Not only do humans introduce harmful chemicals into aquatic habitats, but they also add harmful species. We have purposefully or accidentally introduced many predators that harm frogs around the world. These predators include sport fishes and other frogs. Invasive species have been deposited into aquatic habitats everywhere people live or go to recreate.
Some people think fish from one area should be transported to other areas so the fisheries can be “improved” with more desirable species. Many people prefer fish they had in the area where they grew up to the native species where they live. Thus, we have common carp in the US. This was a regular food and sport fish in Europe, and people thought it would be good idea to introduce it here. Now, it is rarely caught as a sport or food fish in the US, but has spread across the country. Carp stir up aquatic habitats, destroys habitat for other fish, and eat most anything they can fit in their mouths. Likewise, people have introduced bass into many habitats, and trout wherever they can.
We have introduced relatively large predators into virtually all major drainages of the world. Many of these introduced predators will eat frogs or tadpoles. Predator introduction is one more factor contributing to amphibian declines.
A very important example of an introduced species, with respect to amphibian declines, is the cane toad was a native to Central and South America. It has been introduced in many places in the world for pest control. It was introduced into Australia in an effort to control beetles that are pests on sugarcane. Unfortunately, the toad is ineffective against the pests it was introduced to control; the toad lives on the ground, and the beetles are higher up in the sugarcane.
Cane toads are very toxic, native snakes die when they eat the adults, and fish die when they eat the cane toad tadpoles. Having no evolutionary experience with the introduced toad, Australian predators continue to try to eat both the adults and the tadpoles. In addition, the large toad will consume any smaller frogs or other amphibians it comes into contact with as well as any other prey that can be gulped and squeezed past its jaws. The cane toad is one amphibian that, unfortunately, is not in decline.
Bullfrogs have spread from Central and Eastern US all the way to California. Because its eggs and tadpoles are not palatable, this is one of the few species with tadpoles that can avoid being preyed upon by fishes. Being the largest North American frog, bullfrogs can ingest mice, other frogs, toads, worms, salamanders, and many types of insects. These frogs may be hastening the disappearance of some rarer species of frogs, particularly those found in California. The bullfrog is resistant to the chytrid fungal disease to which many other frog species are susceptible. It may actually serve as a reservoir to move the fungus around (known as a vector to epidemiologists). Introduced species not only include animals, but also include diseases.
The chytrid fungus, Batrachochytrium dendrobatidis, causes a lethal infection to many species of frogs. The disease could have initially been spread by African clawed frogs, although multiple introductions have probably occurred. These frogs were transported around the world to be used for pregnancy tests and as research species (such as in the work of Dr. Hayes mentioned previously). The theory is that some frogs escaped and then passed the disease on to local amphibians. The African frog and many frogs from warmer habitats are resistant to the disease, but many other species of frogs are not. The resistant frogs can carry the disease from one habitat to the next.
The fungus infects the frog’s skin and causes a catastrophic loss of ability to control water balance across their skin; frogs need healthy skin to survive and are particularly sensitive to factors that alter the exchange of moisture and oxygen across their skin. The fungus causes the frog’s skin to thicken to the point where they suffocate. Diseased frogs look terrible with their skin sloughing and their beautiful colors hidden behind a thick layer of infected skin. The fungal disease seems to be hitting particularly hard in Central America and Australia, though these are the areas where the main study on the effects of the disease has occurred, and it is possible that other areas have also been hit that are not as well documented.
Global warming might interact with the chytrid fungal disease. Even a slight warming of climate represents a significant change in temperature in the tropics, where temperature extremes are not so wide. Warming paradoxically leads to cooler days at higher altitudes in Central America by pumping more moisture from the oceans and increasing cloud cover. Cloud cover also causes warmer evenings. Dr. J. Alan Pounds, of the Monteverde Preserve in Costa Rica (the same preserve that used to be home to the Golden Toad), found that these climate changes are creating the perfect conditions for spread of the chytrid disease. Over 100 species of frogs have probably gone extinct in Central America from the combination of local effects of global climate change and the disease. Frogs have experienced periods of warming and cooling over their evolutionary history, but never in concert with the chytrid fungus.
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