PS 35-66
Measuring the importance of neighbors: consumer and focal-resource densities mediate associational effects in a model system
Neighbors have the potential to influence individual vulnerability to predation and parasitism in a number of systems (e.g. plant-herbivore interactions, predator-prey interactions, host-pathogen interactions). Such "associational effects"--—those effects on a focal resource at a particular density that are due to the neighboring resource composition—-- likely depend on both the frequency and density of resource organisms as well as the consumer density. Few studies have disentangled these factors, although this is an important first step towards understanding how associational effects contribute to ecological and evolutionary outcomes,. Here we present results from a simple model system in which two resource-organisms, black-eyed peas (Vigna unguiculata) and mung beans (V. radiata), share a common consumer, the bean beetle Callosobruchus maculatus. We manipulated the densities of both resources, allowing us to distinguish between effects of focal resource density and the associational effects that result from neighbor density/frequency. We also manipulated consumer density (high v low) to investigate how this modifies the strength of associational effects.
Results/Conclusions
Our results demonstrate that associational effects can be asymmetrical, can diminish with focal-resource density, and may increase with consumer density. While black-eyed peas were less likely to be damaged by bean beetles in the presence of mung beans, no reciprocal associational effects were observed. That is, there was no influence of black-eyed peas on mung bean damage. Interestingly, as black-eyed pea density increased, the associational effects of mung beans diminished, consistent with a prior field experiment in a plant-herbivore system (Kim 2012). In addition, consumer density increased the magnitude of associational effects at low black-eyed pea density. These findings suggest that, when present, associational effects may be greatest when consumer densities are high and focal-resource densities are low.