Review Date2010-09-29
Change Date2010-09-29
Edition Date2011-04-04
Edition AuthorsHammerson, G.
Threat ImpactVery high - high
Range Extent250-1000 square km (about 100-400 square miles)
Number of Occurrences1 - 5
Rank ReasonsThis frog occupies only 10-20 percent of the historical range and currently occurs in 8 natural and 5 translocation sites in Nevada. Primary threats include water diversions and developments, the presence of non-native predators and competitors, loss and fragmentation of habitat, and low numbers of individuals in metapopulations. Currently, no specific water developments or direct habitat losses are known that could result in impacts to the species, and the numbers of individuals and sites occupied by the frog are increasing through captive-rearing and translocation.
Range Extent CommentsThe known historical distribution includes springs, streams, and wetlands within the Virgin River drainage downstream from the vicinity of Hurricane, Utah; along the Muddy River, Nevada; and along the Colorado River from its confluence with the Virgin River downstream to Black Canyon below Lake Mead, Nevada and Arizona; all historical localities are at or within a few kilometers of these rivers, but this apparent restriction to the proximity of the main rivers may be partially an artifact of historical collecting activities (USFWS 2004). This species also may have occurred at lowland localities along the Colorado River upstream from the confluence with the Virgin River, but no known specimens exist from this area (Relict Leopard Frog Conservation Team 2005).
Relict leopard frogs are currently known to occur in two general areas in Nevada: near the Overton Arm area of Lake Mead, and Black Canyon below Lake Mead. Specimen records date back to 1936 at the Overton Arm area and to 1955 at Black Canyon. These two areas encompass maximum linear extents of only 3.6 and 5.1 kilometers, respectively (USFWS 2009). Relict leopard frog populations may possibly occur in other localized areas (USFWS 2009).
Two leopard frogs have been observed on different occasions in 2000 and 2001 at the fish hatchery at Willow Beach, Arizona, 10 km downstream from Bighorn Sheep Spring in Black Canyon; one of these frogs was collected and confirmed as R. onca based on mitochondrial DNA sequence similarity (C. Fiegel, pers. comm., 2001, cited by USFWS 2009). This individual was likely swept downstream from the occupied sites in Nevada.
Occurrences CommentsThe species is extant in only five locations in two general areas (Bradford et al. 2004, USFWS 2009). It occurs in multiple specific sites in each location. As of 2008, 13 sites (8 natural and 5 translocation sites) supported relict leopard frogs (egg masses observed at 10 of 13 sites) (USFWS 2009).
Threat Impact CommentsThe causes of the decline are not entirely clear, but suggested factors include alteration of aquatic habitat due to agriculture and water development, and the introduction of exotic predators and competitors (Jennings 1988, Jennings and Hayes 1994). The formation of Lake Mead in 1935 and Lake Mojave in 1951 inundated many river miles and adjacent associated wetlands and fragmented some of the remaining populations (USFWS 2009). Connectivity among the extant populations has almost certainly been dramatically reduced as a result of damming the Colorado River (USFWS 2009). The reduction in connectivity is a result of a wider waterbody created when the Colorado River was dammed, thus preventing frogs from moving from one side of the river to the other. Lake Mohave influences the river level such that the canyon floor is never exposed, predatory game fishes are present in the river, and water is continually cool as it emerges from Lake Mead (USFWS 2009). Moreover, wetland habitat has been converted to agriculture or urban development near the Virgin and Muddy Rivers in Utah, Arizona, and Nevada (USFWS 2004). Also, along the Virgin River, the hydrological regime has been substantially changed by upstream impoundments, diversions, and ground water pumping (BIO-WEST, Inc., 2001; USFWS 2009).
Two recent population extinctions occurred concomitantly with encroachment of emergent vegetation into pools; this may have occurred as a result of natural processes in one case, and anthropogenic processes in the other (Bradford et al. 2004).
Exotic species, which are often implicated as serious predators and competitors of native ranid frogs in the western U.S., have become widely distributed along the Virgin, Muddy, and Colorado Rivers. Included among these are the American bullfrog, many species of exotic fishes, and red swamp crayfish (Procambarus clarkii) (Jennings and Hayes 1994). These species potentially prey on all life stages of the relict leopard frog. Bullfrogs also negatively impact native amphibians through competition for prey and coversites. Crayfish and exotic fishes may be important predators on eggs and larvae of relict leopard frogs. [from USFWS 2002]
The relict leopard frog is further threatened by the low numbers of individuals within each population, some of which may not be viable. Amphibians are thought to have a metapopulation structure (i.e., groups of individuals inhabiting a system of habitat patches connected by migration across contiguous habitat). Populations that occur in isolated patches may be extirpated by stochastic events such that recolonization may not occur due to the distance of separation and absence of contiguous habitat. Genetic drift and inbreeding depression may also occur as a result of restricted gene flow associated with small, isolated populations, thus further threatening their persistence. [from USFWS 2002]
Chytridiomycosis, a fungal disease associated with population declines of various amphibian species, although not yet confirmed as a pathogen of relict leopard frogs, must be regarded as a potential threat.
Extirpations
Between 1991 and 1995, habitat change was conspicuous at Corral Springs. The pools that were initially largely open with scattered emergent vegetation became choked with emergent vegetation, primarily native Scirpus spp. By early summer of 1994, most of these pools had virtually no open water. Extirpation of leopard frogs from this site may have been the result of natural processes, because individuals may periodically colonize this site from Rogers Spring during wet periods after the site is scoured by flood waters, and populations may subsequently be extirpated due to shrinkage of aquatic habitat and vegetation encroachment as drier conditions prevail. The demise of the relict leopard frog at Corral Spring may also have been facilitated by the construction of a fence in 1991 to exclude feral burros from most of the site, an action that encouraged overgrowth of emergent vegetation. [from USFWS 2002]
As at Corral Spring, the demise of the population at Littlefield occurred concomitantly with loss of pool habitat due to rapid encroachment of emergent vegetation. Between 1992 and 2001, vegetation cover (primarily Scirpus spp.) had increased dramatically such that no pools of open water remained exposed except for the artificial pond. This rapid encroachment may have resulted from anthropogenic processes. Historically, prior to the establishment of reservoirs in the Virgin River watershed, the emergent vegetation at the Littlefield site would have been scoured periodically by flooding of the Virgin River. Until some years ago, vegetation in part of this area was kept open by light to moderate livestock grazing. Subsequently, with the absence of both flood action and grazing, emergent vegetation grew over virtually all the former open water at the site. Moreover, introduced bullfrogs, which may prey on the relict leopard frog, have become established in wetlands along this portion of the Virgin River (BIO-WEST, Inc., 2001). [from USFWS 2002]