COS 68-3
Associational effects: Going from individual level interactions to population consequences
Interactions between individual consumer and resource organisms can be modified by neighbors, e.g. when herbivory depends on the identity or diversity of neighboring plants. Effects of neighbors on consumer-resource interactions (“associational effects”) occur in many systems including plant-herbivore and plant-pollinator interactions. Unfortunately, we lack appropriate models and data to determine how neighbor effects on individuals contribute to net interactions at population and community levels. As first steps towards making this connection between individual level effects and population consequences, we supply a general definition of associational effects and suggest strategies for empirical studies measuring the impacts of associational effects at the population level. We then describe results of field manipulation of the densities of two old-field plants (response surface design) that demonstrates how associational effects can be measured in a way that will allow linking them to population consequences.
Results/Conclusions
We define associational effects as occurring when consumer effects on individuals of one resource organism type, at a given density of that type, are a function of the neighborhood composition of other resource types; this definition distinguishes these effects from effects of focal resource density (resource concentration or dilution effects). We suggest that measuring associational effects thus requires manipulating the densities of both focal and neighbor resource species simultaneously. In our field experiment, we found that total herbivore damage to our focal plant species (Solanum carolinense) did depended on the frequency of neighboring Solidago altissima plants, indicating that there are associational effects in this system. In this case the effect of neighbors was to increase herbivore damage, indicating associational susceptibility.