Theory predicts that native plant species should exhibit latitudinal gradients in the strength of their interactions with their herbivores. We hypothesize that if an invasive plant species exhibits a different latitudinal gradient in response to herbivores (e.g., a non-parallel gradient), it can create large-scale heterogeneities in community resistance/susceptibility to the invasive species. We conducted a study of latitudinal variation in the strength of herbivory and defenses of native genotypes of Phragmites australis in North America (NA) and Europe (EU) and European invasive genotypes in NA. Within NA, we tested whether 1) invasive genotypes are better defended and suffer less herbivory than co-occurring native genotypes, 2) herbivory and defenses of native P. australis decreases with increasing latitude; and 3) invasive genotypes exhibit either no latitudinal gradient, or a non-parallel latitudinal gradient in herbivory and defenses compared to native genotypes. For the European genotypes, we tested two additional hypotheses: 4) defenses, nutritional condition and herbivory differed between the native (EU) and invasive ranges (NA) and 5) latitudinal gradients in defenses and herbivory were similar between ranges. Within NA, chewing damage, internal-feeding incidence and aphid abundance were 650%, 300% and 70% lower, respectively, on invasive than native P. australis genotypes. Genotypes in NA also differed in nutritional condition (%N, CN ratio), but there was little support for invasive genotypes being better defended than native genotypes. For the European genotypes, herbivory was significantly lower in the invaded than native range, supporting the enemy-release hypothesis. Defense levels (leaf toughness, total phenolics) and tissue %C and %N were higher in the invaded than native range for European genotypes. Overall, latitudinal gradients in P. australis nutritional condition, defenses and herbivory were common. Interestingly, chewing damage and stem-feeder incidence decreased with latitude for native P. australis genotypes in NA and EU, but no latitudinal gradients in response to herbivores were evident for invasive genotypes in NA. Non-parallel latitudinal gradients in herbivory between invasive and native P. australis suggest that the community may be more susceptible to invasion at lower than at higher latitudes. Our study points strongly to the need for invasion biology to include a biogeographic perspective.
Keywords: biotic resistance, enemy-release hypothesis, herbivory, invasive species, latitudinal gradients, Phragmites australis, plant defenses