THE SOUTHWESTERN NATURALIST 49(2):218–228 JUNE 2004 POPULATION STATUS AND DISTRIBUTION OF A DECIMATED AMPHIBIAN, THE RELICT LEOPARD FROG (RANA ONCA) DAVID F. BRADFORD,* JEF R. JAEGER, AND RANDY D. JENNINGS United States Environmental Protection Agency, Landscape Ecology, Branch, P.O. Box 93478, Las Vegas, NV 89193-3478 (DFB) Department of Biological Sciences, University of Nevada, 4505 Maryland Parkway, Las Vegas, NV 89154-4004 (JRJ) Department of Natural Science, Western New Mexico University, P.O. Box 680, Silver City, NM 88062 (RDJ) *Correspondent: Bradford.David@epa.gov ABSTRACT The relict leopard frog (Rana onca) was once thought to be extinct, but has recently been shown to comprise a valid taxon with extant populations. We delineate the minimum his- torical range of the species based on records from 24 localities, report the extinction of 2 of 7 populations extant in the 1990s, and estimate total population size. The 5 remaining populations occurred in 2 areas: near the Overton Arm of Lake Mead and in Black Canyon along the Colorado River below Lake Mead, Nevada. These 2 areas are only 3.6 and 5.1 km long, respectively. The 5 extant populations inhabited spring systems with largely unaltered hydrology and no introduced American bullfrogs (Rana catesbeiana) or game fishes. In a mark-recapture study conducted in the Overton Arm area, the estimated number of adult frogs averaged 36 over 555 m of stream habitat, and estimated annual survivorship of adults averaged 0.27. A single mark-recapture estimate for the size of the largest population within Black Canyon, at a site approximately 450 m in length, was 637 adult frogs. An estimate for the total number of frogs at all sites, based on mark-recapture data, visual encounter surveys, and extent of habitat, was approximately 1,100 adults (range 693– 1,833). The 2 recent population extinctions occurred concomitantly with encroachment of emer- gent vegetation into pools. We speculate that this occurred as a result of natural processes in one case, and anthropogenic processes in the other. RESUMEN La especie Rana onca previamente fue considerada extinta, pero recientemente se ha demostrado que es un taxo ´n va ´lido con poblaciones actuales. Delimitamos el rango mı ´nimo histo ´ rico de distribucio ´n de la especie basado en registros en 24 localidades, registramos la extin- cio ´n de dos de las siete poblaciones presentes durante los an ˜os de 1990 y estimamos el taman ˜o total de la poblacio ´n. Las cinco poblaciones restantes se encontraron en dos a ´reas: en la vecindad del brazo de Overton del Lago Mead y en el Can ˜o ´n Negro a lo largo del rı ´o Colorado rı ´o abajo del Lago Mead, Nevada. Estas dos a ´reas son de longitud de so ´ lo 3.6 y 5.1 km, respectivamente. Las cinco poblaciones actuales habitaron un sistema de manantiales sin perturbaciones y sin la rana exo ´tica rana toro (Rana catesbeiana) u otros peces exo ´ ticos. En un estudio de marca-recaptura conducido en el a ´rea del brazo de Overton, el nu ´ mero estimado de ranas adultas promedio ´ de 36 en una distancia de 555 m de ha ´bitat ripario y la supervivencia anual estimada de adultos promedio ´ de 0.27. Una sola sesio ´ n de marca-recaptura estimo ´ que el taman ˜o de la poblacio ´ n ma ´s grande del Can ˜o ´n Negro, en un sitio de aproximadamente 450 m de longitud, fue de 637 ranas adultas. Un ca ´lculo para el nu ´ mero total de ranas en todos los sitios, basado en registros de marca- recaptura, muestreos visuales y extensio ´n del ha ´bitat, fue de aproximadamente 1,100 adultas (con un rango de 693–1,833). La extincio ´ n de dos poblaciones coincidio ´ con la invasio ´n de vegetacio ´n emergente en charcos. Especulamos que esto ocurrio ´ como resultado de procesos naturales en un caso y procesos antropoge ´nicos en otro. The relict leopard frog (Rana onca), once thought to be extinct, comprises a valid taxon with extant populations ( Jaeger et al., 2001). The historical range of this frog, as described in earlier literature (e.g., Jennings, 1988; Jen- nings and Hayes, 1994), has been confused be- cause of taxonomic uncertainties regarding various leopard frog populations within the re- gion, but genetic and morphological evidence indicates that it occurred at least within the