Abstract
It has long been hypothesized that biotic interactions, including herbivory, are most intense at lower latitudes. However, this generalization has recently been challenged with studies showing that latitudinal gradients in damage may be rarer than previously believed. Additionally, most studies have focused on herbivory of native species, so it remains unknown whether natives and exotics follow similar patterns. This study compares rates of aboveground herbivory of multiple native and non-native Asteraceae across a latitudinal gradient, with a more detailed investigation of a focal exotic, Cirsium arvense. Herbivory of multiple tissue types was quantified for all species across an 815 km transect in Ontario, Canada. The native Asteraceae included in the survey typically experienced a decline in folivory with increasing latitude. Herbivory patterns for the exotic species were less clear; while most experienced high damage at the southernmost site, some also experienced high damage rates at mid-latitudes. For the focal species C. arvense, leaf and stem herbivory declined with increasing latitude, although seed damage showed strong regional variation across the invaded range. These results show that latitudinal variation in herbivory is highly dependent on the plant species being investigated, the tissue type being measured, and the type of herbivore(s) causing the damage. In some cases, populations in marginal areas might benefit from reduced damage by some groups of herbivores. In other cases, factors such as the availability of suitable habitat, the biology of specific enemies, and the origin of the host plant may override the influence of latitude on host performance.