Effect of land use on the community composition of amphibians in playa wetlands

Populations of amphibians are declining world-wide, likely as a consequence of anthropogenic (e.g., pollution, habitat alteration and loss, global climate change) and natural (e.g., global climate change, diseases, and invasive species) changes in the landscape. A number of natural and anthropogenic stressors may affect the distribution and abundance of amphibians on the Southern High Plains (SHP). Common stressors on the SHP include spatial and temporal variation in precipitation and temperature, the presence of pesticides, and deposition of sediment in playas. Sedimentation reduces hydroperiod of playas and is likely the most significant anthropogenic stressor on amphibians in playas, and a stressor that logically interacts with climatic variation. However, the interactions of these factors and their effects on amphibian communities are largely unexplored. In light of continued intensive agriculture and projected global climate change, there is a need to better understand the driving factors affecting amphibians in playa ecosystems. Objectives of this study were to: (1) determine the relationship between land use (cropland vs. native grassland), hydrologic factors, habitat characteristics, playa density, and the amphibian community (richness and composition). Two sets of 40 playas were studied in 2003 and 2004, evenly split between land uses (i.e., cropland and grassland). Playas were sampled every two weeks following initial inundation until dry or October 15 to determine presence/absence of amphibians. Vegetation was sampled during the month after initial inundation. Playa characteristics and sedimentation were measured once playas were dry. Amphibian species richness was different between years, but not land uses. Land use was not a good predictor of amphibian species richness (AICc relative importance < 0.395) and community composition (Mantel statistic r = -0.018, p = 0.725), however, hydroperiod was related to amphibian species richness and community composition. Hydroperiod was a strong predictor of amphibian species richness in 2004 (D=52%), however the relationship was weak in 2003 (D=3.4%). Precipitation likely affected the relationship difference between hydroperiod and amphibian species richness as 2003 was a drought year whereas 2004 was a deluge year. Habitat characteristics (vegetation cover, structure, and height, plus presence of modifications) influenced the relationship between hydrologic factors and amphibian community composition in 2003 (Mantel statistic r = 0.169, p = 0.055). Corrected AIC models of species richness which included habitat characteristics had some support (Äi = 4.18-7.33). Models of species richness which contained playa density consistently had some support in both years (AICc Äi > 0.63). Playas in the medium-textured soil zone had deeper sediment, and thus reduced volume. Also, most cropland playas have lost over 100% of their original playa volume due to sedimentation, facilitating a reduction in hydroperiod.

Land use was not determined to be the main factor affecting amphibians, probably because it does not capture the variability inherent in playas with differing hydrologic regimes. Continued cultivation of the SHP and predicted climate changes likely will interact to influence amphibian communities in playa wetlands. Climate models predict increases in annual mean temperature, which coupled with sedimentation, will accelerate drying of cropland playas. Given the current gradient of hotter, dryer conditions in the southwestern reaches of the SHP compared to the northeastern portions, I predict that loss of species from playas will occur in a non-random pattern, beginning in the southwestern SHP with species which have long larval development periods. In order to maintain amphibian diversity in the SHP, a reduction in sediment deposition in playas must be attained and hydroperiod should be optimized by implementing buffers.