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Southeast Ecological Science Center

Amphibian Research And Monitoring
At The Okefenokee National Wildlife Refuge

 Lora L. Smith and Kevin G. Smith

U.S. Geological Survey, Florida Integrated Science Center
7920 NW 71st Street, Gainesville, Florida, 32653, USA


In response to concerns about amphibian population declines, the Department of Interior (DOI) received funding from Congress to institute a national Amphibian Research and Monitoring Initiative (ARMI). The goal of the initiative is to promote long-term surveys on the status and trends of amphibian populations on DOI lands.  The Okefenokee National Wildlife Refuge (ONWR) is one of five primary index sites in the southeastern United States where amphibian research and monitoring efforts will be focused. The research initiative at ONWR began in August 2000 and includes a baseline inventory to determine which species are present on the Refuge and how they are distributed among available habitats.  The initiative also includes sampling at permanent monitoring sites within the Refuge. The sampling protocol at these sites includes the use of traditional methods of estimating abundance such as capture-recapture as well as the evaluation of new methods for monitoring amphibian populations such as quadrat-based species richness sampling and estimations of proportion of area occupied.  The latter methods require the use of a variety of sampling techniques to determine species richness, as well as the development of estimates of detectability for each individual species.  Since the project is still in its infancy, the data reported here are preliminary.

Description of the study Site. The Okefenokee Swamp is a 200,000 ha freshwater wetland located in southeast Georgia and extreme northeast Florida.  The National Wildlife Refuge encompasses approximately 158,000 ha of the swamp and 98% of the Refuge is a designated National Wilderness Area. The swamp is famous for its cypress (Taxodium spp.) dominated wetlands, but also contains dense shrub thickets, extensive wet prairies, lakes, and is surrounded by pine flatwoods. The first investigations of the amphibians of the Okefenokee Swamp were undertaken in the early 1900s by A.H. Wright and students from Cornell University (Wright, 1932). In the 1960s and 1970s researchers from the University of Georgia made extensive collections of amphibians from the region (Laerm et al., 1980). The swamp and surrounding area contain 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. Human activity in the swamp has historically included logging, dredging, peat mining, and water impoundment, all of which might be expected to have had an influence on amphibian populations. Fires (both natural and anthropogenic) also have played a large role in shaping the character of the swamp.

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Southern toad - Bufo terrestris - click to enlarge Southern toad eggs - click to enlarge

Female southern toad, Bufo terrestris, and southern toad egg strands in ONWR.

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The research initiative at ONWR includes monthly sampling to determine species presence, distribution, and habitat association and quarterly intensive sampling at permanent monitoring sites. During monthly sampling, five to ten randomly chosen sites (called extensive survey sites) are sampled using a variety of techniques to determine species presence.  Sixteen permanent monitoring sites, stratified across the major vegetation communities within the Refuge, are sampled four times per year. At extensive survey sites, sampling methods include the use of automated recording systems, time- and area-constrained searches and trapping (funnel traps, minnow traps, crayfish traps). At permanent monitoring sites, additional methods, including PVC pipes and coverboards, also are used to detect species. Each of the individual sampling methods is described below.

Automated Recording Systems.  Automated recording systems (frog loggers) are being used to monitor anuran breeding activity (Peterson and Dorcas, 1994) (Figs. 1, 2).  The recorders are programmed to record for 1-minute per hour for a 14-hour period (from 1800 hrs – 0700 hrs EST) and loggers are set for 2-5 consecutive nights. The recordings are used to identify which species were calling and calls are assigned a North American Amphibian Monitoring Program (NAAMP) calling intensity code.  Phenological data will be used to describe environmental and temporal patterns in anuran breeding activity at wetlands on the Refuge (Fig. 3).

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Figure 1.  The general design of a frog logger - click to enlarge Figure 2.  A frog logger, as deployed in the field - click to enlarge
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Figure 1 (left). The general design of a frog logger, similar to those used in this project.  Modifications and improvements to this design continue to be made, allowing for longer periods of data acquisition and a higher quality of recording.

Figure 2 (right).  A frog logger, as deployed in the field.  The electronic components reside in a weatherproof ammunition can which also provides for convenient storage and transportation within a single unit. A plastic 2-liter bottle serves the dual purpose of protecting the microphone from moisture and amplifying rain sounds, making them audible to the listener.

