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Arizona Leopard Frogs: Balanced on the Brink?

The low humidity, high summer temperatures, and other natural forces that helped shape the saguaro, spectacular canyonlands, and other familiar features of the Southwest would seem to be inhospitable to the highly aquatic frogs of the genus Rana, known as ranids. In spite of these conditions, leopard frogs were, until recently, common inhabitants of Arizona's wetland and riparian ecosystems; in fact, the Southwest boasts the greatest diversity of leopard frog species in the United States. The recent dramatic declines of some members of this species complex need to be examined and addressed immediately.

   

Establishing a Historical Baseline

 

In the Southwest, native members of the true frog family (Ranidae) include the leopard frogs and the Tarahumara frog. Arizona's leopard frog fauna, among the largest in North America, is surprisingly diverse and includes at least five (possibly six) native leopard frogs and one introduced species (Platz et al. 1990; Table). While it is unclear exactly when declines of these frogs began, they were first noticed during the 1970's, when the status of the Tarahumara frog was of concern. Population studies of the Tarahumara frog in the mid-1970's indicated that the future existence of populations of this species in Southeast Arizona and the northern Sonora could be in jeopardy (Hale and May 1983; Hale and Jarchow 1988; Hale 1992). During these studies, researchers suspected that native leopard frogs were also declining, but there were no baseline data to support this suspicion. One of the first studies to investigate the status of Southwest leopard frogs on a large landscape was that of Clarkson and Rorabaugh (1989), who surveyed 56 historical and 7 new localities between 1983 and 1987 for four species of native leopard frogs.

   

Although this seminal investigation concluded that all species of leopard frogs examined were declining, several points relevant to status determination and conservation planning were not addressed by the authors. First, by focusing on historical localities, the data set gathered by Clarkson and Rorabaugh (1989) was biased toward concluding that declines have occurred (P. Geissler, U.S. Geological Survey, Laurel, Maryland, unpublished report). Second, because the study was not specifically designed to identify new localities or systematically evaluate threats, this information, which is important to status assessment and recovery planning, was not discussed.

   

In an effort to gather such data, the Arizona Game and Fish Department began assessing the status and current distribution of all native Arizona ranids by conducting statewide visual encounter surveys (in the sense of Crump and Scott 1994) of historical and high potential habitats, and by recording detailed habitat data and herpetofauna observations. Because of the confused taxonomic history of this group (Hillis 1988) and the likelihood that specimens of this complex are misidentified even in museum collections (Jennings 1994), biologists at the Arizona Game and Fish Department gathered historical locality data from selected sources in the published literature or local museums whose collections reflect the current taxonomy. Our data base presently contains over 4,000 herpetofaunal observations, collected from more than 1,500 localities.

   

I present a preliminary analysis of our survey data for all species of Arizona ranids except the Tarahumara frog, whose status has been recently reviewed elsewhere (Hale et al. 1995). Presence or absence of frogs at historical localities has been determined since 1990, the beginning of our statewide survey efforts. Because large temporal differences exist in activity of many amphibians, determining their presence or absence from a locality is difficult and requires multiple visits. Corn and Fogleman (1984), studying northern leopard frogs in Colorado, suggested that if frogs or reproduction are not observed at a locality over a three- year period, a population can be considered extirpated. In most, but not all instances, I have determined the presence or absence of leopard frogs at a site by examining survey results from three or more visits to that site during times of peak activity (April through October).

   

Preliminary Status of Arizona Leopard Frogs

 

Rio Grande Leopard Frog

 

Researchers believe that this species was inadvertently released into the Colorado River near Yuma, Arizona, in the 1960's during sport-fish stockings (Platz et al. 1990). Although all native leopard frogs have declined in at least some part of their Arizona ranges, it is ironic that this nonindigenous leopard frog continues to expand its Arizona range and is now known from the Colorado River near Yuma, the Gila River up to its confluence with the Salt River, and the Gila, Salt, Agua Fria, and Hassayampa rivers and adjacent agricultural areas near Phoenix (M. J. Sredl, Arizona Game and Fish Department, unpublished data; J. C. Rorabaugh, U.S. Fish and Wildlife Service, Phoenix, Arizona, personal communication).

   

Plains Leopard Frog

 

Most of the range of the plains leopard frog occurs in the central and southern Great Plains, where it is an inhabitant of aquatic habitats in prairie and desert grassland ecosystems (Stebbins 1985). In Arizona, this species is restricted to the Sulphur Springs Valley in southeastern Arizona (Frost and Bagnara 1977), an area separated from the main portion of the range of this taxon by about 350 kilometers (Mecham et al. 1973). Specimens have also been collected from Ashurst Lake, southeast of Flagstaff in northern Arizona (J. E. Platz, Creighton University, Omaha, Nebraska, personal communication), but there have been no recent leopard frog records to verify this species at that site. Within the Arizona range of the plains leopard frog, Arizona Game and Fish biologists have conducted 98 surveys at 85 localities and found the species absent from nearly 93% of the localities surveyed historically (Table). Recent surveyors have only found this species at three localities, two of which were found after 1990.


