Declines in amphibian populations have been documented for many regions and habitat types worldwide (Alford and Richards, 1999). No single cause for declines has been determined, and it seems likely that several factors interact to threaten populations (Carey and Bryant, 1995). In response to concerns about amphibian population declines, the Department of Interior (DOI) received funding from Congress to institute long-term surveys of the status and trends of amphibians on DOI lands. The U.S. Geological Survey has been conducting inventories of amphibian species in Everglades National Park (ENP) and Big Cypress National Preserve (BCNP) from 2000 to present.

Together ENP and BCNP comprise a contiguous protected area of more than 890,000 ha. This landscape, unique in the United States, contains a diverse array of different wetland and upland habitats. The amphibian fauna of the Everglades and Big Cypress region comprises 15 native species, all derived from temperate zone fauna, and three non-indigenous species from the tropics. Although historical and recent survey and inventory work on amphibians has been done on DOI lands in south Florida (Duellman and Schwartz, 1958, Meshaka et al., 2000), there is a need for a current inventory and estimate of the abundances of the amphibians of ENP and BCNP. This project uses data on detection of amphibian species after repeated sampling at a site to estimate the occupancy rate of various amphibian species (MacKenzie et al., 2002).

Sampling locations were chosen randomly throughout ENP and BCNP using a Geographic Information System (GIS), and all sampling was stratified by major habitat type. We divided the parks into six natural habitats: pineland, rocky glades, tropical hardwood hammock, mangrove forest, cypress dome, and freshwater slough. These habitat designations were based loosely on the vegetation classification scheme of Madden et al. (1999), and their map was used as the basis for site selection. We selected points at random within each of the major habitat types. Six of these sites in each habitat type at each park were used as monthly sampling locations. These were visited once within each calendar month for one year. Other random locations were sampled at least twice during times when they were accessible. A total of 118 sites in ENP and more than 75 sites in BCNP were sampled.

The primary method of sampling was a standard visual encounter survey (VES) technique combined with a 10-minute auditory survey for calling anurans (Heyer et al., 1994). Each sampling event lasted 30 minutes and all were conducted after dark. Each area searched was a 20-m radius circle (1,256 m² area) around the randomly chosen point. Each individual found was identified to species, sex, age, and snout-to-vent length. We measured the air temperature and relative humidity using a digital thermohygrometer. Cloud cover, wind speed, whether the plot was inundated with water, and water temperature (if applicable) were also noted.

Data were organized into capture history matrices where detection of species was denoted by a "1" for a given site and month. Non-detection of a species was denoted with a "0", and a "-" was used in cases where a site was not visited during a particular month. These data were analyzed using the program PRESENCE to calculate a proportion of sites occupied (PAO) by each species within each habitat (MacKenzie et al., 2002).

We observed 1,788 individual amphibians in ENP and more than 2000 in BCNP during VES surveys. Not all of the anurans were observed a sufficient number of times to allow calculation of proportion of sites occupied in ENP. Data collection and analysis for BCNP is still underway. For some of the more abundant anurans it was possible to estimate the site occupancy by habitat (table 1).

The results obtained for site occupancy may serve as a baseline for long term monitoring of amphibians in the national parks of south Florida. Estimating the total abundance of any of these species is practically impossible due to the large area involved and complicating variables like weather and phenology of amphibian behavior. However, by estimating the site occupancy rate of each species at randomly chosen sites within each habitat, it is possible to produce an estimate of the proportion of that habitat in which a species occurs. This number does not indicate the abundance of individuals, but it does permit estimates of the abundance of populations. This can serve as the basis for tracking changes in population abundance over time.

The sampling protocol developed here for these parks is considered appropriate for future monitoring of amphibians in the Everglades ecosystem. The methods are relatively inexpensive, and replication should be straightforward. Tracking changes in site occupancy over time will help identify trends in amphibian populations and serve as an early warning if any amphibian populations are truly declining in south Florida. Plans for future work include using PAO with hydrologic models of the Everglades ecosystem to predict how populations of species might react to hydrologic change. With estimates of the proportion of sites occupied by each species in amphibian assemblages in areas with different hydrology, it will be possible to predict how the assemblage will change during restoration. This will assist managers in assessment of proposed hydrological changes and restoration success.

REFERENCES

Alford, R. A., and S. J. Richards. 1999. Global amphibian declines: a problem in applied ecology. Annual Review of Ecology and Systematics 30:133-165.

Carey, C., and C. J. Bryant. 1995. Possible interrelations among environmental toxicants, amphibian development, and decline of amphibian populations. Environmental Health Perspectives 103:13-17.

Duellman, W. E., and A. Schwartz. 1958. Amphibians and reptiles of southern Florida. Bulletin of the Florida State Museum 3:179-324.

Heyer, W. R., M. A. Donnelly, R. W. McDiarmid, L. C. Hayek, and M. S. Foster. 1994. Measuring and monitoring biological diversity: standard methods for amphibians. Smithsonian Institution Press, Washington, DC.

MacKenzie, D. I., J. D. Nichols, G. B. Lachman, S. Droege, J. A. Royle, and C. A. Langtimm. 2002. Estimating site occupancy rates when detection probabilities are less than one. Ecology 83:2248-2256.

Madden, M., D. Jones, and L. Vilchek. 1999. Photointerpretation key for the Everglades vegetation classification system. Photogrammetric Engineering & Remote Sensing 65:171-177.

Meshaka, W. E., W. F. Loftus, and T. Steiner. 2000. The herpetofauna of Everglades National Park. Florida Scientist 63:84-103.

Table 1: Proportion of sites occupied by amphibian species by habitat in ENP.
Habitat	Acris gryllus	Bufo quercicus	Bufo terrestris	Gastrophryne carolinensis	Hyla cinerea	Hyla squirrella	Rana grylio	Rana sphenocephala
Pineland	0.63	1.00	1.00	0.64	0.95	0.94	0.33	0.38
Rocky Glades	0.84	0.60	0.21	0.46	0.99	0.61	0.90	1.00
Hammock	0.85	0.00	0.49	0.67	0.96	0.39	0.91	0.76
Cypress	0.84	0.00	0.00	0.66	0.90	0.97	0.78	0.68
Mangrove	0.00	0.00	0.47	0.40	0.49	0.53	0.27	0.35
Slough	1.00	0.00	0.39	0.28	1.00	0.24	1.00	0.76

Contact: Kenneth G. Rice, U.S. Geological Survey, UF-FLREC, 3205 College Ave., Ft. Lauderdale, FL 33314, Phone: 954-577-6305, FAX: 954-577-6381, e-mail: ken_g_rice@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

Back to Project Homepage