Abstract
The cooccurrence of species is a crucial component for maintaining the stability and diversity of ecosystems. This has been observed to be particularly prevalent in ecosystems prone to recurring disturbance that disrupts equilibrium. Dynamic and highly variable seasonal wetlands, or vernal pools, are characterized by annual cycles of inundation and drying. Once widespread in the California Central Valley, vernal pools have been reduced from their historical coverage by over 90%. Among the many endangered and endemic species found only in California vernal pools are the vernal pool tadpole shrimp (Lepidurus packardi) and the California tiger salamander (Ambystoma californiense). These species have been observed to cooccur throughout vernal pools in the Central Valley and play key ecological roles in vernal pool communities. Data gaps exist with regards to L. packardi and A. californiense and potential factors that may drive cooccurrence and individual growth. With both species known to have gape-limited predation, cooccurrence may also interact with body size trends of these species. This study examined the patterns of spatial and temporal occurrence and body size growth of A. californiense and L. packardi were associated with the presence of one another and with the cooccurring versatile fairy shrimp (Branchinecta lindahli). Data was drawn from annual aquatic species surveys conducted by the US Fish and Wildlife Service’s Don Edwards San Francisco Bay National Wildlife Refuge between January and April of 2015. The surveys were conducted with dip and seine nets to collect and measure aquatic organisms in 58 pools over 181 surveys. Patterns of cooccurrence were assessed using Fisher Exact Tests. Body size trends of each species over time were modeled using linear mixed models in R, with pool depth, inundation period, and pool identity as additional variables. While presence of B. lindhali was highly associated with L. packardi and A. californiense was highly associated with L. packardi, B. lindhali and A. californiense were less strongly associated. These occurrence correlations may be the result of overlapping aquatic habitat criteria and the preference A. californiense larvae show towards predating L. packardi. Mean and maximum body sizes over time of both species were not observed to vary based on cooccurrence with the second species, nor did body size trends over time vary based on cooccurrence. Body sizes for both species did not show significant variation based on pond depth or between individual pools, although body size was correlated with pool inundation duration. Although A. californiense and L. packardi are known to interact through a predator-prey relationship, these results suggest that there are additional interspecific interactions or environmental factors that influence cooccurrence patterns between these species and with B. lindahli that are beyond the scope of this study. Given the imperiled status of vernal pool ecosystems in California and the need for conservation of associated endemic species, the results of this study not only outline avenues of continued research on cooccurrence and body size patterns but also may inform vernal pool conservation and restoration efforts through improved occurrence mapping and prediction.