Impacts of Invasive Species on Amphibians in the U.S.

When I hear the words “invasive species”, my mind automatically jumps to Burmese Pythons in the Everglades. Large pythons and bizarre snakehead fish make for sensational news stories, but there are many other “invasive species” all around us that go unnoticed. For instance, did you know that the Dandelion flower (Taraxacum officinale) is not actually native to the United States? It was actually introduced as food for honeybees, but has now become one of the most common weeds in the U.S. Another species that was introduced with good intentions but may be harmful to our native forests and their amphibian residents is the earthworm.

There are only a few species of native earthworms in the United States, and most of these are not found in very high concentrations in the soil. However, when the settlers first arrived in the Eastern United States, they brought with them heaps of European Earthworms in hopes of improving the soil for agriculture. Today, many of our eastern soils are full of both Asian and European species of non-native earthworms. These earthworms affect their environment by mixing the soil layers, eating leaf litter and increasing rainwater penetration into the soil through their burrows. This may not seem like a problem initially, but studies are now showing that our native hardwood forests may suffer from the effects of these subterranean invaders.

Redback salamanders (Plethodon cinereus), one of the most common salamanders in the Southeastern United States, are thought to be experiencing population decline in lowland areas partially due to the large amounts of earthworms in these areas. Earthworms may also be helping the spread of invasive plants, which also alter these salamanders’ habitats. Stokegrass, an invasive plant from Japan, is thought to also contribute to higher juvenile American Toad (Anaxyrus americanus) mortality by allowing one of their predators, wolf spiders, to exist in more dense populations. Finally, introduced rainbow and brown trout fishes may have negative impacts on the populations of one of our most endangered salamander species, the Ozark Hellbender (Cryptobranchus alleganiensis bishopi). Juvenile hellbenders typically can recognize and respond to predatory fish, but they may not be able to recognize these voracious non-native predators. However, because trout are typically stocked into rivers for fishing and do not really reproduce there, correlating the timing of trout stocking so that it does not impact Hellbender reproduction may reduce negative effects from these species.

Only time will tell if these impacted ecosystems will balance out or if the populations of amphibians described will be sustained despite the presence of harmful invaders. These cases show that sometimes what may seem to us to be a perfectly innocuous or useful species, like the earthworm, can still cause negative consequences when introduced into areas where they are not native. The only thing that can really be done is to study the ecosystems where these non-native species of plants and animals are already present and try to prevent them from spreading to undisturbed habitats.

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