Data from: Limited role of aquatic habitat complexity in mitigating the ecological impact of invasive bullfrogs on native amphibians?

1. Biological invaders pose severe threats to biodiversity worldwide. Spatial overlap in habitat use and the structural complexity of habitats can exert contrasting influences on interspecific interactions, thereby shaping the ecological impact of biological invaders. Incorporating the influence of local environmental conditions on the ecological impact of biologic invasions into risk assessments is thus key for the development of effective impact mitigation strategies. Yet, the relationship between environmental conditions and impact severity is not well understood. 2. In this study, we investigated how pond-level habitat characteristics shape the ecological impact of non-native American bullfrog (Lithobates catesbeianus) on native amphibians. We surveyed 134 ponds in Belgium using environmental DNA (eDNA) quantitative barcoding and metabarcoding to assess bullfrog density and native amphibian community structure, respectively, and recorded 25 biotic and abiotic pond characteristics. We investigated (i) environmental niche overlap between native amphibians and bullfrogs, (ii) species-habitat associations, and (iii) the interactive effect of aquatic habitat complexity and bullfrog density on native amphibians. 3. Our findings show that the environmental niche of native amphibians was substantially smaller than and included within that of bullfrogs, narrowing the environmental window for impact mitigation through niche differentiation. Native species with smaller niches and stronger habitat associations were generally more susceptible to increasing bullfrog densities than less habitat-specific species. Although structurally complex habitats benefited most native amphibians, their capacity to buffer the impact of bullfrogs was limited, with the negative effects of bullfrog density often overriding the advantages of habitat complexity. 4. Synthesis and applications. Increasing aquatic habitat complexity can help native species withstand the effects of biological invaders on native species and improve ecosystem resilience. However, our results show that its ability to reduce the impact of non-native bullfrogs varies depending on the native amphibian species involved and the specific conservation objectives being pursued—whether oriented toward maintaining amphibian species richness or ensuring the persistence of focal species. Increasing aquatic habitat complexity alone is thus not sufficient to protect native amphibians from non-native bullfrogs, meaning that conventional eradication strategies are still necessary.