Latitudinal variation in plant palatability and herbivore pressure in two old-field habitat species

Background/Question/Methods A longstanding theory in biogeography is that herbivore pressure in southern latitudes is higher than in northern latitudes, resulting in latitudinal gradients in plant palatability and defenses.   Although some experimental studies have supported these patterns (mainly in simple systems), there have been many disagreements on their generality.  The goal of this study was to examine whether the geographic pattern persists in a relatively diverse system focusing on two perennial herbaceous plants (Solanum carolinense and Solidago altissima).  Specifically, I asked (1) Are there latitudinal gradients in local plant abundances and plant community composition in old-field habitat? (2) Do plant palatability and leaf nutrient content vary with latitude? (3) Does insect herbivore community structure vary with latitude?  In 2008, 18 old-fields were surveyed at six latitudinal locations ranging from Florida to New York.   Local abundance of S. carolinense and S. altissima were recorded, along with plant growth, reproduction, leaf tissue damage, and surrounding plant community composition.  Surveyed plants were collected, oven-dried, and ground to a fine powder for insect herbivore bioassays to assess leaf palatability.  To determine leaf nutrient content, carbon to nitrogen ratios were determined.  Insect herbivores were also collected via sweep-net and identified to species. 
Results/Conclusions Results show that there was no significant relationship between plant abundance, plant community composition, and overall leaf tissue damage with latitude.  However, when plants species were examined separately, S. carolinense showed a negative relationship between leaf tissue damage and latitude, whereas S. altissima showed the reverse pattern.  Preliminary herbivore performance bioassays indicate that leaf palatability varied with plant species, as well as with the type of herbivores used (e.g. generalists versus specialists).  Specifically, specialists on S. carolinense showed no significant relationship with latitude and their relative growth rates, whereas generalists showed a positive relationship.  Bioassays on S. altissima are currently being investigated.  Changes in insect herbivores community structure for both plant species were also observed across latitude, which could help elucidate why different patterns of abundance and leaf tissue damage were observed.   This study shows that many factors (e.g. leaf nutrient content, plant palatability, and insect species turn-over) can influence latitudinal patterns of herbivory and evidence for such patterns may depend on the types of insect herbivores used.