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Figure 3.  Phenogram detailing frog vocalization activity - click to enlarge
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Figure 3.  Phenogram detailing frog vocalization activity for 5 species during one night of monitoring (12 June 2001 to 13 June 2001). These data were acquired through the use of a frog logger placed at a forested wetland in ONWR.  Symbols represent different NAAMP calling intensities: box1 = Code 1, box2 = Code 2, box2a = Code 3.  Species codes are:  Hfe = Hyla femoralis, Agr = Acris gryllus, Rgr = Rana grylio, Poc = Pseudacris ocularis, Bte = Bufo terrestris.

Time and Area-constrained Searches.  In terrestrial habitats and wetlands with little or no standing water, leaf litter, logs, and other debris are turned in search of ground-dwelling amphibians.  In aquatic habitats where standing water is present, dipnets are used to survey the site.  These searches are either area constrained (confined to a 50 m by 50 m area) or time-constrained (person-hours). The number of individuals and life stage of each species observed are recorded, as well as the number of person-hours invested at each site.

Trapping. A variety of trapping methods are being used in both aquatic and terrestrial habitats on the Refuge.  Minnow traps and modified crayfish traps are used to detect larval amphibians and aquatic salamanders (Fig. 4).  Screen funnel traps are placed at ecotones and along natural barriers within dry and inundated terrestrial sites to catch adult amphibians (Fig. 5).  Traps typically are set for 3-5 nights per trapping session.  A mark-recapture study of aquatic salamanders (Siren lacertina, Siren intermedia, Pseudobranchus striatus, and Amphiuma means) using crayfish traps and screen funnel traps is being conducted at four sites on the Refuge.

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Figure 4. A crayfish trap - click to enlarge
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Figure 4 (left). A crayfish trap (manufacturer: Lee Fisher, Inc., Salt Springs, FL), used in ONWR primarily for surveys of large, aquatic salamanders.  Modification of these traps by the insertion of a 9.5 mm Vexar liner allows for the capture of smaller species (e.g., Pseudobranchus striatus) and larvae.

Mosquito - click to enlarge
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Figure 5.  Screen funnel trap - click to enlarge

Figure 5.  Screen funnel traps are placed in both dry and inundated terrestrial habitats.  They are placed along natural barriers, such as logs, to maximize trap efficacy.

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PVC Pipes.  At the permanent monitoring sites, PVC pipes have been placed to detect hylid treefrogs (Boughton et al., 2000) (Fig. 6).  At each site, fifteen pipes (5 cm diameter x 60 cm long, capped at the bottom) are mounted approximately 2 m above ground on tree trunks and 15 pipes (5 cm diameter x 91 cm long) have been placed in the ground with approximately 60 cm of pipe above ground. At each sampling session, frogs captured in the pipes are identified to species, measured, marked and released at the point of capture (Fig. 7).  Frogs are individually marked by clipping a unique series of toes (Heyer et al., 1994).  Data from the mark-recapture study of tree frogs will be used to monitor population trends.

Coverboards. Coverboards (60 cm x 60 cm x 1 cm in size) have been placed at permanent sampling sites either in a 5 m square grid or along the edge of lakes or ponds.  The boards are made of untreated lumber and were allowed to weather at the sites for 2-3 months prior to being checked.

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Figure 6. A forested wetland - click to enlarge Figure 7. A pinewoods treefrog, Hyla femoralis - click to enlarge

Figure 6 (left). A forested wetland in ONWR, showing the placement of PVC pipes on trees and in the ground (red arrows).  Tree pipes are capped at the bottom to hold water and ground pipes are recessed 30 cm, to provide a cool, dark microclimate for hylid treefrogs.

Figure 7 (right). A pinewoods treefrog, Hyla femoralis, caught in the act of taking refuge within a PVC pipe in ONWR. All captured frogs are weighed, measured, toe-clipped with a unique mark, and released.

Results and Discussion

To date, 17 species of amphibians (13 species of frogs and 4 species of salamanders) have been observed at ONWR. Most species were encountered incidentally either by direct observation or by detection of calls during the course of our field activities (Table 1).  However, the most effective directed sampling methods were dip netting, passive trapping (with crayfish and screen funnel traps) and frog loggers.  Methods such as PVC pipes, which targeted specific taxa, also were productive.  Litter searches (both area- and time-constrained) and coverboard monitoring were labor intensive and ineffective for detecting amphibians at ONWR

Crayfish traps were particularly useful for capturing two-toed amphiuma (A. means). For example, capture success for two-toed amphiuma with this method was as high as 15% at one site.  Greater siren (S. lacertina) and larval anurans also were caught in crayfish traps at the Refuge.  This technique is being used as part of a graduate research project funded by ARMI involving a mark-recapture study of aquatic salamanders.

