Amphibian and Reptile Monitoring in the Pontchartrain-Maurepas

B. Crother and C. Fontenot

Project Overview

Amphibians and reptiles are good bio-indicators of environmental health because they are in the middle of the food chain, as both predators and prey. In addition, the larval and adult life of amphibians is tied to water, particularly reproduction. Consequently, changes in water quality are often reflected by changes in amphibian populations. This study monitors amphibian and reptile populations in the Lake Ponchartrain-Maurepas Region so that changes in their ecology can be detected.

This study is in two parts:

Monitoring via transect and Hwy 51 Road Survey

A standard transect approach has been undertaken in the Lake Maurepas region (Alligator Island) and in the Pontchartrain region (Manchac swamp) to survey herpetofaunal species diversity and population density estimates. In addition, Highway 51 was driven from Ponchatoula to LaPlace and back every third night from January 2004 through June 2005 in search of amphibians and reptiles on the road.

Effect of Salinity on a Common Frog Species

Salinity tolerance of one of the most abundant frogs in the Manchac swamp area, Hyla cinerea (Green Treefrog), was investigated.  Anurans are ideal for such a study because of their importance as indicators of environmental health.  This study investigates salinity tolerance by conducting a lethal concentration test of H. cinerea larvae as well as look at developmental rate and other effects on ontogeny of the two populations.  One population is prone to saltwater intrusion from Lake Pontchartrain (Manchac area) and considered intermediate salinity marshland.  The second population is freshwater (West Lake Maurepas) and supports a healthy swamp.  Seven salinity levels are used with three replicates of each for each population.

The lethal concentration test showed divided results between the predominately freshwater site and intermediate salinity site. Both populations exhibited minimal mortality at 0 and 2ppt, no mortality at 4 and 6ppt. At 10 and 12ppt both populations experienced 100% mortality within 48 to 96 hours. However, at 8ppt a noticeable difference between the populations occurred.  For instance, the Manchac population experienced 5% mortality while the West Lake Maurepas population experienced 50% mortality. This indicates that the frogs may be increasing resistance through exposure to heightened levels of salinity.

Citation: In press. B. Crother,T. Schriever, J. Ramspott, C. Fontenot. Disturbance to Southeastern Louisiana Wetlands and its Effect on Herpetofauna.