Breeding Phenology of the Lowland Leopard Frog (Rana yavapaiensis): Implications for Conservation and Ecology

Abstract

We monitored breeding phenology and population levels of Rana yavapaiensis by use of repeated egg mass censuses and visual encounter surveys at Agua Caliente Canyon near Tucson, Arizona, from 1994 to 1996. Adult counts fluctuated erratically within each year of the study but annual means remained similar. Juvenile counts peaked during the fall recruitment season and fell to near zero by early spring. Rana yavapaiensis deposited eggs in two distinct annual episodes, one in spring (March-May) and a much smaller one in fall (September-October). Larvae from the spring deposition period completed metamorphosis in early summer. Over the two years of study, 96.6% of egg masses successfully produced larvae. Egg masses were deposited during periods of predictable, moderate stream flow, but not during seasonal periods when flash flooding or drought were likely to affect eggs or larvae. Breeding phenology of Rana yavapaiensis is particularly well suited for life in desert streams with natural flow regimes which include frequent flash flooding and drought at predictable times. The exotic predators of R. yavapaiensis are less able to cope with fluctuating conditions. Unaltered stream flow regimes that allow natural fluctuations in stream discharge may provide refugia for this declining ranid frog from exotic predators by excluding those exotic species that are unable to cope with brief flash flooding and habitat drying. RESUMEN-Investigamos la fenologia de reproducci6n y los niveles poblacionales de Rana yavapaiensis con censos mfiltiples de masas de huevos y con observaciones directas en Agua Caliente Canyon, cerca de Tucson, Arizona, de 1994 a 1996. El nfimero de adultos fue variable cada afio del estudio pero los promedios anuales fueron similares. El nuimero de juveniles fue mais alto en la 6poca otofial de reclutamiento y baj6 casi hasta cero en la primavera temprana. Rana yavapaiensis puso huevos en dos 6pocas distintas del afio, una en la primavera (marzo a mayo) y una mais pequefia en el otofio (septiembre a octubre). Larvas de la puesta de la primavera llegaron a la metamorfosis temprano en el verano. En los dos afios del estudio, el 96.6% de las masas de huevos produjeron larvas. Las ranas pusieron huevos en periodos cuando el arroyo tenia corriente moderada pero no en los periodos de sequia ni de inundaci6n. La fenologia de reproducci6n de R. yavapaiensis conviene para la vida en los arroyos del desierto con patrones naturales de flujo, que incluyen frecuentes inundaciones y sequias predecibles. Los depredadores ex6ticos de R. yavapaiensis soportan menos las condiciones fluctuantes. Arroyos con fluctuaciones naturales de inundaciones y sequias pueden proveer refugios de depredadores ex6ticos para esta rana en decremento por excluir a las especies ex6ticas que no pueden soportar estas condiciones. Reports of amphibian population declines around the world have focused scientific attention on the paucity of ecological and natural history information available for most species. As species ranges collapse, small and isolated population remnants become increasingly important for species survival and maintenance of genetic diversity. Population declines have been reported for all native frogs of the genus Rana in Arizona, and local extinctions have been observed in apparently suitable habitats (Clarkson and Rorabaugh, 1989; Sredl, 1997). Causes of ranid frog declines have been elusive despite much attention. Explanations for declines in the desert Southwest have included predation by exotic species (Vitt and Ohmart, 1978; Rosen et al., 1996a), pollution from smelters (Hale and Jarchow, 1988; Clarkson and Rorabaugh, 1989), and habitat salinification (Jennings and Hayes, 1994) and desertification (Rosen et al., 1996a, 1996b). A combination of these and other causes may be to blame (Kiesecker and Blaustein, 1995). Limited natural history data are available for This content downloaded from 150.135.115.53 on Thu, 3 Jul 2014 11:06:36 AM All use subject to JSTOR Terms and Conditions 268 The Southwestern Naturalist vol. 45, no. 3 the declining species and reproduction and population dynamics of leopard frogs (Rana pipiens species complex) in the southwestern United States are poorly known. Rana yavapaiensis is a medium sized member of the R. pipiens species complex (mean snout-vent length [SNV] = 59.7 mm; Platz and Frost, 1984) that formerly occurred in southeastern California and southern Arizona in the United States and northern Sonora, Mexico (Stebbins, 1985). It has been extirpated from nearly half of its historic geographic range (Clarkson and Rorabaugh, 1989), remaining abundant only in central Arizona, south and west of the Mogollon Rim. Why this species has persisted in some regions, while other native ranids in Arizona are severely threatened, is a key question we address here. Little has been published on the ecology of R yavapaiensis, especially regarding small, isolated populations. Collins and Lewis (1979) reported on collections of tadpoles and egg masses over a two-year period at several localities in central Arizona. Frost and Platz (1983) reported finding egg masses from late February through late April and in October. No data have been published on population dynamics or egg mass success rates of lowland leopard frogs and there are no precise data on reproductive timing. We report observations over a two-year period on a population of R. yavapaiensis occurring in a semi-desert canyon bordering suburban Tucson, Arizona. Conditions at this site are characteristic of many current refuges for leopard frogs in Arizona which have relatively small, isolated populations. METHODS AND MATERIALS--We studied a single R. yavapaiensis population in a 2 km segment of Agua Caliente Canyon in the Santa Catalina Mountains of southern Arizona (Pima Co.) near Tucson, from February 1994 to November 1995 and again in June 1996. Elevation at the site ranges from 850 to 940 m. Vegetation is transitional between Arizona Upland Sonoran Desert scrub (Brown and Lowe, 1980) at lower elevations, and a sparse oak-juniper-grassland assemblage at higher elevations and on northfacing slopes. Canyon walls consist of granitic cliffs and steep slopes with numerous rock slides and boulder piles. A narrow strip of riparian habitat occupies the canyon bottom along a seasonally intermittent stream. Here riparian-associated trees such as velvet ash (Fraxinus velutina), netleaf hackberry (Celtis reticulata), willows (Salix), and Arizona cottonR. yavapaiensis II[ I III III l l fil |ll IIIIL1 r AAI AA l Mll ll11111lf1111 11