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

Consumer-resource interactions are often influenced by other species in the community, such as when neighbouring plants increase or reduce herbivory to a focal plant species (known as associational effects). The many studies on associational effects between a focal plant and some neighbour have shown that these effects can vary greatly in strength and direction. But because almost all of these studies measure associational effects from only one or two neighbour species, we know little about the actual range of associational effects that a plant species might encounter in a natural setting. This makes it difficult to determine how important effects of neighbours are in real field settings, and how associational effects might interact with competition and other processes to influence plant community composition. In this study, we used a field experiment with a focal species, Solanum carolinense, and 11 common neighbour species to investigate how associational effects vary among many co-occurring neighbour species and to test whether factors such as neighbour plant apparency, phylogenetic proximity to the focal species, or effects on focal plant defence traits help to explain interspecific variation in associational effect strength. We found that some neighbour species affected S. carolinense damage and attack by specialist herbivores, but associational effects of most neighbours were weak. Associational effects increased herbivore attack on average earlier in the season (associational susceptibility) and reduced herbivore attack on average later in the season (associational resistance) relative to S. carolinense in monoculture. We found some evidence that a neighbour's associational effect was related to its biomass and phylogenetic proximity to the focal species. While neighbour species differed in their effects on physical leaf traits of focal plants (trichome density, specific leaf area, and leaf toughness), these traits did not appear to mediate the effects of neighbours on focal plant herbivory. Synthesis. Our results suggest that the distribution of associational effect strengths in natural communities are similar to those observed for other interaction types, and that multiple mechanisms are likely acting simultaneously to shape associational effects of different neighbour species.