Using Proportion of Area Occupied to Estimate Abundance of Amphibians in
Everglades National Park and Big Cypress National Preserve
Kenneth G. Rice, U.S. Geological Survey, Center for Water and Restoration
Studies, Ft. Lauderdale, FL
J. Hardin Waddle and H. Franklin Percival, U.S. Geological Survey, Florida
Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville,
FL
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 (1256 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 1788 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.
Literature Cited
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
Bufo
Gastrophryne Hyla
Hyla
Rana
Rana
quercicus terrestris carolinensis cinerea squirrella grylio 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
Kenneth G. Rice, U.S. Geological Survey, UF-FLREC, 3205 College Av., Ft.
Lauderdale, FL 33314, Phone: 954-577-6305, FAX: 954-577-6381, e-mail:
ken_g_rice@usgs.gov