Automated Acoustic Sampling

FISC - Biology

Results of automated acoustic sampling of anurans
at Okefenokee National Wildlife Refuge

Jennifer S. Staiger, William J. Barichivich, Lora L. Smith¹,
Steve A. Johnson, and C. Kenneth Dodd, Jr.

U.S.G.S. Florida Integrated Science Center, 7920 N.W. 71^st St., Gainesville, FL 32653
¹Joseph W. Jones Ecological Research Center, Rt. 2, Box 2324, Newton GA 31770

Poster presentation. 2004 Joint Meeting of the Herpetologists' League, American Elasmobranch
Society, Society for the Study of Amphibians and Reptiles, and American Society of Ichthyologists
and Herpetologists, Norman, Oklahoma.

Abstract: As part of the Department of Interior's initiative to survey amphibian populations on federal lands, we established a project at the Okefenokee National Wildlife Refuge, Georgia. One of the many sampling techniques we used was automated frog call data loggers. We sampled 16 survey sites, representing six distinct habitat types, for four consecutive days during quarterly periods from March 2001 to January 2002. The call data loggers recorded one minute every hour from approximately 18:00 to 06:00. The loggers detected 10 anuran species, including one (Carpenter Frog, Rana virgatipes) not detected with other methods. Southern Cricket Frogs (Acris gryllus), Pig Frogs (R. grylio), Southern Leopard Frogs (R. sphenocephala), and Carpenter Frogs were detected most frequently, whereas Southern Toads (Bufo terrestris) and Squirrel Treefrogs (Hyla squirella) were detected least often. The greatest frequency of calling occurred during the spring and summer quarters, with more species recorded at the wet prairie sites than other habitats. Calling intensity varied considerably among species during the 12-hour recording cycles. Application of these data to anuran inventory and monitoring projects is discussed.

Introduction

To address concerns about amphibian population declines, the United States Department of the Interior (DOI) instituted the national Amphibian Research and Monitoring Initiative (ARMI). The goal of the initiative is to promote the inventory and long-term monitoring of the status and trends of amphibian populations on DOI lands. The Okefenokee National Wildlife Refuge (ONWR) was one of several sites in the southeastern United States where research and monitoring efforts have been focused. The research initiative at ONWR included a baseline inventory to determine presence and distributions of amphibians, as well as sampling to determine species richness, habitat associations, and proportion of area occupied (PAO).

A variety of amphibian sampling techniques were used at intensive monitoring sites in ONWR, including automated acoustic sampling of anurans. Automated sampling allows for data collection at several sites concurrently and over extended time periods, thus increasing the chance of detecting rare species (Barichivich 2003, Peterson and Dorcas 1994). Also, the use of automated recorders decreases observer disturbance to calling anurans and provides a sampling record that can be reviewed and archived (Bridges and Dorcas 2000). The data obtained with this method in ONWR are used to evaluate its utility and applicability to anuran inventory and monitoring projects.

Methods

Description of the study site: The Okefenokee Swamp is a 200,000 ha freshwater wetland located in southeast Georgia and extreme northeast Florida. The ONWR encompasses approximately 158,000 ha of the swamp, with 98% of the refuge designated as a National Wilderness Area. The refuge is known for its cypress (Taxodium spp.) dominated wetlands, but also contains dense shrub thickets, extensive wet prairies, lakes, and surrounding pine flatwoods. The area contains significant amphibian biodiversity (21 anurans and 17 salamanders, including 5 species of concern) and the region represents the southern limit of the range of several species.

Sixteen intensive monitoring sites, stratified across the major vegetation communities within ONWR, were sampled quarterly from March 2001 to January 2002. Because of limited access to some sites during certain times of year, not every site was sampled during each quarter. The monitoring sites represent six habitat types: forested wetland (FW1-4) (Fig. 1), lake (L1-2), pine flatwoods (PFW1-2) (Fig. 2), ephemeral pond (P1-2), shrub wetland (ShW1-2), and wet prairie (Pr1-4) (Fig. 3).

At each site an automated frog call data logger (ACDL) was deployed for an average of four consecutive days. For a detailed description of the ACDL used, see Barichivich 2003 (Fig. 4). The loggers were set to record one minute every hour from approximately 18:00 to 06:00.

In the laboratory, two reviewers independently listened to each tape using a stereo cassette player and headphones. Species heard calling and call ranks (after the North American Amphibian Monitoring Program calling index) were recorded for each interval. Any discrepancies between the two listeners in the results were resolved by a third listener.

Results and Discussion

The ACDLs recorded a total of 2872 minutes, of which 634 minutes (22%) contained vocalizations of at least one anuran species. Two or more species were calling during the majority of intervals containing vocalizations, with two, three, or four species recorded in 54% of intervals with vocalizations. No more than four species were recorded in the same interval.

Ten anuran species were detected using ACDLs. Southern Cricket Frogs (Acris gryllus ) were recorded calling in 75% of intervals containing vocalizations. Pig Frogs (Rana grylio , Fig. 5) followed, with vocalizations detected in 56% of intervals with calling activity, Southern Leopard Frogs (R. sphenocephala , Fig. 6) in 14%, and Carpenter Frogs (R. virgatipes , Fig. 7) in 13% of intervals with calls. Pine Woods Treefrog (H. femoralis) and Little Grass Frog (Pseudacris ocularis) were detected in 9% and 6% of intervals, respectively. Four species were recorded in less than 5% of intervals containing vocalizations: Bronze Frog (R. clamitans , 4%), Green Treefrog (Hyla cinerea, 3%), Southern Toad (Bufo terrestris, 1%), and Squirrel Treefrog (H. squirella , 0.4%).

