Invasive species

How do invasive species affect amphibians?



Figure 1 - created by Heidi Rockney using data referenced at bottom of page

Invasive species are a major problem to ecosystems around the globe. When non-native animals, plants, bacteria or fungi enter a new ecosystem and thrives, it completely disrupts the normal food chain. This can cause extinctions of local animals as well as alter nutrient recycling. The relationships that all the animals in a particular habitat have established are often thrown out of balance. Invasive species are often organisms that thrive in disturbed habitats and are extremely efficient at utilizing nutrient resources. In normal ecosystems, there are natural predator and prey dynamics that keep population dynamics in check. When a novel organism enters a new area they have no natural predators and often take nutrients from the habitat that other animals and plants need. There are four forms of invasives that dramatically affect amphibians: vegetation, predatory, competitor, and pathogen invaders (see figure 1 - created using data compiled from Oregon Spotted frog research, references at bottom of page).


 

Vegetation

On the right is a picture of prime Oregon Spotted frog habitat during the breeding season.  Can you see the two frogs hiding in the vegetation? The arrow points to a similar habitat that has been taken over by reed canary grass. Photo credit: Heidi Rockney (top)http://extension.usu.edu/rangeplants/htm/reed-canarygrass (bottom)
Invasive plants can completely change an entire landscape. Many animals rely on the plants and features in their habitat for food, camouflage from predators and appropriate nursing grounds for their young. An example of the devastating affects of introduced plant species can be seen with the endangered Oregon Spotted Frog. The Oregon Spotted Frog requires very low lying vegetation around its shallow breeding pools. This allows optimum camouflage from birds and other predators that regularly seek out Oregon Spotted Frogs for a meal. They also are almost entirely aquatic, and not very efficient at moving on land. If they need to move across their habitat to a new water source for any reason, the low lying vegetation allows them to move unimpeded to a new home. Unfortunately, Reed Canary Grass has invaded wetland habitats throughout the Oregon Spotted Frog's already diminishing range. This grass is tall, fast growing and out competes indigenous low lying plants that normally border wetlands. In the areas that the grass has invaded, Oregon Spotted Frogs are no longer seen as they cannot migrate to and from breeding sites through the tall, dense grass. The Oregon Spotted Frog is highly endangered and this additional stress has completely disrupted their ability to recover population numbers.

 

Competitors, predators, pathogens

Bullfrog eating a bird. Photo credit:animalseatinganimals.

Bullfrog gorging on another frog. Photo credit:
flickr

One invasive species found all over the globe is the American Bullfrog. Bullfrogs were introduced around the world over the last 100 years. Bullfrogs are edible, and many people across the globe enjoy eating these large frogs. Not only have they have been introduced on purpose for a food and game source, but also accidentally via escape from bullfrog farms. Bullfrogs have been known to have devastating effects on the ecology of local habitats. They are the largest amphibian in the United States and have voracious appetites. They prey on local amphibians, including tadpoles and adults and compete with other amphibians for food and habitat. Bullfrogs breed extremely fast and can quickly dominate any water source they inhabit, often wiping out many animals in an area besides local amphibians. Bullfrogs also are carriers for the deadly chytrid fungus that has been spreading worldwide. Bullfrogs themselves are not as affected by the chytrid fungus, however many other amphibians are and bullfrogs carry the fungus into previously uninfected sites.  American bullfrog gorging on another frog.


Another major invader is in the form of predatory fish. Many amphibians historically will not lay eggs in water with predatory fish and have no adaptations to combat the effects of water predators. Predatory fish have been (and continually are) introduced all over the world into freshwater systems that amphibians rely on for breeding. They eat amphibian eggs, larvae and adults if they can and will sometimes completely wipe out entire populations of amphibians.

Next

References:

Hayes, Marc P., Rombough, Chrsitpher J., Padgett-Flohr, Gretchen E., Hallock, Lisa A., Johnson, James E., Wagner, R. Steven, Engler, Joseph D. 2009. Amphibian Chytridiomycosis in the Oregon Spotted Frog (Rana pretiosa) in Washington State, USA. Northwestern Naturalist, 90 (2) pp 148-151.

Pearl, Christopher A., Adams, Michael J. Leuthold, Niels. 2009. Breeding habitat and local population size of the Oregon spotted frog (Rana pretiosa) in Oregon, USA. Northwestern naturalist. 90 (2) pp 136-147.

Pearl, Christopher A., Hayes, Marc P. 2004. Habitat Association of the Oregon Spotted Frog (Rana pretiosa) A Literature review. Final Report. WDFW Olympia, Washington 44p.

Stuart, Simon N., Chanson, Janice S., Cox, Nail A., Young, Bruce E., Rodrigues, Ana S.L., Fischman, Debra L., Waller, Robert W. 2004. Status and Trends of Amphibian Declines and Extinctions worldwide. Science Magazine. 306 (5702) pp 1783-1786.

Watson, James W., McAllister, Kelly A, Pierce, John D., Alvarado, Amy. 2000. Ecology of a Remnant Population of Oregon Spotted Frogs (Rana pretiosa) in Thurston County, Washington. Final Report. WDFE Olympia, Washington 85p.

Watson, James W., McAllister, Kelly R., Pierce, John D. 2003. Home ranges, movements, and habitat selection of Oregon spotted frog (Rana pretiosa) Journal of Herpetology. 37 (2) pp 292-300.