NORTHWESTERN NATURALIST 88:147–154 WINTER 2007 A COMPARISON OF GROUND-BASED AND TREE-BASED POLYVINYL CHLORIDE PIPE REFUGIA FOR CAPTURING PSEUDACRIS REGILLA IN NORTHWESTERN CALIFORNIA CALE HMYERS,LISA EIGNER,JAKE AHARRIS,ROBERT HILMAN,MATTHEW DJOHNSON, RYAN KALINOWSKI,JORDAN JMUIR,MELISSA REYES, AND LESLIE ETUCCI Humboldt State University, Department of Wildlife, Arcata, California, USA 95521 ABSTRACT—Creating artificial refugia through the use of polyvinyl chloride (PVC) pipes to capture hylid treefrogs has been examined in the Eastern United States and Caribbean, but has not been evaluated in the Pacific Northwest. We compared the effectiveness of ground-based and tree-based PVC pipes for capturing the Pacific Treefrog (Pseudacris regilla) in northwestern California. We recorded a total of 464 P. regilla captures. Probability of use increased asymp- totically over our 28-d sampling period and was higher (6–20%) for tree-based than ground- based pipe refugia. Tree-based pipe refugia caught 81% more P.regilla than ground-based pipe refugia but latency to first detection did not vary significantly. Probability of use was higher for pipe refugia that had been used in a previous study than for new pipes, but age did not influence capture rate or latency to detection. Our results support the hypothesis that tree-based pipe refugia are more effective than ground-based pipes for capturing P. regilla, as well as the hy- pothesis that P. regilla actively discriminate among refugia. Using tree-based PVC pipe refugia to monitor P.regilla may be more effective than current techniques for addressing many research questions. Key words: Pacific Treefrog, Hyla regilla, Pseudacris regilla, polyvinyl chloride, pipe refugia, northwestern California Traditional amphibian sampling techniques, accurately monitor P. regilla populations, spe- such as the use of pitfall traps, drift fences, and cific and efficient capture techniques are need- visual encounter surveys are ineffective for ad- ed (Johnson 2005). dressing many research questions for hylid Constructing artificial refugia by means of treefrogs due to their ability to evade pitfall polyvinyl chloride (PVC) pipes has been estab- traps and drift fences, and observer biases as- lished as an effective monitoring technique for sociated with visual encounter surveys (Dodd hylids in the eastern United States and Carib- 1991; Moulton and others 1996; Lamb and oth- bean (McComb and Noble 1981; Stewart and ers 1998; Boughton and others 2000). Despite Pough 1983; Meshaka 1996; Moulton and oth- these findings, traditional techniques are still ers 1996; Boughton and others 2000), but to our employed to assess trends of the Pacific Tree- knowledge there have been no studies investi- frog (Pseudacris regilla, also referred to as Hyla gating PVC pipe refugium design for captur- regilla; family Hylidae), the most abundant am- ing hylids in the Pacific Northwest or for P. r e - phibian in western North America (Matthews gilla in particular. Our objective was to monitor and others 2001). and assess how ground-based and tree-based Urbanization, habitat destruction, and the in- positioning of PVC pipe refugia influence cap- troduction of exotic trout have reduced abun- ture success for P. regilla. dances and distributions of P. regilla in many Previous studies examining pipe refugia for parts of its range (Bull 2002; Pearl and others other hylids have demonstrated that pipe de- 2003; Knapp 2005; Pearl and others 2005; Reid sign and positioning can influence capture suc- 2005; Riley and others 2005). In addition, P. r e - cess (Townsend 1989; Moulton and others 1996; gilla populations show great natural fluctua- Boughton and others 2000; Zacharow and oth- tion, making it difficult to assess trends of the ers 2003; Bartareau 2004). Pacific Treefrogs species (Matthews and others 2001). In order to seek refuge beneath ground cover and within 147 148 NORTHWESTERN NATURALIST 88(3) trees (Nussbaum and others 1983), but it is un- were white, schedule 40-weight, 60 cm in known if tree-based PVC pipe refugia are more length, 5.08 cm in diameter, open on the top effective than ground-based pipe refugia for and capped on the bottom. To ensure a moist, capturing the species. Hylids have shown a humid environment within the confines of the preference for elevated retreat sites in other pipe, we drilled a drain-hole 15 cm from the studies examining pipe refugium design bottom of the pipe and filled it to the hole with (Townsend 1989; Greenberg and others 1996; water. To hang pipe refugia on screws, we Boughton and others 2000), and we hypothe- drilled another hole approximately 10 cm from sized that tree-based PVC pipe refugia would the top of the pipe. We mounted tree-based be more effective than ground-based PVC pipe pipe refugia vertically on tree trunks