Reptile and Amphibian Population Decline Report
Report Prepared by Pennsylvania Researchers
Researchers around the world are reporting marked declines in the abundance of many amphibian species, including frogs. People have also reported declines of local amphibian populations with which they have personal experience. We have heard quotes such as, "I don't hear frogs calling along the river as I did 20 years ago," or "When I was a kid, we went down to the pond and caught frogs all the time; now you rarely see one."
There is no doubt that these observations are accurate. There have been many reasons offered and theories proposed to explain these noted declines. One of the most obvious is that habitat is lost or altered at an alarming rate. In Pennsylvania, we have lost a majority of the wetlands that were present at the turn of the 20th century. All of Pennsylvania's frogs require water and wetlands in which to lay their eggs so that they may hatch into tadpoles and develop into juvenile frogs. Loss of wetland habitat results in the loss of wetland-dependent species such as frogs.
Even though outright wetland destruction or degradation is obvious even to the casual observer, some other environmental alterations may be less visible. New highways have been built, and vehicular traffic has increased on most older highways. Frogs and salamanders that need to cross roads to reach ancestral breeding grounds often become nighttime mortalities, left on the road as food for crows and raccoons. These losses are not obvious to most people because scavengers often consume the carnage by daylight.
Amphibian road mortality studies were conducted at the Hawk Mountain Sanctuary in Berks County in 1997. On seven rainy nights during spring and early summer, more than 100 amphibians (including frogs) per night, representing up to 13 species, were recorded as mortalities on some three miles of road. When one considers the vast amount of road mileage that bisects or borders wetlands and forested areas in Pennsylvania, the potential losses of amphibians to highway mortality are staggering.
Research conducted in central Pennsylvania has unequivocally linked acid precipitation to the decline of wood frogs and several species of woodland salamanders. These amphibians require seasonal temporary ponds for reproduction and rearing of young. Temporary ponds do not contain water year-round, so they do not support fishes that could be predators on the larval amphibians. Thus, the survival rates of larvae would be expected to exceed those from ponds that do contain fish. However, many of these ponds occur in mountainous areas where the soil has a low pH (highly acidic), and where the soil is naturally poorly buffered. Thus, a further drop in pH caused by acid deposition causes the pH to drop to lethal levels, causing the eggs or larvae to die. Additionally, sublethal effects can cause deformities in the animals that do survive.
Depletion of the ozone layer has been shown to allow certain wavelengths of harmful ultraviolet (UV) radiation to reach ground levels. This radiation was probably filtered by the ozone layer to a greater extent in the past than it is today. Researchers in the northwestern United States have documented that damage to the DNA of certain amphibians can result from exposure to these radiation levels. Some species can withstand higher exposure levels than other species. Similarly, some species are able to repair radiation damage to their cells, while others cannot. The early theories are that those species in greatest decline may also be those that are less able to repair radiation-damaged DNA. Researchers are testing various amphibian species to determine if certain patterns are evident.
Another reason for the decline of many amphibian species is that herbicides, pesticides, and other chemicals in the environment have been linked to deformities and death in certain amphibians. Most amphibians and nearly all frogs breathe through their skin in addition to their lungs. They travel along the ground and are always in contact with the substrate. Because this is where many toxins accumulate, frogs are prime candidates for toxin-loading when harmful agents are found in their environment. Even though it may be difficult to determine which chemicals are harmful, just think of all the herbicides, pesticides, deicing agents, cleaners, and solvents that are in use today. Today's "harmless" chemicals could be tomorrow's DDT!
Other less obvious chemicals may be affecting our frogs. Recently, deformities in fish and amphibians have been linked to human female hormones that have found their way into the environment via birth control usage ("the pill") and the local sewer system.
Most recently, scientists working independently in the United States and Australia have discovered a new fungus genus known as a chytrid fungus. It is believed to be killing frogs and toads around the world. There are still several unanswered questions about this latest discovery. It is known that this fungus has been found on dead frogs. However, it is unclear whether this fungus caused death or appeared after the frogs were weakened by some other agent, such as ultraviolet radiation damage. Additional research is needed to be certain.
The research conducted so far suggests that the decline of frogs and other amphibians may not be caused by any single factor, but instead by a combination of environmental alterations. That may explain why some leopard frog populations still exist and others have disappeared completely.
The natural environment has been compared to the workings of an automobile. Certain parts of a car, like the wipers, radio, or some wiring, can be removed and the car will still runmaybe not as well as before, but it will still run. However, at some point a critical part, such as the fuel pump, battery or distributor coil, might be removed, thus causing the car to stop working. Perhaps we are damaging or removing critical parts in the northern leopard frog's environment. In some areas they are gone already. In other areas it may be only a matter of time. Hopefully, the right answers will be found before it's too late.