Southeast Ecological Science Center
Effects of predatory fish on Rana capito
Denise R. Gregoire
Presented at the Joint Meeting of Ichthyologists and Herpetologists
Southern Leopard Frogs Rana sphenocephala and Gopher Frogs R. capito are two species of anurans found in the southeastern United States whose larvae are very similar morphologically. Leopard Frogs are habitat generalists occurring in virtually all aquatic habitats within their geographic range, whereas Gopher Frogs are a species of conservation concern and only breed in semi-permanent fishless ponds. The introduction of predatory fish into historically fishless breeding sites has been suggested as a cause of Gopher Frog population declines.
The purpose of this study was to evaluate the effect of predatory fish on the survival and behavior of Leopard Frog and Gopher Frog tadpoles.
Leopard Frog and Gopher Frog tadpoles were reared from egg masses collected from a semi-permanent fishless pond on the Ordway-Swisher Biological Station in Putnam Co. Florida. The three species of fish used were warmouth sunfish (Lepomis gulosus), banded sunfish (Enneacanthus obesus) and the eastern mosquitofish (Gambusia holbrooki). All are commonly found in Leopard Frog breeding sites.
I conducted a randomized factorial experiment to examine the survival rate and behavior of the tadpoles when exposed to predatory fish. All possible combinations of the two tadpole species (Leopard Frog and Gopher Frog) and four predator treatments (warmouth, banded sunfish, mosquitofish and no predator) were represented. Each treatment was replicated five times.
Forty aquaria (50 x 25 x 30cm) were filled with well water. Two artificial refuges constructed from mesh screening were added to each tank to create cover for the animals. Each tank contained 30 tadpoles (total length 14.65 + 1.90 mm) of either species, and one predator according to the treatment.
Observation of tadpole hiding behavior (number of tadpoles underneath or inside a mesh refuge) were recorded periodically during the experiment. The experiment was terminated before any treatments reached 100% mortality. Surviving tadpoles were removed, counted and examined for injuries.
I also conducted choice experiments to examine tadpole survival when both species were simultaneously exposed to a predator. The choice experiments consisted of two treatments: predator (warmouth or banded sunfish) and no predator. Aquaria were set up the same as in the first experiment. 15 tadpoles (total length 22.94 + 3.20 mm) from each species were added to each tank along with one predator.
Tadpole survival was significantly affected by predator treatment (ANOVA, F3,24 = 16.07, P <0.01). Survival of both tadpole species was lower with warmouth than with either banded sunfish or mosquitofish.
Mosquitofish inflicted more injuries than any other predator (ANOVA, F3,24 = 7.87, P = 0.01), and injured significantly more Gopher Frog than Leopard Frog tadpoles (ANOVA, F3,24 = 4.0, P = 0.02).
In all predator treatments, Gopher Frog tadpoles hid significantly more than Leopard Frog tadpoles (ANOVA F3,24 = 20.31, P < 0.01). Both species showed an increase in hiding behavior when exposed to warmouth and mosquitofish. (ANOVA F3,24 = 3.0, P = 0.05)
Warmouth consumed significantly more Gopher Frog than Leopard Frog tadpoles. (Paired t-test, t = -3.05, df = 7, p=0.02). Mean number of Leopard Frog survivors with warmouth was 11.0 + 3.01 compared with only 7.75 + 2.62 Gopher Frogs. Alternatively, survival of Gopher Frogs with mosquitofish did not differ significantly from Leopard Frog survival. (Paired t-test t = -0.41, df = 7, p=0.70).