Factors affecting hylid treefrog use of PVC refugia in north central Florida

Jennifer S. Staiger¹ and Robin G. Boughton<sup>2

 1</sup> Department of Wildlife Ecology and Conservation
University of Florida, PO Box 110430, Gainesville, FL 32611
² Department of Physiological Sciences, College of Veterinary Medicine
University of Florida, PO Box 100144, Gainesville, FL 32611

Presented at the 1999 Joint Meeting of the American Society of Ichthyologists and Herpetologists, American Elasmobranch Society, Herpetologists' League, and Society for the Study of Amphibians and Reptiles, Penn State University, State College, Pennsylvania.

Abstract

Traditional herpetofaunal sampling techniques may overlook arboreal anuran species, particularly during the non-breeding season. The use of artificial refugia is helpful in sampling hylid frogs during non-breeding periods and away from breeding sites.  Artificial refugia constructed of PVC tube sections approximately 60 cm long were placed on hardwood trees in a xeric oak hammock/longleaf pine stand adjacent to an ephemeral pond on the Katharine Ordway Preserve–Carl Swisher Memorial Sanctuary, Putnam County, Florida. At each cardinal point on a tree, one of four tube types was hung vertically.  The tubes were checked regularly for one year.  Four frog species utilized the refugia, Hyla squirella, H. cinerea, H. femoralis, and H. gratiosa.  The bottom capped tubes holding a few centimeters of water captured the highest number of frogs.  Tubes on the north side were more effective than those on other sides.  Frogs utilized the refugia throughout the year, with captures generally increasing over time.  One advantage of this sampling method is the lack of trap mortality, allowing the tubes to be left in the field for extended periods.

Introduction

Because they can easily trespass over drift fences, climb out of pitfall traps, or escape detection in dense vegetation, arboreal anurans are often underrepresented in studies utilizing traditional herpetofaunal sampling techniques (Dodd 1991, Greenberg et al. 1994, Moulton et al. 1996).  Most field studies of wetland breeding amphibians have been focused at breeding sites during breeding seasons (see Duellman and Trueb, 1985).  Consequently, little is known of these species away from these sites or during the non-reproductive season.

Based on anecdotal reports of hylids using artificial refugia and studies employing nest boxes or pipes in the ground to specifically sample treefrogs (e.g. McComb and Noble 1981, O'Neill 1995) a novel method for sampling hylid frogs has been developed (Boughton 1997).  Boughton (1997) demonstrated the effectiveness of using polyvinyl chloride (PVC) tubes hung in trees as artificial refugia for arboreal frogs. That study considered the effects of factors such as tube length, height of tube in tree, type of tree, and surrounding microhabitat on frog usage. The study presented here was designed to supplement those results by determining if  tube type and orientation affects usage.

Materials and Methods

The Katharine Ordway Preserve-Carl Swisher Memorial Sanctuary is 3,750 hectare property owned and managed by The Nature Conservancy and the University of Florida Foundation.  Located in Putnam County, Florida, it includes oak hammocks, sandhills, permanent and ephemeral wetlands, and ruderal sites. The study site is a mixed xeric oak hammock and long leaf pine stand adjacent to an ephemeral sinkhole pond (Porter's Hole). Tree species at the site include long leaf pine (Pinus palustris), laurel oak (Quercus hemispherica), water oak (Q. nigra), sand live oak (Q. geminata), sweetgum (Liquidambar styraciflua), black cherry (Prunus serotina), and tupelo (Nyssa biflora).

Artificial refugia were constructed of 60 cm lengths of white schedule 20 PVC tubing.  Four tube designs were used: 1) open at both ends with an inside diameter of 4.3 cm (open, Fig. 1), 2) capped on the top with a 4.3 cm inside diameter (top, Fig. 2), 3) capped on the bottom, 4.3 cm inside diameter, and holding a few centimeters of water (bottom, Fig. 3), and 4) open at both ends with an inside diameter of 1.75 cm (narrow, Fig. 4).  The bottom tubes included a small hole 15 cm from the bottom to allow excess water to drain.

