Some neighbors are better than others: Variation in associational effects among plants in an old field community

Nora Underwood, Jacob M. Heiling, Jessie Mutz, Monica Paniagua Montoya and Brian D. Inouye, Biological Science, Florida State University, Tallahassee, FL

Background/Question/Methods
Associational effects occur when neighboring plants influence interactions of a focal plant with herbivores or pollinators. The many studies on associational effects between a focal plant and some neighbor have shown that these effects can vary greatly in strength and direction. But because virtually all of these studies measure effects from only one or two neighbor species or genotypes, we know little about the range of associational effects that a plant species or genotype might encounter in a natural setting. This makes it difficult to determine the importance of neighbor effects in field settings, and how associational effects might interact with competition and other processes to influence plant community composition. In this study, we measured the effects of 11 neighbor species on damage, size and traits of our focal plant species, Solanum carolinense, relative to having only conspecific S. carolinese neighbors. All neighbors are typically found in proximity to S. carolinense in N. Florida, where this research was conducted. Six S. carolinense, of known genotypes, were planted into 1.6m diameter circular plots. Each plot also contained nine plants of one of the 11 neighbor species, or nine additional S. carolinense. There were six replicates of each plot type. We measured damage to plant leaves, SLA, trichomes, leaf toughness and size of focal S. carolinense, as well as neighbor plant size. We also counted individuals of specialist herbivore beetle Leptinotarsa juncta.
Results/Conclusions
We addressed three questions: 1) how does the strength of associational effects vary among heterospecific neighbor species, relative to conspecific effects, 2) are these effects generally positive or negative, and 3) to what extent are neighbor effects mediated by some plant traits commonly related to herbivory? We found that neighbor identity significantly affected damage and number of L. juncta on S. carolinense. These associational effects ranged widely in effect sizes, with the distribution of effect sizes fairly even. There was no relationship between strength of effects and relatedness of the neighbor species to S. carolinense. Nearly all effects were negative (associational resistance). Neighbors affected all measured plant traits; size and specific leaf area of focal plants both influenced damage. Solanum carolinense genotype affected all response variables, but only rarely interacted with effects of neighbors. Our results suggest that associational effects on insect herbivory can be highly variable, and may not match the expectation from studies of other types of interactions that most effect sizes should be weak.