Functional response predicts invasiveness but not trophic impact.

Biological invasions are a major driver of biodiversity erosion mainly because invasive species show greater trophic impact than their non-invasive counterparts. The experimental paradigm for assessing this trophic impact is the functional response (FR) test that describes the relationship between per capita consumption rate and resource density. Two key parameters are then assessable and comparable between populations and species: the space clearance rate (attack rate, a) measuring predatory efficiency at low prey densities, and handling time (h) representing the time required to capture, handle and digest prey. This test is frequently conducted to compare non-invasive and invasive species and shows that invasive species have a higher FR than non-invasive species (characterized by higher space clearance rates and lower handling times) which would explain both their invasion success and their ecological impact. However, it appears that whether FR parameters differ between invasive species sampled in their native versus invasion range has never been tested, implicitly assuming that FR measures can be extrapolated to the entire range of distribution. Using a phylogenetically corrected comparative analysis of 269 FR observations from 45 freshwater fish species (23 non-invasive species and 22 invasive species), we confirm that invasive species exhibited higher FR than non-invasive species. However, this pattern holds true only when considering invasive species sampled in their native range. Invasive species studied in their invasion range displayed functional responses comparable to non-invasive species, with similar space clearance rates and handling times. Additionally, space clearance rates decreased with temperature in non-invasive species but tended to increase in invasive species from invasive introduction ranges, suggesting that climate warming may exacerbate competitive asymmetries. Together, these results indicate that high FR predispose species to invasiveness, but also challenge the assumption that FRs measured in the native range of a species can be directly extrapolated to predict its trophic impacts elsewhere. Our findings call for greater consideration of biogeographic context when using functional responses to assess invasion risk and ecological impact.