Environment-competition interactions in an invaded amphibian assemblage

Background/Question/Methods: Environmental variation underlies important diversity-maintenance mechanisms, including the spatial storage effect. Taxa with life stages that are highly responsive to environmental variation, such as pond-breeding amphibians, are most likely to exhibit environment-competition interactions that underlie the spatial storage effect. We examine how competition (broadly defined as negative biotic interactions) changes with species densities across two structurally different habitats – open water and vegetated ponds. Our focal species are two larval amphibian intraguild predators: introduced Barred Tiger Salamanders (Ambystoma tigrinum mavortium) and endemic California Newts (Taricha torosa).

Results/Conclusions: We document evidence of the three basic requirements for the spatial storage effect to function in this system: 1) niche differences, 2) interactions between environmental factors and species densities, and 3) density-dependent covariance between environmental and competitive responses (covEC). We illustrate how a common experimental design (factorial experiment) can be used to measure covEC, and how multiple ecological interactions (competition, intraguild predation, and cannibalism) can contribute to a single, quantifiable mechanism (the spatial storage effect). Our results suggest that the spatial storage effect may offset negative effects of A. t. mavortium (which are larger and more predatory than endemic California tiger salamanders, A. californiense) on Taricha torosa. Differences between A. t. mavortium and A. californiense, in response to vegetation structure and rates of growth and cannibalism, indicate that this introduction may alter the function of coexistence mechanisms that operate through environment-competition interactions in this system.