Standing on an invasion front: predicting impacts of praying mantids using fuzzy interaction webs

Developing tools for rapidly predicting which introduced species will become invasive is essential for effective management. It’s also notoriously difficult. Quantitative models exist but often require extensive data, precluding timely application. A qualitative modeling approach, Fuzzy Interaction Webs (FIW), offers a practical alternative. FIWs hold the potential to predict impacts, guide early responses, and identify research needs using the limited data commonly available on an invasion front. As a case study, we applied FIW to a European praying mantis (Mantis religiosa) invasion in grassland communities of western Montana, USA. Using available field data, published data, and expert opinion, we modeled mantid impacts on this community. We also explored how abiotic conditions (winter severity) and natural enemies (parasitoids) might independently and interactively influence mantid abundance. Our model predicted that current mantid densities, which are comparatively low, produce negligible impacts. However, increasing mantid abundance could cause general declines across invertebrate guilds, with the greatest impacts at high densities, especially on invertebrate predators, herbivores, and pollinators. Mantids were also predicted to reduce songbird abundance via indirect effects. However, if wild parasitoids establish, our model predicted they would maintain mantids below impact thresholds, even under mild winter conditions. The potential adverse effect of these parasitoids on native mantids is unknown, warranting monitoring, laboratory, and field experiments. We also recommend monitoring M. religiosa abundance to trigger management actions if they approach impact thresholds. Our case study demonstrates the potential usefulness of FIWs as a tool for evaluating invasion fronts.