Multiple-Predator Effects and Functional Redundancy of Pond Predators
Functional redundancy occurs when different predator species have similar effects on the diversity, abundance, and composition of a prey community. When multiple predators coexist, their interactions can alter prey survival and ultimately diversity through emergent multiple-predator effects (MPEs). MPEs can be exacerbated by differences in predator behavior; however, little is known about the magnitude of MPEs when predators compete for the same prey or have similar behavioral traits. To understand functional redundancy and the interactions of multiple predators in similar niches, as well as their impacts on the composition of a prey community, we conducted two experiments using two ambystomatid salamander predators (Ambystoma opacum and Ambystoma annulatum). We exposed a suite of tadpole prey (Anaxyrus americanus, Rana sphenocephala, Rana sylvatica, Pseudacris feriarum, and Pseudacris crucifer) to different experimental food webs in a mesocosm experiment, and a single prey species (R. sphenocephala) in microcosm experiment with substitutive and additive designs to test for MPEs. We found no evidence for functional redundancy between the two predators: A. annulatum selectively preyed on competitively dominant prey species (A. americanus) and did not alter community diversity. Ambystoma opacum decreased prey diversity relative to the control because of natural phenological mismatch with A. americanus. Interactions between the two predators (e.g., predator inference) were lacking, indicating that predation risk from each predator was independent for the one prey species we tested. A better understanding of community and ecosystem-level effects by A. annulatum can inform future conservation efforts and management decisions regarding this endemic species.ABSTRACT
Mean proportional survivorship of (a) Anaxyrus americanus, (b) Pseudacris crucifer, and (c) Rana sphenocephala across predator treatments in the multiple-prey experiment. Lower-case letters above bars identify pairs of means that are significantly different. Red lines indicated expected survival values based on the additive design of the MRM. Values shown are treatment means ± 1 SE. n = 4 in all cases.
Proportion of total tadpoles surviving in the multiple-prey experiment is shown by total bar height. Species composition of surviving individuals is broken down by food web.
Mean species diversity among predator treatments as represented by the Simpson's diversity index in the multiple prey experiment. Lower-case letters above bars identify pairs of means that are significantly different. n = 4 in all cases.
Prey (Rana sphenocephala) survival from the single-prey experiment. Survival is a function of predator treatment (different colors and shapes) and mean predator head width (mm) in low-density (a) and high-density (b) treatments. AO = Ambystoma opacum and AA = A. annulatum. (c) Shows observed mean survival (± 95% CI) based on predator treatment and prey density (gray bars = high density, white bars = low density), compared with expected survival (dashed horizontal lines) for pairs of con- and heterospecifics. Red horizontal lines indicated expected values based on the additive design, whereas the blue lines indicate the substitution design of the multiplicative risk model.
Contributor Notes
