Decreases in herbivore damage approaching northern range limits in native and exotic Asteraceae

Background/Question/Methods

How herbivore impacts change over latitudinal gradients is a long-standing problem in ecology which has recently attracted new attention. For instance, it is unclear whether trends would be similar for native species and for exotics, which have often lost many of their native-range herbivores during the invasion process. If natives and exotics follow different rules, they may differ in future range expansion, particularly in response to a changing climate.

In this study, we report preliminary results from a multispecies survey of herbivore damage to 10 commonly occuring native and exotic plants of the family Asteraceae. We surveyed herbivore damage to natural populations over an 800 kilometer latitudinal transect from the Carolinian forest of Southern Ontario to near the end of their distributions in the northern Boreal Forest zone. We measured damage by both foliar herbivores and pre-dispersal seed predators, and compared these results with previous work.

Results/Conclusions

Overall, the species studied exhibited decreased herbivore damage as they approached their northern range limits. Natives and exotics followed broadly similar patterns, with both leaf and seed damage declining with latitude. This pattern likely reflects climatic constraints on herbivores and/or difficulties in their finding and persisting in small or isolated marginal host populations. There was, however, considerable interspecific variation; damage rates in some species, such as Eupatorium maculatum, declined by more than 50% from interior to marginal populations, while others such as Rudbeckia hirta were subject to >90% less herbivory near the limits of their distributions. These latitudinal patterns may move or disappear in future as insect herbivores and their host plants move north in response to climate warming; until then, marginal populations of many natives and exotics may benefit from reduced levels of damage.