Consumptive and non-consumptive predator effects from native fish but not invasive crayfish on Ambystoma salamander larvae

Introduced species impose a variety of negative impacts on native biodiversity, including as consumers of native species. Invasive consumers can also impact native species indirectly through non-consumptive predator effects, where mere presence of the predator causes a reduction in prey fitness. Yet, when prey lack strong responses to non-native predators, non-consumptive effects may be imposed only by native predators. White River crayfish ( Procambarus acutus ) were recently introduced into Canada, including on Pelee Island, Ontario which is home to the country’s only population of small-mouthed salamander ( Ambystoma texanum ). Leveraging data from 105 surveys conducted over four years in 36 distinct waterbodies we test for evidence of consumptive and non-consumptive effects of P. acutus on larval Ambystoma salamanders. We concurrently evaluate, and control for, consumptive and non-consumptive effects from native fish species, density dependence, water temperature effects, and variation in habitat conditions. Additionally, we develop a habitat suitability map for P. acutus to identify potential areas of interaction with salamanders and to guide future monitoring. We found considerable overlap between areas of high habitat suitability for P. acutus and areas where Ambystoma salamanders occur. Between 2022–2025 there was a significant increase in the proportion of surveyed waterbodies where P. acutus was detected and a concomitant decrease in the proportion of surveyed waterbodies where Ambystoma larvae were detected. Nevertheless, we found no evidence that presence of P. acutus was associated with a reduction in catch-per-unit-effort (CPUE, larvae/sweep) or larval snout-vent length. In contrast, native fish presence was associated with an 8.4-fold reduction in CPUE and a 6.6% reduction in snout-vent length. Larval body size was also negatively impacted by high conspecific density and warmer water temperature. While we are relieved that P. acutus does not appear to affect salamanders in the ways we investigated, we remain concerned about other routes through which P. acutus may impact Canadian amphibians and other native biodiversity. Determining the true impacts of introduced species on native ecosystems hinges on objective evaluation of threats and remains essential to and safeguarding native biodiversity in human-affected landscapes.