One species, Carpenter Frog, was detected with certainty at ONWR only with this method. However, two species detected with other sampling techniques at intensive sites were not recorded using ACDLs. River Frog (R. heckscheri) larvae were captured in aquatic funnel traps at some sites and Oak Toads (B. quercicus) were seen and heard calling during the day at several sites, but their vocalizations were not detected on ACDL tapes. It should be noted that malfunctions did occur, and it is possible some of these species may have been present and calling at sites during periods for which we have no usable tapes.

Anuran calling activity in ONWR varied among seasons, habitats, and species. Call activity was measured as the number of intervals in which a species was recorded calling during a sampling period. The greatest frequency of calling was recorded during the spring and summer sampling periods (Fig. 8), with more species recorded at the wet prairie sites than in other habitats (Fig. 9). Cricket Frog and Pig Frog calling activity was influenced by season and habitat (Two-way ANOVA; season: p=.002, .000; habitat: p=.000, .000). Cricket Frogs called more often in spring and summer, and their calling activity was higher at wet prairie sites than other habitats (Tukey's HSD, significant at the .05 level). Pig Frogs called more during spring than summer, and more in summer than in fall and winter (significant at the .05 level). Wet prairie and lake sites produced significantly more Pig Frog vocalizations than other habitats (significant at the .05 level). Southern Leopard Frog and Carpenter Frog calling activity did not vary significantly among habitats or among seasons.

Within the 12-hour recording cycles there was interspecific temporal variation in calling activity. In the spring samples, Cricket Frogs called throughout the evening and early morning, peaking between 21:00 and midnight, whereas Pig Frogs called more often during the very early morning (between midnight and 03:00 am) (Fig. 10). Southern Leopard Frog and Carpenter Frog call activity increased through the evening but was highest in the morning hours, peaking after 03:00 am (Fig. 10). In the summer samples, Cricket Frog and Pig Frog calling followed that same pattern, with peak activity occurring after 03:00 am (Fig. 11). Southern Leopard Frog and Carpenter Frog activity was negligible in the summer samples, with each species recorded in only six intervals during that period (Fig. 11).


		Fig. 1. Forested wetland (dry).

		Fig. 2 Pine flatwoods.

		Fig. 3. Wet prairie.

					Fig. 4. Automated frog call data logger.

				Fig. 5. Rana grylio.

				Fig. 6. Rana sphenocephala.

			Fig. 7. Rana virgatipes.

	Fig. 8. Calling activity for each anuran species by season. Spring = Apr, May 01; Summer = Jun, Jul 01; Fall = Sep, Oct 01; Winter = Dec 01, Jan 02.

	Fig. 9. Calling activity for the most frequently recorded species by habitat type.

Fig. 10. Nightly calling patterns for the most frequently recorded species, Spring 01. Early PM = 18:00-20:59, Late PM = 21:00-23:59, Early AM = 0:00-2:59, Late AM = 3:00-6:00.

Fig. 11. Nightly calling patterns for the most frequently recorded species, Summer 01. Early PM = 18:00-20:59, Late PM = 21:00-23:59, Early AM = 0:00-2:59, Late AM = 3:00-6:00.

The data obtained in ONWR using ACDLs is useful for determining fine scale and seasonal temporal variation in anuran calling activity, as well as variation in calling activity across habitats. Information of this kind can be used by researchers to maximize sampling effort, target rare species, avoid temporal biases, and minimize observer disturbance when conducting population monitoring projects (Barichivich 2003, Bridges and Dorcas 2000). In anuran inventory projects ACDLs are most effective when deployed across all possible habitats and during different seasons. However, it must be noted the data obtained from this method cannot be used to assess the status of females in a population or to determine breeding success (Barichivich 2003, Rand and Drewry 1994).

Literature Cited

Barichivich, W.J. 2003. Guidelines for building and operating remote field recorders (automated frog call data loggers), Appendix IV. In: Monitoring Amphibians in Great Smoky Mountains National Park. C. K. Dodd, Jr. U.S. Geological Survey Circular 1258.

Bridges, A.S., and M.E. Dorcas. 2000. Temporal variation in anuran calling behavior: implications for surveys and monitoring programs. Copeia 2000(2):587-592.

Peterson, C.R., and M.E. Dorcas. 1994. Automated data acquisition, p. 47-57. In: Measuring and monitoring biological diversity: standard methods for amphibians. W.R. Heyer, M.A. Donnelly, R.W. McDiarmid, L.C. Hayek, and M.S. Foster (eds.). Smithsonian Institution Press, Washington, DC.

Rand, A.S., and G.E. Drewry. 1994. Acoustic monitoring at fixed sites, p. 150-152. In: Measuring and monitoring biological diversity: standard methods for amphibians. W.R. Heyer, M.A. Donnelly, R.W. McDiarmid, L.C. Hayek, and M.S. Foster (eds.). Smithsonian Institution Press, Washington, DC.

Acknowledgments

We thank the rest of the Okefenokee field crew: K.G. Smith, K. Sorensen, M.J. Zacharow, A.K. Owens, G.L. Hill, and R.J. Lewis. A special thanks to Kevin G. Smith for statistical advice. We also thank A.D. Hester and K.L. Keefe for their fieldwork and the refuge personnel for permission to work there.

For more information: jennifer_staiger@usgs.gov.

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