at a refugia for capturing P. regilla. We evaluated height of 2.5 m above the ground while ground- this hypothesis by testing the following predic- based pipe refugia were mounted vertically on tions: relative to ground-based pipe refugia, wooden stakes (1.27 ϫ 5.08 ϫ 122 cm) so that tree-based pipe refugia should have (1) a high- the bases of the pipes were resting on the er probability of use, (2) a higher capture rate, ground. All pipe refugia faced north to control and (3) a shorter latency to detection for P. r e - for potential differences stemming from ori- gilla. entation. We positioned tree-based pipe refugia on alder trees at approximately 5-m intervals, METHODS while ground-based pipe refugia were also po- We conducted our study on the Humboldt sitioned at 5-m intervals, 1 m from each tree- Bay National Wildlife Refuge, located on the based pipe refugium and directly in line with coast of northwestern California, approximate- the transect. We obtained 100 PVC pipes from ly 150 km south of the Oregon border. The ref- a previous study (summer 2006) and purchased uge has a variety of habitats including marshes, 100 additional pipes for our study (hereafter, seasonally flooded wetlands, riparian wet- ‘‘old’’ and ‘‘new’’ pipe refugia). We distributed lands, streams, and forests (Kwansy 2000), and old and new pipe refugia evenly among tree P. regilla are known to inhabit these types of and ground sites. During the course of our habitats at elevations ranging from sea level to study, we noticed that 10 tree-based pipe re- 2440 m above sea level (Corkran and Thoms fugia were placed onto trees used in this pre- 1996). Our study site consisted of a forested ri- vious study. To reduce possible biases associ- parian area with a flowing water source (Salm- ated with these locations, we removed these on Creek). Red Alder (Alnus rubra) and willows pipe refugia from our analyses. (Salix spp.) dominated the canopy layer while Pipe refugia were deployed on 30 September Salmonberry (Rubus spectabilis), Twinberry 2006 and we inspected pipe refugia on every (Lonicera involucrata), and California Blackberry (Rubus ursinus) comprised the under story veg- 3rd day from 2–29 October 2006, resulting in a etation. Pseudacris regilla have been observed in total of 10 surveys. To avoid predation and des- the area year-round, as the area supports a iccation, hylids seek out retreats during the day large amount of invertebrate prey (Verhey (Boughton and others 2000); therefore, we con- 1992) and wet areas for breeding (Stebbins ducted surveys between 09:00–16:00 PST. With 2003). The climate throughout the region is rel- the aid of flashlights, survey teams (consisting atively cool and moist, with annual rainfall av- of 2 field technicians at each transect) tallied the eraging 99 cm and mean annual temperatures number of frogs in each pipe. Due to budget ranging from 9.5 to 15Њ C (NOAA 2006). constraints, we were unable to mark any P. r e - gilla in this study. We designed our methods to Sampling Scheme be minimally intrusive so frogs were not sexed, We utilized a systematic sampling scheme aged, weighed, or removed from pipe refugia consisting of 2 separate 250-m transects. Each at any time during this study. We removed de- transect consisted of 50 ground-based and 50 bris only if it interfered with our ability to ac- tree-based pipe refugia. Pipe refugia were de- curately observe or count the number of frogs signed following previously successful proto- in each pipe refugium. We refilled each pipe to cols for other species of treefrogs (Boughton the 15 cm mark with water during each survey and others 2000; Staiger and others 2002): pipes when necessary. WINTER 2007 MYERS AND OTHERS: PVC PIPE REFUGIA 149 Data Analyses tory. We considered pipe refugium positioning (tree or ground) and pipe refugium age (old or We measured 3 response variables to deter- new) as factors potentially influencing␺ˆ and pˆ. mine if the positioning (ground vs. tree) and In addition, we modeled pˆ as a function of time age (old vs. new) of pipe refugia influenced (survey number 1–10) by examining a linear their effectiveness: probability of use, capture trend in pˆ over survey number, an asymptotic rate, and latency to initial detection (LTD; increase in pˆ over survey number, a unique val- Foresman and Pearson 1998). Our unit of rep- ue for pˆ at each survey, or a unique value for pˆ lication was a pipe (n ϭ 190). Because capture for surveys 1–5 and 6–10. We used Akaike’s In- rates can be difficult to interpret if the inde- formation Criterion (AIC) for model selection pendence of captures is unknown, probability and considered models with ⌬AIC values of Ͻ3 of use and LTD are useful in comparing pipe to be competitive models (Burnham and An- refugium efficacy (Gompper and others 2006). derson 2002).