Table 1. Species identified by various survey techniques - click to enlarge

Table 1. Species identified by various survey techniques during the first year of amphibian monitoring at ONWR.  *"Unusual species" include those which have been identified by only one or two of the implemented techniques.

Adult broad-striped dwarf siren, Pseudobranchus striatus striatus - click to enlarge

Adult broad-striped dwarf siren, Pseudobranchus striatus striatus, from ONWR. This species, Amphiuma means, Siren lacertina, and Siren intermedia are currently being monitored in a mark-recapture study at four sites on the Refuge.

Three species, pinewoods treefrogs (Hyla femoralis), green treefrogs (Hyla cinerea), and squirrel treefrogs (Hyla squirella) used pipes regularly at ONWR.  A total of 229 captures of the three species was recorded during two quarterly monitoring sessions, of which 100 individuals were recaptures (overall recapture rate of 43.7%). Recapture rates at individual sites ranged from 27.6- 61.5%. Long term monitoring of PVC pipes has the potential to provide estimates of population size and survival of these species. 

The automated recording systems allowed us to confirm the presence of carpenter frogs (Rana virgatipes), a species of concern, at two sites where they were not detected by any of the other survey methods.  Preliminary analysis of recordings at one site revealed interspecific variation in breeding phenology (Fig. 3).  Southern cricket frogs (Acris gryllus) were in chorus throughout most of the night, whereas pinewoods treefrogs and southern toads (Bufo terrestris) were recorded during the early morning hours—a period of time rarely sampled by human listeners.

Although still in the preliminary stage, the research initiative at ONWR has led to information on patterns of distribution of amphibians within the Refuge. The greatest absolute amphibian species richness was observed in wet prairie habitat (N=15) (Fig. 8).  Several species, including southern cricket frog and pinewoods treefrog, were nearly ubiquitous in distribution.

Amphibian species richness at different habitat types - click to enlarge

Figure 8. Absolute amphibian species richness at different habitat types in ONWR after one year of monitoring. The "other" habitat category includes canals and maintained canoe trails.

The use of passive methods of sampling, such as automated recording systems, traps, and PVC pipes has enabled us to detect difficult-to-observe species with minimal investment of time.  Much of the Okefenokee Swamp is extremely difficult to access and future field efforts will include the expanded use of these methods in remote areas of the Refuge.

Literature Cited

Boughton, R.G., J. Staiger, and R. Franz. 2000. The use of PVC pipe refugia as a sampling technique for hylid treefrogs. Am. Midl. Nat. 144:168-177.

Donnelly, M.A., C. Guyer, J.E. Juterbock, and R.A. Alford. 1994. Appendix 2: Techniques for marking amphibians. pp. 277-284. In Measuring and Monitoring Biological Diversity: Standard Methods for Amphibians. Heyer, W.R., M.A. Donnelly, R.W. McDiarmid, L.C. Hayek, and M.S. Foster (eds.). Smithsonian Institution Press, Washington, D.C.

Laerm, J., B.J. Freeman, L.J. Vitt, J.M. Meyers, and L. Logan. 1980. Vertebrates of the Okefenokee Swamp. Brimleyana 4:47-73.

Peterson, C.R., and M.E. Dorcas. 1994. Automated data acquisition. pp. 47-57. In Measuring and Monitoring Biological Diversity: Standard Methods for Amphibians. Heyer, W.R., M.A. Donnelly, R.W. McDiarmid, L.C. Hayek, and M.S. Foster (eds.). Smithsonian Institution Press, Washington, D.C.

Wright, A.H. 1932. Life-histories of the frogs of Okefinokee Swamp, Georgia. The Cayuga Press, Ithaca, NY.


We thank the rest of the Okefenokee field crew: William J. Barichivich, Kristina Sorensen, Maya Zacharow, Gary L. Hill, Bob Lewis, and C. Kenneth Dodd, Jr., the regional coordinator of Southeast ARMI. We also thank Amy D. Hester for fieldwork and the Refuge personnel for permission to work in the swamp. The frog loggers were built using a design modification developed by William J. Barichivich (

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