Landuse and hydroperiod influences on amphibian community structure and the role of larval amphibians in the playa food web

Abstract

Playa wetlands are the primary breeding sites for amphibians in the Southern High Plains (SHP). Most playas have cultivated watersheds, which causes sediments to accumulate in playa basins, reducing their hydroperiods. Little is known about the influences of landuse practices and hydroperiod on amphibian community structure. I investigated potential influences on amphibian populations for cumulative (entire sampling season) and short term sampling period 2-day values. For cumulative analyses, I examined the influence of landuse (cropland versus native grassland), hydroperiod (how long a playa contains water), vegetation cover, and playas size on cumulative amphibian metamorph diversity (H’), amphibian adult diversity, Spea spp. metamorph abundance, and Spea spp. adult abundance. To examine short-term influences on amphibian communities, I tested for the influence of landuse, time (week within a season), weekly water loss (ranging from positive (i.e., water gains) to negative (water loss) values), vegetation cover, and playa size on sampling period amphibian diversity, sex ratios (proportion of males), metamorph Spea spp. abundance, and adult Spea spp. abundance by developing predictive models. I also examined how landuse influenced metamorph survival, capture and recapture rates, emigration probability, and population size. Because aquatic food web studies can provide useful information regarding relationships among trophic levels and wetland conservation efforts, I tested hypotheses that concerning how hydroperiod and landuse influenced playa trophic structure, and whether this structure is best modeled as a web or a chain. I sampled 12 playas each year in 2003, 2004, and 2005 in the SHP. I did not observe a landuse effect on cumulative amphibian metamorph diversity, amphibian adult diversity, Spea spp. metamorph abundance, or Spea spp. adult abundance. However, H’ for metamorphs was greater in 2004 than 2003. Spea spp. metamorph and adult abundance was greater in 2003 than 2004. I also did not observe a hydroperiod, playa size, or vegetation cover effect on cumulative amphibian metamorph diversity, Spea spp. metamorph abundance, and Spea spp. adult abundance. Hydroperiod and amphibian adult H’ were positively associated in 2003 or for both years combined. Results are difficult to interpret because power is low in these tests due to high variation and low sample size. Contrary to other studies, I did not observe a landuse effect on sampling period Spea spp. abundance. I also observed no evidence of landuse effects on sampling period amphibian diversity or sex ratios. Sampling period metamorph diversity increased with weekly water loss, whereas adult diversity decreased with weekly water loss. Sampling period proportion of males decreased with weekly water loss. Sampling period Spea spp. metamorph abundance increased with weekly water loss, whereas sampling period Spea spp. adult abundance decreased with weekly water loss. There were weekly differences in Spea spp. abundance, but no differences in weekly diversity or sex ratios. Vegetation cover was positively related to metamorph diversity and had no influence on adult diversity. Vegetation cover had an overall positive influence on sampling period proportion of males. Spea spp. metamorph abundance was positively associated with playa vegetation cover in 2003 and negatively associated with vegetation cover in 2004. Spea spp. metamorph abundance was positively associated with vegetation cover for both years combined. Vegetation cover had no influence on adult abundance. Playa size had no influence on Spea spp. abundance and proportion of males, but had a positive influence on diversity of adults in 2004. However, the relevance of results related to adult proportion of males, diversity, and Spea spp. abundance are unclear because I was unable to sample playas immediately following rain events, when adults were most abundant. Spea spp. metamorph survival could not be estimated effectively using current model estimation techniques, as data were overdispersed. Whereas thousands of metamorphs were initially captured, few metamorphs (mean among playas = 1.86%) were recaptured among weeks. When emigration could be estimated, it was usually high (> 60%). Population size also varied considerably among playas. Because of poor model fit, testing for differences in population parameters between cropland and grassland landuses could not be performed. The hypothesis that playa trophic structure was best modeled as a food web over a food chain was supported. However, models that included environmental variables such as hydroperiod and vegetation cover had higher Akaike weight than food web or chain models alone, meaning food web models should include foraging functional group and environmental variables. Hydroperiod was an important variable in structuring playa communities, wheras landuse was not. Long hydroperiod playas had high Ambystoma tigrinum mavortium and low tadpole densities, compared to short hydroperiod playas. Short hydroperiod playas had higher proximate invertebrate and amphibian richness. Because all playas dry completely and refill at some point, this food web study is applicable to data collected over a short time period. Long-term sampling of playas would reveal that long hydroperiod playas initially have communities similar to short hydroperiod playas and then shift to communities of long hydroperiods. Therefore, long hydroperiod playas should have a cumulative richness greater than short hydroperiod playas. My results suggest the proximate mechanism influencing amphibian communities in playas is hydroperiod. Anything that alters hydroperiod, such as playa sedimentation or excavation, could negatively influence amphibian communities, and those activities should be avoided. Given their high densities, larval amphibians are important members of the playa community. Therefore, conservation efforts for playas should include amphibians. Because amphibians use terrestrial and aquatic habitats, I suggest that the best strategy requires conservation of playas and adjacent upland areas. Conservation of uplands will reduce playa sedimentation and ensure amphibians have upland habitat to forage, emigrate, and hibernate.