The tubes were hung vertically on nails placed 2 m high on hardwood tree trunks. One of each tube type was hung in each of 25 trees. The tubes were placed on the north, east, south, and west sides of the trunks, with tube order alternating among trees. All trees used were greater than 18 cm DBH and within 55 m of the pond. 

Tubes were checked twice weekly (on non-consecutive days) from October 1995 through September 1996.  Several night surveys were conducted to determine nocturnal tube usage. Frogs were removed from tubes by gently shaking the tube while holding a plastic bag over one end (Fig. 5).  Captured frogs were marked with a toe clip pattern to distinguish recaptures.  Weight and snout-urostyle length were measured and sex was determined when possible.  Frogs were then returned to the tube in which they were captured (Fig. 6).

Results and Discussion

A total of 1760 captures of four frog species were made over the course of this study. Hyla squirella accounted for the most captures (1567, or 89%), followed by H. cinerea (171, 9.7%), H. gratiosa (12, 0.7%), and H. femoralis (10, 0.6%). A fifth hylid species, Pseudacris crucifer, was heard calling at the site but was never captured in the tubes. This may be due to its habit of staying high in the canopy in the non-breeding season.  No frogs were found in tubes during night surveys.

Tube type significantly affected the number of frogs captured (Tables 1 and 2).  The bottom tubes were preferred by H. squirella and H. cinerea (too few H. femoralis and H. gratiosa were captured to determine preference), and accounted for 74.3% of all captures. Open tubes were used by 9.1% of all captured frogs, followed by top tubes at 10.5%, and narrow tubes at 6.1%. The water in the bottom tubes likely accounts for the effect by providing an attractive diurnal retreat.

While tubes on the north side did account for more captures, the effect of tube orientation is not significant. North tubes accounted for 27% of all captures, followed by 25% for south tubes, and 24% each for east and west. The interaction of tube type and orientation is slightly significant when considering all species and H. squirella alone (Tables 1 and 2).  Open and top tubes captured more frogs at north than at other points.

The number of frogs captured increased initially, then dropped at the beginning of winter (Fig. 11).  This could be due to the late fall placement of the tubes.  Individuals may be less likely to search for new retreat sites during extreme temperatures.  The captures increased over spring and declined again during the early summer. This decline could be due to individuals leaving the trees to breed, to the creation of other attractive retreat sites by the summer rains, to higher temperatures in the tubes, or to some combination thereof.

Hylid frogs utilized the PVC refugia throughout the year, including the winter non-breeding season, because the tubes provided suitable diurnal retreats.  Tube design and placement are important factors affecting captures.  By determining other factors affecting tube usage, the number of captures may be maximized and particular species could be targeted.  The method described here could be valuable in studying arboreal frogs outside the breeding season and away from breeding sites. Other advantages are its low cost and the lack of trap mortality.

Literature Cited

Boughton, R.G. 1997.  The use of PVC refugia as a trapping technique for hylid treefrogs. M.S. Thesis, University of Florida, Gainesville.

Dodd, C.K. Jr. 1991.  Drift fence associated sampling bias of amphibians at a Florida sandhill temporary pond. J. Herpetol. 25:296-301.

Duellman, W.E. and L. Trueb. 1985.  Biology of Amphibians.  McGraw-Hill, New York.

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.  J. Herpetol. 28:319-324.

McComb, W.C. and R.E. Noble. 1981.  Herpetofaunal use of natural tree cavities and nest boxes. Wildl. Soc. Bull. 9:261-267.

Moulton, C.A., W.J. Fleming, and B.R. Nerney.  1996. The use of PVC pipes to capture hylid frogs. Herpetol. Rev. 27:186-187.

O'Neill, E.D.  1995. Amphibian and reptile communities of temporary ponds in a managed pine flatwoods. M.S. Thesis, University of Florida, Gainesville.

Acknowledgements

We would like to thank Richard Franz for his guidance and enthusiasm, C. Kenneth Dodd, Jr. and Melvin Sunquist for their support, and John F. Eisenberg and the board of the Katharine Ordway Preserve - Carl Swisher Memorial Sanctuary for allowing us to conduct this study. We would also like to thank the Wildlife Field Techniques students for their field assistance and William J. Barichivich for his invaluable advice and support.