R. pipiens faces a number of threats.
One identified threat is chemical contamination, particularly from agricultural enterprises.
One of the most extensively cited contaminants is the herbicide atrazine, the most commonly used herbicide in the United States and one of the most widely used throughout the world. The breeding season for R. pipiens follows the annual peak period for the application of atrazine in the U.S., and thus coincides with the annual peak of atrazine contamination of water sources (Hayes et al. 2002). In a laboratory study of the effects of water-borne atrazine on R. pipiens, Hayes et al. found that 10-92% of exposed males developed gonadal abnormalities such as retarded development and hermaphroditism. Hermaphroditism was also found in wild R. pipiens specimens collected in a transect from Utah to Iowa. The Hayes study found males with testicular oocytes in all areas where local atrazine sales exceeded .4 kilograms per square kilometer, and water-borne atrazine exceeded 2 parts per billion.
It is believed that atrazine may induce the production of an enzyme that converts androgens into estrogens, and can cause males to produce estrogens at the expense of androgens. This would explain both the presence of oocytes and the inhibition of spermatogenesis. At one site in the Hayes study, a wild population exposed continuously to atrazine exhibited fewer abnormalities than a population exposed intermittently, leading to the suggestion that adaptive resistance may be occurring in the more frequently exposed populations. Atrazine is a significant threat to R. pipiens, and possibly to amphibians in general, because most water sources in the U.S., including rain, contain atrazine at higher levels than those needed to induce abnormalities in laboratory specimens (Hayes et al 2002).
Another agriculture-related threat to R. pipiens is nitrate contamination of water sources. In a laboratory study, tadpoles of four amphibian species were exposed to levels of nitrate commonly exceeded in agricultural areas around the world. Effects varied across the species, but included reduced activity, lower rates of metamorphosis, and physical abnormalities (Hecnar 1995).
Some environmental conditions do not directly cause death, but may induce behavior that increases the likelihood of predation by other species. Exposure to organochlorines, the remnants of such pollutants as DDT, has been suggested as a possible cause of population declines. While organochlorines do not appear to have the same dramatic effects on amphibians as they do on large animals such as predatory birds, exposure to these compounds does appear to induce behavior that might cause a population to decline. In a laboratory study, R. pipiens tadpoles exposed to organochlorines tended to spend more time resting and less time feeding, behavior that could reduce the ability of tadpoles to consume scarce resources. Organochlorines may also affect the production of certain hormones that help R. pipiens respond to local environmental changes such as pond drying. Most products that contain organochlorines are now banned, but their derivatives are common and persistent pollutants even today (Glennemeier et al. 2001).
The decline of R. pipiens in areas where it used to thrive has also been attributed in recent years to infectious diseases, whose prevalence may be exacerbated by environmental stresses such as acidification (Brodkin et al. 2003). Acidification is a problem for this species because the breeding season, during which the frogs spend a great deal of time in the water, coincides with the highest levels of acidity in the lakes and streams of the northeastern United States. The breeding season also directly follows the period of winter hibernation, during which cold-exposure weakens the immune system of frogs. In a laboratory study by Brodkin et al., both the degree of cold exposure and the level of acidity to which frogs were exposed correlated to the health of the frogs immune systems. Frogs exposed to acid, and frogs exposed to acid after being exposed to cold, demonstrated higher levels of bacterial colonization of the spleen and higher rates of mortality. Brodkin et al. conclude that acidic conditions of pH 5.5 and below contribute to this contamination by: 1) damaging the intestinal epithelium, thereby allowing bacteria to pass from the intestinal tract to the bloodstream and spleen, and 2) reducing the number and viability of white blood cells. Cold exposure alone did not damage frogs immune systems, and pH levels of 6.0 or above, while damaging to the intestinal epithelium, were not sufficient to induce high levels of mortality. While it is fairly certain that acid exposure is a threat to R. pipiens, it is not clear why it has become a problem in the last twenty years or so (Brodkin et al. 2003).
There has been a flurry of reports in recent years on the widespread prevalence of limb deformities among many species of frogs, even those in seemingly pristine environments. R. pipiens is one of the species most commonly reported to exhibit such deformities. A number of hypotheses have emerged to explain this trend.
One suggestion is that increased exposure to ultraviolet light may be responsible, because ultraviolet light is known to cause damage to cellular DNA. The validity of the hypothesis when applied to conditions in the wild requires further study. Preliminary work has been done to assess how UVB light penetrates aquatic environments, and to determine what other environmental factors affect amphibians exposure to that light (Peterson et al. 2002).
Another hypothesis suggests that environmental stresses have increased the vulnerability of amphibians to parasites such as trematodes (Schothoeffer et al. 2003). In a laboratory study of the interaction between R. pipiens tadpoles and the larvae of the trematode Ribeiroia ondatrae, infection of the frog tadpoles by R. ondatrae led to a number of different types of malformations. Depending on the stage of development at which the tadpole is infected and the intensity of the infection, the developing R. pipiens may develop extra limbs, digits, or phalanges; its limbs or digits may be smaller than normal; bones may bridge, skin may web, and the ilium may be reduced or misshapen. The timing of infection appeared to be crucial, as R. pipiens exhibits different levels of vulnerability as it develops. It appears to be most vulnerable at the larval stage, pre-limb bud stage, and limb bud stage. Infections at the paddle stage appear to have no effect on limb development. In addition to mortality due to limb malformations, infected tadpoles may also die as a result of the infections themselves. It is not yet known what environmental factors relate to the timing of infections, or what factors affect the length of amphibian larval periods and tadpole vulnerability (Schothoeffer et al. 2003).
It is also possible that R. pipiens is suffering decline due to the type of fungal infections that have been found responsible for the decline of other species. The chytrid fungus Batrachochytrium dendrobatidis, for example, was found to cause oral abnormalities in the species Rana muscosa, a species in the Sierra Nevada that has drastically declined recently (Fellers et al. 2001).
Introduced species may also be contributing to the decline of this species (Lannoo et al. 1994). A 1994 study investigated amphibian populations in Dickinson County, Iowa, and found that since a study of the same area in 1923, several populations had declined and two had disappeared. In Dickinson County around the year 1900, R. pipiens was collected and exported at a rate of about 20 million specimens per year. In their 1994 report, Lannoo et al. reported an estimated population of about 50,000 for Dickinson County. They cite the introduction of the common carp and predatory bullfrogs as a partial reason for the decline, in addition to disturbance and loss of habitat (Lannoo et al.).
On the subject of habitat loss, James P. Gibbs reports that by the late 1980s, the lower 48 states had lost 53% of the wetlands that existed in the 1780s. Naturally, water-dependent species such as R. pipiens are negatively affected (Gibbs 2000).
In a related matter, R. pipiens decline has been attributed to vehicular traffic, particularly in areas where wintering areas are separated from breeding areas by roads. The full extent of this risk has not been established (Linck 2000).
