Amphibians across the world are rapidly declining. Numerous studies have addressed causes of the decline, but very few have looked at the effects of invasive plants. Dan Saenz, Southern Research Station (SRS) research wildlife biologist based in Nacogdoches, Texas, is working with collaborators to determine the effect of Chinese tallow tree on Texas frog species.
Following its introduction in the late 1700s, Chinese tallow tree (tallow) rapidly took over the Gulf Coast that stretches from Florida to Texas, covering over 500,000 acres in Texas and Louisiana alone. SRS Forest and Inventory Analysis data suggest that tallow increased 174 percent in east Texas and more than 500 percent in Louisiana since the early 1990s; the aggressive invasive is now the fifth most common tree species in east Texas and Louisiana.
Although tallow can grow in almost every habitat and soil type, it is often found in wetter areas. Forming a monoculture in the areas it invades, tallow literally chokes out the native trees around the wetlands frogs breed in, blanketing the bottoms of pools with its leaves in the fall.
“Tallow leaves decompose much faster than the leaves of native wetland trees and plants,” says Saenz. “The process of leaf decomposition and the release of tannins from the leaves can affect water quality and specifically dissolved oxygen, which could adversely affect frogs in the egg or tadpole stages.”
A recent study by Saenz and SRS wildlife biologist Cory Adams on the effects of tallow leaf litter on the hatching success of southern leopard frog eggs is the first of its kind. “To our knowledge, no work has been conducted on the effects of invasive species on amphibian eggs,” says Saenz. “Amphibian eggs are immobile and one of the most vulnerable stages of development.”
For the study, the researchers exposed southern leopard frog eggs at various stages of development to different concentrations of tallow leaf litter. Results,published late summer in the Canadian Journal of Zoology, showed that eggs in the earliest stages of development exposed to tallow leaf litter died, regardless of the concentration, while some eggs in later stages of development survived.
“We found that the greater the concentration of tallow tree litter, the lower the dissolved oxygen and the more acidic the water,” says Saenz. ”We suggest that changes in these water quality factors are the cause of the death of frog eggs in our experiments. This has profound implications for amphibians in wetland areas where tallow has taken over.”
In an earlier study published in June in the Journal of Herpetology, Saenz and colleagues at Stephen F. Austin University reported findings from introducing tadpoles from four different frog species into pools containing leaf litter from tallow or from one of two native trees. Results were mixed, suggesting that the breeding season of a species may determine how well its members survive and develop in an environment with tallow leaf litter. “Chinese tallow leaf litter breaks down faster than native species,” says Saenz. “Because of this, negative effects might be short-lived, but could pose a threat to species that breed soon after leaf fall.”
For more information, email Dan Saenz at dsaenz@fs.fed.us