Declines in herbivore damage towards northern range limits of native and non-native plants
It long has been hypothesized that populations of plants closer to the equator experience more damage from insect herbivores than populations at higher latitudes. Additionally, marginal populations may experience less damage than more central populations because they escape enemies that fail to find them or cannot persist at these sites. Both of these ideas remain controversial, with recent studies both supporting and questioning them. As well, it is unclear whether non-native species (which may have lost many of their natural enemies during the invasion process) would follow the same model as co-occurring natives.
The objective of this study was to document whether insect damage declines approaching the northern range limits of native and non-native Asteraceae. We surveyed populations of four natives and four non-natives at several locations over an 800 km transect from temperate southern to boreal northern Ontario and assessed them for damage by insect folivores and pre-dispersal seed predators. We also measured indicators of both vegetative and reproductive performance. Finally, we conducted more intensive surveys of a focal invasive species, Canada thistle (Cirsium arvense).
Results differed among species, and did not generally follow a simple relationship with latitude. However, in most cases, the highest levels of leaf damage occurred in southern populations, and the lowest in more northern sites. Patterns for seed damage were less clear, but when significant, damage also tended to be lowest at the northernmost sites. Performance followed a variety of patterns. Intensive sampling of C. arvense indicated a strong decline in leaf damage with latitude, while stem galls were present only at southern sites. Seed damage varied irregularly, reflecting the presence of one pre-dispersal seed predator (Larinus planus) primarily at southern sites, and another (Terellia ruficauda) mostly at northern locations.
These results indicate that populations of plants often escape natural enemies near their northern range limits. This pattern is clearer for folivores than seed predators, but applies to both natives and non-natives. Escape from enemies near range margins may relax biotic selection pressures and accelerate further spread by both natives and co-occurring non-natives in response to climate change.