The critically endangered dusky gopher frog, Lithobates sevosus
Amphibian species worldwide are in rapid decline. One of the most common reasons for this decline is the spread of the chytrid fungus. The dusky gopher frog Lithobates sevosus is more threatened than any other species of amphibian found in the United States; L. sevosus has been listed as a Critically Endangered Species by the IUCN Red List of Threatened Species and as Endangered by the US Endangered Species Act. However, in contrast with many other amphibian species L. sevosus is not affected by the chytrid fungus. Therefore it is important to understand the unique requirements of this species in each stage of its life cycle as well as the variety of factors to which it is vulnerable. The synonym Rana sevosa is commonly encountered in the historical literature on the taxon dusky gopher frog.
Terrestrial surroundings are vital to amphibians as they provide habitat outside of the breeding season, in addition to a means of migration to breeding ponds. During the breeding season, Lithobates sevosus requires temporary, upland ponds. For the remainder of the year this species occupies longleaf pine ecosystems, which are maintained by periodic fires. While in the terrestrial habitat, L. sevosus utilizes burrows, predominantly those excavated by the gopher tortoise, Gopherus polyphemus. These burrows provide dusky gopher frogs with protection from predators and unfavorable weather conditions, and also serve as a source of food supply. The suppression of natural fires has caused modification in the longleaf pine ecosystem by promoting tree growth where they formerly did not occur. This alteration contributes to a decreased density of available burrows which negatively impacts L. sevosus and other species that depend on these refuges.
Demography and distribution
There are two known, documented populations of dusky gopher frogs. The more well-known population consists of approximately 100 adults and is located at Glen’s Pond in Harrison County, Mississippi. The second population is located at Mike’s Pond in Jackson County, Mississippi. These two populations are geographically isolated from one another at a distance of about 32 km.
In 2000, the dusky gopher frog was found throughout the region known as the Coastal Plain in the Florida panhandle and extended throughout Alabama and Mississippi. By the following year the distribution had been critically reduced to a single known location in De Soto National Forest in Harrison County, Mississippi. The Mike’s Pond population, which consists of fewer than fifty individuals, may no longer exist.
Though the dusky gopher frog is critically endangered, the southern leopard frog Lithobates sphenocephalus is neither reduced nor endangered. The possibility of competition between the southern leopard frog, which is widely distributed throughout the southeastern United States, and the dusky gopher frog has been considered as a cause for decline. However, there has been no evidence found to support the hypothesis, as observations indicated an increase in the survival of dusky gopher frog tadpoles in aquatic enclosures with southern leopard frog tadpoles.
The dusky gopher frog breeds in temporary, upland ponds. Outside of the breeding season they occupy nearby longleaf pine forests and utilize burrows made by small mammals and the gopher tortoise as well as holes left by tree stumps. This species has been observed at rest outside of selected burrows in the soil and in direct sunlight. Observations of L. sevosus movements outside of the breeding season found that adult individuals travel relatively short distances of under 300 m and most of their movements are associated with rainfall events. Less information is known on the movement behaviors of juvenile dusky gopher frogs, though observations of the closely related Rana capito have shown juveniles to emigrate much greater distances. Since gopher frogs breed from winter to early spring, L. sevosus does not hibernate.
The reproductive success of the breeding season may depend on the water levels and length of time that such levels are maintained. A hydroperiod must have a duration of at least 195 days in order for L. sevosus tadpoles to complete metamorphosis. An inefficient length of time likely results in tadpole mortality. The cause of a short hydroperiod is often linked to an insufficient amount of annual rainfall.
Many amphibians have an assortment of antimicrobial peptides in their skin secretions that serve as a method of defense which are released as a response to injury. Seven histamine-releasing peptides of the four families brevinin-1, ranateurin-2, esculentins-1 and esculentins-2 are present in the skin secretions of the dusky gopher frog. These peptides were shown to hinder growth of two strains of bacteria, M. luteus and E. coli. While there were no differences in antimicrobial behaviors of peptides between male and female specimens, an additional histamine-releasing peptide was observed in female skin secretions. An immunomodulatory peptide Tyrosine Arginine, the second member of a newly discovered class of peptides associated with formation of various types of blood cells was also isolated in L. sevosus. Genetic variation of the single remaining L. sevosus population has been compared to two closely related species, Rana capito and Rana areolata, since no other metapopulations of L. sevosus were available for comparison.
Genetic variation was found to be lower in L. sevosus than the non-isolated populations they were compared to. L. sevosus had less genetic variation in individual loci and both lower observed heterozygosity, referring to having two or more genes on the same chromosome, and expected heterozygosity for six of seven loci. L. sevosus was the only species of the three to show an allele frequency shift as a result of losing low-frequency alleles that indicated a recent bottleneck occurred in this species.
