Diets of Newly Metamorphosed Amphibians in West Texas Playas

Although amphibians are the dominant vertebrate inhabiting playa wetlands during spring and summer in the Southern Great Plains, little is known about their diet and role in playa trophic structure. Because new metamorphs are more numerous than other terrestrial life stages of amphibians, we describe the diets of recently (,2 weeks) metamorphosed Great Plains toad (Bufo cognatus), barred tiger salamander (Ambystoma tigrinum), and New Mexico spadefoot (Spea multiplicata). Cultivation has greatly altered most playa watersheds (.95%); therefore, we also examined this influence on diets of Great Plains toad metamorphs. Diets of all 3 species were dominated by coleopteran taxa, and diversity of prey taxa did not vary among species. Amount of specific prey consumed varied among species, with Great Plains toads preying more on scarab beetles and formicid ants than New Mexico spadefoots and tiger salamanders. New Mexico 258 vol. 49, no. 2 The Southwestern Naturalist spadefoot metamorphs consumed more chrysomelid and elaterid beetles than tiger salamanders or Great Plains toads. Given the high numerical abundance of metamorphs, they likely have a strong influence on secondary production in playas. Great Plains toad metamorphs in playas with grassland watersheds had a much more diverse diet than those using playas with cultivated watersheds. Conservation of amphibians in the Southern Great Plains should focus on preserving playas with native grassland watersheds or restoring those with cultivated watersheds. RESUMEN Los anfibios son los vertebrados más abundantes durante la primavera y el verano en los lagos playa de las Grandes Planicies Sureñas. Sin embargo, poca información existe sobre su dieta y posición en la estructura trófica. Porque los recién metamorfoseados son más abundantes que los otros estadios terrestres de anfibios, describimos las dietas de animales recientemente metamorfoseados (,2 semanas) de las especies del sapo de las Grandes Planicies (Bufo cognatus), la salamandra tigre (Ambystoma tigrinum), y el sapo montı́cula de espuela (Spea multiplicata). El cultivo de la tierra ha alterado de gran manera el hábitat de la mayorı́a de los lagos playa (.95%), por lo que también examinamos el efecto de este factor sobre las dietas de metamorfoseados de B. cognatus. La presa principal de las tres especies fue coleóptera y no hubo diferencias entre especies en la diversidad de presas. La cantidad de presas especı́ficas consumidas sı́ varió. Bufo cognatus consumió más escarabajos escárabos y hormigas formı́cidas que S. multiplicata y A. tigrinum. Los recién metamorfoseados de S. multiplicata consumieron más escarabajos crisomélidos y elatéridos que A. tigrinum o B. cognatus. Debido a la gran cantidad de metamorfoseados, hay una alta posibilidad que ellos tengan una influencia fuerte sobre la producción secundaria en los lagos playa. Metamorfoseados de B. cognatus que residen en lagos playa con cuenca pastizal tuvieron una dieta más diversa que los en lagos playa con cuenca de cultivos. Esfuerzos para la conservación de anfibios en las Grandes Planicies Sureñas deben enfocarse en la preservación de lagos playa con cuencas de pastizales nativas o en la restauración de los que han sido alterados. Playa wetlands are the primary wetland habitat for amphibians in the Southern Great Plains (Anderson et al., 1999a). Playas are shallow (generally ,2 m), depressional wetlands that only receive water from precipitation or runoff and lose water only through evaporation, transpiration, and recharge (Smith, 2003). There are approximately 25,000 to 30,000 playas in the region (Osterkamp and Wood, 1987), although they only occupy about 2% of the extensively cultivated landscape (Haukos and Smith, 1994). Given the semi-arid climate of the region, water availability in playas for successful reproduction of amphibians is highly variable on a seasonal and annual basis (Bolen et al., 1989). Cultivation of the watershed surrounding playas has further influenced the availability of water in playas by increasing sedimentation and altering hydroperiods (Luo et al., 1997). These influences on hydroperiod can affect population and community attributes of the entire amphibian assemblage (Skelly, 1997; Gray, 2002). Although amphibians are the most numerous vertebrate group inhabiting wet playas of the Southern Great Plains during spring and summer (Smith, 2003), their importance in playa trophic structure is unknown. Great Plains toad (Bufo cognatus), New Mexico spadefoot (Spea multiplicata), and barred tiger salamander (Ambystoma tigrinum) are typically dominant (Anderson et al., 1999a; Gray, 2002). Information on the diets of these species is necessary to better understand amphibian life history, the importance of amphibians in playa wetland trophic structure, and the potential influence of landscape modification on amphibian population and community structure (e.g., Toft, 1981; Beebee, 1996; Anderson et al., 1999b). The objectives of this study were to describe and compare the diets of recently emerged sympatric barred tiger salamander, Great Plains toad, and New Mexico spadefoot metamorphs in playa wetlands from West Texas, and to examine potential influence of anthropogenic landuse on diets of metamorphs of the Great Plains toad. Moreover, we selected new metamorphs because they emerge in large numbers (e.g., .10,000/night/6-ha playa; Gray, 2002) and are more numerous than adults, potentially intensifying competitive relationships among species at this life stage (Werner and Gilliam, 1984; Flowers and Graves, 1995). Because the historical landscape of the Southern Great Plains was primarily shortgrass prairie (Haukos and Smith, 1997)