Federal and state agencies are attempting to project the impact of hydrologic restoration on the Everglades ecosystem. Current modeling efforts have focused on primary producers (e.g. macrophytes and periphyton) or organisms of higher trophic levels (e.g. panther, deer, birds). Amphibians are a vital intermediate link within the Everglades food web (ATLSS 1997, Ligas 1960). Even though amphibians are known to comprise a large portion of the faunal biomass within wetland systems, they remain underrepresented in restoration efforts (Burton and Likens, 1975). The biphasic lifecycle of pigfrogs (Rana grylio) makes them extremely sensitive to hydrologic change. Unfortunately, little is known about their abundance, ecology, and importance to the greater Everglades ecosystem. This study investigated temporal changes in Rana grylio abundance and diet in south Florida wetlands from August 2000 to September 2002. The goal of this study was to link changes in hydrology to the abundance and condition of this species throughout south Florida wetlands.

A total of 551 one kilometer transects were surveyed at night via airboat to compare the monthly abundance of R. grylio across large areas of Water Conservation Areas 3A, 3B, and Everglades National Park (ENP). Abundance estimates were calculated using a double-observer approach (Nichols et. al., 2000). Capture-recapture techniques were used within three one-hectare plots at each site in order to estimate abundance and growth parameters on a smaller scale. We collected 684 frogs from throughout the study areas from 1999-2002 to examine stomach content among sites. A drought occurred during 2001, which allowed us to assess how this species responds to a severe prolonged dry-down.

Stomach contents were greatest during the mid to late dry season at all three sites (fig. 1). Frogs in 3A had less food in their stomachs than those in either 3B or ENP. Growth was highest from December to March. Frogs were most abundant from March through September (fig. 2). Recruitment of metamorphs appears to peak from July through September. It appears that as water levels decrease, food may become more available to the frogs, allowing them to grow or invest in reproduction during this time. The study sites in WCA 3A rarely dried down. These frogs rarely contained fat bodies or food in their stomachs; this may be due to prolonged flooding in this area. Frogs are both smaller and more abundant in WCA 3A than in ENP or WCA 3B. This is probably due to a combination of both frog harvesting and high water throughout the year. Final results including models of the effects of hydrology on this species will be available in Fall 2003.

Figure 1. Mean total stomach volume of Rana grylio from three study sites in the Everglades during 2001-2002. [larger image]

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Figure 2. Mean abundance (animals/km) of Rana grylio from Water Conservation Area 3A. [larger image]

REFERENCES

ATLSS. 1997. Across trophic level ecosystem simulation: An approach analysis of south Florida Ecosystems. USGS-BRD. Miami, Florida.

Burton, T. M. and G.E. Likens. 1975. Salamander populations and biomass in the hubbard Brook Experimental Forest, New Hampshire. Copeia 3:541-546.

Ligas, F.J. 1960. The everglades bullfrog: Life history and management. Florida Game and Freshwater Fish Commission. Tallahassee, Florida.

Nichols, J.D., J.E. Hines, J.R. Sauer, F.W. Fallon, J.E. Fallon, and P.J. Heglund, 2000. A double-observer approach for estimating detection probability and abundance from point counts. Auk 117: 393-408.

Contact: Cristina A. Ugarte, Department of Biological Sciences, OE 167, University Park, Florida International University, Miami, 33199, Phone: 305-348-1235, e-mail: ugartec@fiu.edu

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

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