Coexistence within an endangered predator–prey community in California vernal pools

Abstract

Globally endangered ecosystems, such as ephemeral wetlands, are often critical habitat for multiple interacting imperilled species. To conserve this biodiversity, managers must consider both species‐specific resource requirements and mechanisms for endangered species coexistence under variable habitat conditions. We examined communities native to California playa pools, ephemeral wetlands that have declined by >90% from their historic extent. Specifically, we describe the diet of a federally Threatened amphibian (Ambystoma californiense), and characterise interactions between this amphibian and two federally Endangered macrocrustaceans (Branchinecta conservatio and Lepidurus packardi) across three rain years to understand how these imperilled species coexist. We examined the dietary preferences of A. californiense larvae, metamorphs, juveniles, and breeding adults, and determined how prey electivity by larval A. californiense differs between natural playa pools and human‐modified stock ponds. Within playa pools, both breeding adult and larval A. californiense preyed extensively on L. packardi, whereas fully terrestrial juveniles had relatively empty stomachs. We provide evidence for size‐moderated optimal foraging by larval A. californiense, and show that larval prey differed dramatically between playa pools and stock ponds. Additionally, an ontogenetic progression from smaller to larger prey gave the relatively large, endangered macrocrustaceans an early‐season temporal refuge, during which they reached maturity in all three rain years. Consistent with complex life cycle theory, our results suggest that ephemeral wetland habitat offers abundant food resources for A. californiense relative to terrestrial habitat. Our findings also suggest that diet flexibility facilitates the persistence of this imperilled amphibian in human‐modified stock ponds. Temporal offsets in prey maturation rates and ontogenetic shifts in predator diets are likely to facilitate coexistence among the focal endangered species. We highlight the importance of accounting for spatial and temporal variation in interspecific interactions when predicting the effects of environmental change on biodiversity, particularly in highly threatened ecosystems.

Managing invasive hybrids through habitat restoration in an endangered salamander system

“…Across hydroperiods, differential survival shifted the distribution of successful individuals from the initial proportions of larval source pond and family groups (Figure 4). (23). This would likely increase rates of cannibalism, which has rarely been documented in CTS, but is common in other members of the tiger salamander complex, including BTS (23,24,67).…”

“…Second, previous studies have already explored the optimal conditions for purely native populations (e.g. 23,36,40,69), and our study was focused on the dynamics that occur when hybrids dominate or are actively competing with native genotypes under different hydroperiod regimes. In doing so, our experimental conditions represent the conservation scenarios where the greatest opportunity exists to reverse selection for non-native genotypes.…”

“…(23). This would likely increase rates of cannibalism, which has rarely been documented in CTS, but is common in other members of the tiger salamander complex, including BTS (23,24,67). If selective cannibalism that avoids close kin occurs in long hydroperiod ponds with a mix of hybrid and native CTS, it could create a positive feedback loop where native CTS experience progressively greater predation as the number of hybrids in a population grows.…”

“…This evidence predicts that hybrid salamanders will eventually replace native CTS at least regionally, resulting in genome-level extinction (34). Supporting this view, hybrids are not ecologically equivalent to native CTS, causing trophic cascades that disrupt vernal pool communities (25) and shift the abundance of other native taxa (24) including endangered prey (23). This body of work has emphasized the need for management strategies that reduce or eliminate invasive hybrids in CTS populations.…”

“…The California tiger salamander (Ambystoma californiense; hereafter "CTS") is an endemic California species restricted to vernal pool grassland habitats in central California (22). As aquatic larvae, CTS are important apex predators that help shape vernal pool communities and trophic webs (23)(24)(25). This endangered species is protected at both the federal and state levels based on population declines throughout its range (26,27).…”