Non-native species often disrupt community dynamics and ecosystem functions, posing challenges to native species conservation. However, some endemic consumers can utilize introduced species as alternative resources. When invasive species induce a mix of positive and negative effects on native species, this can result in a management conundrum between invader control and conservation of imperiled species. Such conundrums can arise due to reciprocal intraguild predation (RIGP) between invasive and native species. In RIGP, non-native predators harm imperiled predators through juvenile predation and competition for shared prey; in turn, juvenile invaders benefit the imperiled predators as alternative prey. Eradication of invaders can be arduous, economically costly, and could lead to unanticipated consequences. These management backfires can include abrupt prey depletion for native predators, which could cascade to reductions in scarce, recovering native prey populations. Therefore, optimal invader control regimes require an intricate balance between reducing the invader population and minimizing the risks of backfires on non-target natives.The management conundrum in a RIGP system is exemplified by the invasive American bullfrog (Lithobates catesbeianus), threatened giant garter snakes (Thamnophis gigas), and the native Pacific tree frog (Hyliola regilla) in California's Sacramento Valley wetlands. Giant garter snakes are threatened predators that prefer tree frogs but also consume juvenile bullfrogs. Bullfrogs are voracious predators of native anurans and juvenile snakes. This RIGP is further complicated due to the snakes and bullfrogs also consuming exclusive supplemental resources: introduced fish species, for snakes, and introduced crayfish, for bullfrogs. While the presence of exclusive, supplemental prey can relax persistence conditions for each predator, it could limit the persistence of shared prey. Controlling bullfrogs could also lead to an overcompensatory increase in bullfrogs, if partial culling of cannibalistic adults reduces juvenile mortality and intraspecific competition.
We address these conundrums by bridging experimental methods and dynamic modeling to explore the efficacy of various bullfrog culling strategies within a RIGP framework, using the Sacramento Valley RIGP system as an example. In Chapter 1, we explore a dynamic central to RIGP: size-dependent predation. We investigate whether giant garter snakes select particular size classes of juvenile bullfrogs, to understand how size-dependent predation might occur in the RIGP between these native and invasive predators. We find that giant garter snakes preferentially consume smaller bullfrogs (≤30% of snake gape limit), potentially due to decreased handling time as snakes grow. However, larger snakes are more likely to strike at larger bullfrogs, highlighting the complexity of predator-prey interactions in this system. In Chapter 2, we parameterize dynamic models of RIGP, with supplemental prey for each predator, to investigate the impacts of stage-specific invader culling on native predator and prey recovery, at the end-states (equilibria) of the system. We include system-specific parameters quantified from Chapter 1 and field data. We find that adult bullfrog removal generally supports native predator and prey recovery more effectively than juvenile removal. However, when adult culling is insufficient, simultaneous juvenile culling becomes crucial. In Chapter 3, we employ time-dependent culling strategies and investigate how quickly and turbulently each native species returns to equilibria. We find that earlier culling initiation leads to faster recovery and lower volatility for native populations. Interestingly, intermediate culling levels can reduce turbulence during the recovery of giant garter snake populations, highlighting the need for carefully calibrated interventions. By considering the timing, intensity, and stage-specificity of culling regimes, conservation efforts can be refined to achieve better outcomes at management-relevant time scales. Overall, these findings can inform more nuanced and effective approaches to ecosystem management in the face of biological invasions in the Central Valley and beyond.
