Plant damage and herbivore performance change with latitude for two old-field plant species, but rarely as predicted

A long standing hypothesis in biogeography is that latitudinal gradients in plant defenses (LGPD) should arise because selection for plant defenses is greater in the tropics compared to temperate areas. Previous studies have focused on plant traits thought to confer resistance, yet many traits may not actually confer resistance (putative resistance) or interact to influence herbivore performance. In this study, I used a multi-trophic approach to examine relationships between latitude, herbivore pressure, and plant resistance (measured as the growth rates of herbivores) of two old-field plant species (Solanum carolinense and Solidago altissima) using a field survey across a 12 degree gradient in the eastern US combined with laboratory bioassays measuring the performance of generalist and specialist herbivores. I used structural equation modeling to examine the direct and indirect pathways by which latitude influences herbivore pressure and plant resistance. A latitudinal gradient in plant damage was observed in the expected direction for S. caroliense (damage decreased with latitude), but the opposite relationship was observed for S. altissima. Damage to both plant species was mediated by herbivore abundances, which was in turn influenced by predator abundances. Resistance to herbivores also varied with latitude but the form of the relationship was dependent on herbivore and plant species. There were direct, non-linear relationships between latitude and resistance (for Spodoptera exigua and Schistocerca americana feeding on S. altissima; S. exigua and Manduca sexta feeding on S. carolinense). Herbivore growth rates were also mediated by the density of S. carolinense for Leptinotarsa juncta and S. americana feeding on S. carolinense. There was no relationship between plant resistance and herbivore pressure and no indication of feedbacks. Results from this study indicate that latitudinal variation in plant resistance is complex, possibly constrained by resource availability and tradeoffs in plant defenses.