Human activities, such as deforestation, development and controlled burning are known to have negative effects on amphibians. The longleaf pine ecosystem is maintained by periodic fire, so mortality due to fires is not of main concern. The habitat around Glen’s Pond has been clearcut and there are indications that this area was utilized by the dusky gopher frog prior to alteration but has been avoided since. The development of a retirement community 200 miles north of Glen’s Pond was initiated in 2001, in addition to the clearcut site. Other construction plans have been proposed in nearby areas that include a dam, sewage treatment plant and highway expansion. It is likely that the distribution of the dusky gopher frog has been reduced by surrounding areas of development. This species may also be exposed to fertilizer run-off and affected by recreational activities.
Conservation and management
Movements of L. sevosus outside of the breeding season suggest that adjacent forests be protected in addition to the breeding area. A buffer zone of 1000 m of land around ponds may be sufficient to incorporate the longleaf pine ecosystems. There are two ponds near Glen’s Pond which are not being utilized by the dusky gopher frog but could potentially be colonized if these areas are protected from human development, runoff, and other disturbances.
Movements of the gopher frog may be a good comparison to determine if emigration behavior of juvenile frogs are affected by fire-suppressed habitat within a longleaf pine ecosystem. Juvenile gopher frogs are more likely to utilize areas maintained by fires and avoid areas where hardwoods occurred, a characteristic of a fire-suppressed area. Areas maintained by fires often contain a greater density of available burrows than areas affected by fire-suppression. Juveniles may return to natal pond if suitable terrestrial habitat is unavailable or inaccessible. Movement behaviors of R. capito also suggest that amphibians may utilize unpaved roads during emigration, which cause the frogs to become more vulnerable to vehicles.
L. sevosus tadpoles have a higher success rate in open-canopy temporary ponds than shaded areas. Since these types of ponds rarely occur in Mississippi, prescribed fires may be used to open up closed-canopy ponds. This approach would have to be done carefully as to avoid negative impact on the habitat and requires that this be done in an area that is historically open.
The utilization of well water to maintain levels of Glen’s Pond is a potential solution for the premature decrease in water levels. There have been successful studies with the supplementation of well water, though further research is required before this method can be implemented as a conservation strategy.
The loss of genetic diversity in the isolated Glen’s Pond population of L. sevosus suggests the necessity for conservation strategies to focus on an increase in genetic variation. To do so, egg masses raised in captivity should be sampled and selected for genetic diversity by genotyping a small number of eggs in each mass before being introduced into Glen’s Pond. Individuals of the Mike’s Pond population could also be transplanted into Glen’s Pond to increase genetic variability. Since it is not known whether or not the Mike’s Pond population persists, the area should continue to be surveyed for dusky gopher frog individuals. Captive populations could also be transplanted into the wild since wild populations are drastically reduced.
Since the survival of this species depends on the availability of longleaf pine habitats, it is vital to educate the public on the importance of protecting these areas from urban development. Besides education, the most important method of management of L. sevosus is to protect the entire inhabited area, meaning Glen’s Pond and the surrounding forests. Further research should be done on the movement patterns and behavior of dusky gopher frogs to ensure the entire inhabited area is protected from development and other human activities. Since this species has been shown to do poorly in response to human activity, it is vital to provide a habitat with limited human interaction. L. sevosus is also sensitive to chemical runoff, so the protected area must be far enough from development that runoff does not enter the buffer zone.
Further research should also be done to find any possible remaining individuals from the Mike’s Pond population. In addition, suitable habitats located nearby either of the two ponds must be searched for dusky gopher frog individuals. Since the distribution of this species once extended throughout the southern United States, suitable habitats in their former range should be found for population reintroduction. To increase genetic diversity, individuals from the Glen’s Pond location and those raised in captivity could be relocated to Mike’s Pond or other suitable habitats within the protected area. Studies must be done prior to translocation to ensure the potential habitats meet the requirements of L. sevosus and that their presence would not disrupt other species.
Decreased hydroperiod poses a major threat to the reproductive success of L. sevosus, so it is vital to pursue the possibility of well water usage to supplement pond levels. Further research should focus on the rate of survival in individuals following metamorphosis in well water and potential effects of pH alteration. This method should be repeated in natural settings as well as in a laboratory setting to explore all potential outcomes. Since L. sevosus is a Critically Endangered species, closely related species should be used in place during experimental procedures that require the use of live specimens. Previous experiments have used leopard frog tadpoles in place of dusky gopher frogs, since this species is not currently in decline.
A controlled fire regime must be thoroughly researched before being implemented. The objective of conservation for this species should focus on protecting suitable habitat without degrading the habitat or negatively affecting other species. Longleaf pine ecosystems are necessary for the survival of dusky gopher frogs yet this habitat has been up to 98% reduced due to fire-suppression. Since natural fires do not occur frequently enough to support the open-canopy habitat preferred by L. sevosus, a well-planned fire regime, in addition to tree removal, could benefit this species.
Several factors are responsible for the reduction of the population of the dusky gopher frog. Therefore, no single conservation method can be expected to reverse the population decline. The integration of several well-planned methods significantly increases the potential for this species to persist. After efforts have been made to understand the unique requirements of the species, a strategy must be developed that focuses on maintaining suitable habitat for each life stage of L. sevosus and to increase genetic diversity. Also, the importance of public education on species decline should not be overlooked in the design of a conservation plan for this unique species.
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