Common name Number of localities/number of surveys Historical absent Historical present Historical unsurveyed New sites
Rio Grande leopard froga -- 0 1 1 9
Plains leopard frog 85/98 14 1 1 2
Chiricahua leopard frog 679/871 80 18 35 45
Relict leopard frogb --        
Northern leopard frog 477/566 24 2 21 29
Ramsey Canyon leopard frog --   1 0 5
Yavapai leopard frog 648/797 35 28 35 167

a Introduced to Arizona.
b No verifed Arizona records.

Table. For each species of Arizona leopard frog, numbers of localities visited, numbers of surveys conducted within the range of that species, and frequency within each status category are listed. The status of each locality was evaluated relative to pre- and post-1990 surveys.

Chiricahua Leopard Frog

 

Of all of Arizona's leopard frogs, the Chiricahua leopard frog has undergone perhaps the largest, most dramatic decline (Sredl and Waters 1995). The range of this species includes the southern edge of the Colorado Plateau in Arizona and New Mexico, southeastern Arizona, southwestern New Mexico, and the Sierra Madre Occidental in Mexico (Platz and Mecham 1979). The Arizona range of this frog consists of a northern part, which extends from the White Mountains and Mogollon Rim (the southern edge of the Colorado Plateau) in central Arizona, and a southern part in drainages associated with the Madrean oak woodlands and semidesert grasslands of the Madrean Archipelago (Sredl and Howland 1995). To evaluate the status of this frog, Arizona Game and Fish biologists have conducted 871 surveys at 679 localities, 98 of them historical. To date, the Chiricahua leopard frog appears absent from 82% of the historical localities surveyed since 1990 (Table). While surveyors found the Chiricahua leopard frog at 45 new sites, many of these observations consisted of as few as one or two frogs observed at localities adjacent to extant populations, making it likely that these individuals had dispersed from nearby populations.

   

Relict Leopard Frog

 

The relict leopard frog has the dubious distinction of being the first North American amphibian thought to have become extinct (Platz 1984) only to be rediscovered (Jennings 1993). No Arizona records of this species exist, but further clarification of relationships of southwestern leopard frogs may result in populations now classified as other species being included in this species.

   

Northern Leopard Frog

 

Until the late 1960's, all currently recognized leopard frogs, including those found throughout the Southwest, were classified as the species Rana pipiens, now known by the common name of northern leopard frog. During the 1960's, though, taxonomists realized that what was recognized as R. pipiens was really a multispecies complex (Hillis 1988). In Arizona, R. pipiens has been found in the lakes, earthen tanks, springs, creeks, and rivers of the Colorado Plateau in the northeast portion of the state. During these surveys, Arizona Game and Fish biologists visited 477 localities and conducted 566 surveys within the Arizona range of this frog, and found it at 8% of the historical localities visited. Although Arizona Game and Fish biologists found 29 new sites of occurrence, few of these populations are large. Within Arizona, there are probably fewer than five metapopulations, many of which are small.

   

Ramsey Canyon Leopard Frog

 

The Ramsey Canyon leopard frog is the most recently described of Arizona's leopard frog species (Platz 1993). This frog is known only from the Huachuca Mountains in southeastern Arizona, where six populations have been found in three drainages. A conservation team composed of agency and academic biologists as well as biologists from private institutions is developing a conservation agreement that is expected to be implemented soon. The continued viability of this species will depend on swift conservation actions by this group.

   

Yavapai Leopard Frog

 

Within the Southwest, the Yavapai leopard frog occurs in aquatic systems from desert scrub to pinyon-juniper habitats (Platz and Frost 1984). While the continued existence of this species in some parts of its range appears uncertain (Jennings 1995; Jennings and Hayes 1995), the status of this species in central Arizona seems good. Our intensive surveys of the range of this species have revealed that the species was present in 44% of the historical localities we visited. This percentage of occupancy of historical localities is by far the highest of any species of Arizona leopard frog. In addition, this frog has been found at 167 new localities, mostly in central Arizona (Table).

   

Mitigating Threats and Searching for Solutions

 

Analysis of historical and recent information reveals that the status of Arizona's six native ranids falls along a spectrum of endangerment. Although none is federally listed as threatened or endangered, two are candidates or species of special concern, and if declines continue, more species will be added. A general approach to stabilizing these declines or initiating recovery must incorporate traditional wildlife management techniques and techniques from endangered species recovery efforts. To have the greatest chance of success we need to begin formulating these plans now. Conservation and management activities should be implemented long before they become actions of last resort (Griffith et al. 1989).

   
  Author
Michael J. Sredl
Arizona Game and Fish Department
2221 West Greenway Road
Phoenix, Arizona 85023-4399

References


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