vol. 11, No. 1 efficiency (Grayson and Roe 2007, this study), their use likely offsets these disadvantages by reducing the number of traps required for surveys. This study demonstrated that glow sticks increase the probability that funnel traps capture tadpoles and Ambystoma larvae and increase the number of individuals that are captured per trap. Surveys for rare amphibians must rely on methods that enhance detection probability (MacKenzie et al. 2005). Recent comparisons (e.g., Bevelheimer et al. 2008) between dip-net and un-baited funnel-trap surveys demonstrated that dip netting was up to ten times more effective than funnel traps at capturing ambystomatid salamander larvae, particularly those of the federally protected Flatwoods Salamander. We did not incorporate dip-net surveys in this study, but we were nearly nine times more likely to capture Ambystoma larvae in glow stick-baited traps than in un-baited traps. Our findings suggest that the increased capture efficiency associated with the addition of glow sticks potentially balances comparisons of the two techniques, or at least places funnel-trap sampling within the range of effectiveness of dip-net sampling. Thus, we suspect that surveys for rare Ambystoma species will benefit from including glow stick-baited funnel traps in sampling protocols. Figure 1. Average counts of tadpoles and ambystomatid salamander larvae captured per trap, i.e., unbaited (n = 82) and glow-stick-baited funnel traps (n = 82). Data were analyzed using negative binomial regression.Traps were deployed in 12 upland isolated wetlands in the South Carolina Coastal Plain, 2010. 2012 S.H. Bennett, J.L. Waldron, and S.M. Welch 55 Our ambystomatid capture success likely reflected larval activity patterns. Most ambystomatid larvae actively feed at night and retreat to protective cover (e.g., vegetation and leaf litter) to feed during the day (Petranka and Petranka 1980). Although un-baited funnel traps passively capture larvae, glow-stick baited funnel traps appear to actively sample nocturnal foragers that are attracted to light. Dip-net surveys are typically used to conduct diurnal searches for larvae that are hiding in cover, requiring multiple sweeps through vegetation clumps to dislodge secretive species from their cover. To date, little attention has been given to the effects of dip-net surveys on pond microhabitat, even though extensive dip netting visibly disrupts pond microhabitats via increased turbidity and altered vegetation structure. Although the degree to which this activity affects pond microhabitat has not been described, suitable breeding ponds for rare species are lacking, and remaining habitats are vulnerable to alterations (Bennett and Nelson 1991, Semlitsch 2003). Thus, we recommend using glow-stick baited funnel traps in lieu of dip-net sampling for amphibian monitoring to reduce damage to wetland habitats, particularly when surveying for rare species. Potential biases associated with glow-stick baiting may affect the results of trapping efforts in amphibian surveys. For example, larval fish studies have reported that light traps target individuals based on size (Marchetti et al. 2004), age (Hernandez and Lindquist 1999), and taxonomic group (Marchetti and Moyle 2000). Further, issues related to pond turbidity, which potentially affects the ability of larval amphibians to see light, could make it difficult to standardize capture data (Marchetti et al. 2004). Turbidity is influenced by factors, such as season and water depth, that make it difficult to assess the volume of water that is sampled by light traps (Marchetti et al. 2004). These issues should only be a concern when density and abundance estimates are required, and should not affect efforts to assess presence. Additional research would prove valuable in identifying biases and quantifying their effects, providing greater accuracy in density and relative abundance estimates for amphibians, Glow sticks can greatly improve capture success of larval amphibians in funnel traps, at least within the environmental contexts studied here. Although the behavioral mechanisms are unclear, positive phototaxis functionally changes funnel traps from passive to active amphibian samplers. Because glow stick-baited funnel traps have many advantages and are less likely to suffer from surveyor skill-level biases, we recommend their use as an active sampling method. A costbenefi t comparison of dip-net sampling versus glow stick-baited funnel trapping is required to develop fully informed sampling protocols that maximize capture success while minimizing habitat destruction. 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