Spatial Variation and Tradeoffs in Species Interactions

The geographic mosaic theory of coevolution predicts that spatial differences in species interactions result in a patchwork of evolutionary hot and cold spots across a landscape. We used horsenettle (Solanum carolinense L.), a perennial weed with a diverse insect community found in old fields and meadows, to examine local adaptation and resource-mediated selection. The goals of this study were to (1) determine the potential for a selection mosaic by identifying local adaptation through trait-interaction matching with herbivores, pollinations and plant competitors, and (2) determine the potential for indirect selection through resource allocation tradeoffs. The potential for local adaptation was determined by measuring interactions in four populations and relating those findings to plant traits measured on offspring grown from those populations in a ‘common garden.’ Allocation tradeoffs between growth, herbivore resistance, and floral traits were also assessed in the common garden. We found high herbivore damage in the field associated with decreased root:shoot ratios in greenhouse-grown plants, which may indicate an herbivore-mediated effect on life-history through selection for a more annual strategy. By examining allocation tradeoffs we found evidence of two distinct reproductive strategies in this perennial plant. Negative correlations between reproductive traits and both growth and defense suggest that individuals either favor current growth and reproduction over defense, or invest in current survival and defense while delaying reproduction. Overall, this study sheds light on how selection changes over space and time, which are of many of the fascinating traits we find in plants and animals today