Effects of the Nonindigenous Treefrog,
Osteopilus septentrionalis, on Native Hylids in
Protected Areas of South Florida

Marquette E. Crockett³, Amber D. Dove³, Kenneth G. Rice¹,
J. Hardin Waddle⁴, H. Franklin Percival²

¹U.S. Geological Survey, Florida Integrated Science Center
University of Florida 3205 College Ave. Ft. Lauderdale, FL 33314-7799

²U.S. Geological Survey, Florida Cooperative Fish and Wildlife Research Unit
University of Florida. 117 Newins-Ziegler Hall, Gainesville, FL 32611

³U.S. Geological Survey, Everglades National Park Field Station
40001 S.R. 9336, Homestead, FL 33034

⁴U.S. Geological Survey, Big Cypress National Preserve Field Office
33100 Tamiami Trail. Ochopee, FL 34141

Introduction

Cuban Treefrogs, Osteopilus septentrionalis were first reported in the Florida Keys by Barbour (1939). They were later reported on the mainland, near Miami (Schwartz 1952). These frogs have continued to expand their range in Florida.  The range of this species as of 1997 is shown in Figure 1.  Further expansion has been reported by several sources, including this study.

Cuban treefrogs threaten native fauna in protected areas throughout South Florida.  In addition to competing with natives, Osteopilus are predators of hylids. Cuban treefrogs are voracious predators and are cannibalistic (Allen and Neil, 1953). Osteopilus tadpoles are carnivorous and are known to eat other tadpoles (Babbitt and Meshaka, 2000).  Despite circumstantial evidence, no study has shown the extent to which Cuban treefrogs reduce populations of native frogs in natural areas.

Figure 1: Range of Osteopilus septentrionalis in Florida (Northern Prairie Wildlife Research Center, 1997)

The primary goals of this study are to:

• Investigate the recovery of native populations upon removal of Osteopilus
• Quantify predation of Osteopilus on native hylids
• Identify factors that facilitate or obstruct Osteopilus dispersal into natural areas 
• Incorporate this information into an ArcInfo model of the potential spread and impact of Cuban treefrogs on native species in protected areas

Figure 3: Hyla cinerea (shown) and Hyla squirella have been encountered in PVC refugia during this survey.

Results to Date

Based on data collected, Osteopilus is more abundant in sites which are closer to disturbed areas. Hyla cinerea and Hyla squirella, the native treefrogs found in this study, are more numerous at more remote site (Figures 3 and 4).  Estimates of population size and survival rates of all species will be calculated using mark-recapture software. Removal of Cuban treefrogs at these sites is scheduled to begin in Summer, 2002.

Three additional sites were selected in Spring, 2002. Survey in these sites will continue for one year. Removal of Osteopilus will begin in these areas as early as Summer, 2003.

Figure 4:  Numbers of Marked Individuals at Pine Rockland and Mangrove Sites

Methods

The first phase of this project is a mark-recapture survey of treefrogs in selected sites in South Florida. Six plots (5 in Everglades National Park and 1 in Biscayne National Park) have been set up. These plots contain 99 two-inch PVC tree pipes (Figure 2).  They are located in two different habitats, Pine Rockland and Mangrove, and are varying distances away from disturbed areas. The sites are checked bi-weekly and all frogs encountered are weighed, measured from snout to vent, and administered a unique toe-clip for identification. Additionally, specimens over 60mm in snout-vent length are administered a pit tag. Population estimates for both Osteopilus and natives will be derived from this survey.

After sites have been surveyed for one year, Cuban treefrogs will be removed.  We will continue to mark, release and recapture native species during this time. Removals will be conducted for one year. This will provide estimates of native population recovery or reinvasion by Osteopilus.  Each removed animal will be sacrificed for stomach content analysis to quantify presence of native hylids as prey. The information collected will be used to investigate factors influencing Osteopilus invasion and to create an Arc Info spatial model of potential Osteopilus invasion.

Figure 2:  Two-inch PVC pipes are placed on trees in the study sites (3 meters from the ground)  to survey treefrogs. Water collects inside the pipes and creates a refugia which is used by both Cuban and native frogs.

Literature Cited

Allen, E. and W. Neil. 1953. The treefrog Hyla septentrionalis in Florida. Copeia 1953(2): 127-128.

Babbitt, K. and W. Meshaka. 2000. Benefits of eating conspecifics: effects of background diet on survival and metamorphosis in the Cuban treefrog (Osteopilus septentrionalis). Copeia 2000:469-474.

Barbour, T. 1931. Another introduced frog in North America. Copeia 1931(3): 140.

Schwartz, A. 1952. Hyla septentrionalis on the Florida mainland. Copeia 1952(2): 117-118.

Northern Prairie Wildlife Research Center. 1997. Checklist of Amphibian Species and Identification Guide. An Online Guide for the Identification of Amphibians in North America north of Mexico.  North American Reporting Center for Amphibian Malformations. Jamestown, ND: Northern Prairie Wildlife Research Center Home Page. http://www.npwrc.usgs.gov/narcam/idguide/   (Version 27 March, 2002).