Amphibians, Seasonal Ponds, and Drainage Ditches in Coastal Georgia

By Nancy O’Hare
Bucketful of toads

A bucketful of Southern toads and Eastern spadefoots (~125 toads)

My work as a Wormsloe Fellow focuses on amphibians, seasonal ponds, and drainage ditches in conservation lands in the Sea Islands of coastal Georgia.  Amphibians (frogs, toads, and salamanders) have long been considered good indicators of ecological functions since they are sensitive to quality, quantity and connectivity of both upland and wetland habitat. This makes amphibians especially suitable as indicators of ecological function of conservation lands.  However, amphibians have experienced the highest population decline and species loss among vertebrates. The reasons vary, but typically include habitat loss, habitat degradation, loss of spatial connectivity, and climate variability (e.g. rainy versus dry periods). More importantly, the cumulative effect of several factors may be greater than predicted from factors acting independently.

In many regions, the loss of the seasonal freshwater ponds required for amphibian breeding is the most obvious single factor. Seasonal ponds are small (< 0.5 ha) and shallow (1 m), making them difficult to detect remotely or from most digital terrain models. However, seasonal ponds and drainage ditches can be mapped from digital terrain models derived from light detection and ranging (LiDAR). At Wormsloe, all of the historic freshwater ponds are connected to tidal marsh by drainage ditches, a past last-use legacy.  During the past six months, I have demonstrated that these ditches are negatively affecting pond hydrology since saltwater is pushed along the ditches and into the ponds during April and November high tides.

LiDAR based digital terrain model

LiDAR derived digital terrain model. Seasonal ponds and drainage ditches were digitized

Even though the seasonal ponds have a bi-annual tidal influence, somehow adult frogs persist. I have trapped adults of seven species of frogs at Wormsloe. Regardless, no breeding occurred in natural seasonal ponds in spring 2011 or 2012. Rainfall has been below average for the past few years, which likely exacerbates residual salinity from tidal intrusion. However, frogs tend to have explosive breeding events, and may only need to breed successfully every three to ten years to persist. On a rainy evening in March 2012, I captured an explosive breeding attempt by two species of toads. More than 450 toads were captured in ~12 hours. Despite this explosive occurrence of adults, breeding did not occur; it was the next month, in April 2012, that I discovered the salinity issue.

Like Wormsloe, many of the conservation lands throughout coastal Georgia have a similar land-use legacy of drainage ditches. Therefore, this study has regional implications.

Thanks to the Wormsloe Institute for Environmental History and to Craig and Diana Barrow.