Background/Question/Methods: Theory suggests that branching structures in the environment can alter species interaction rates by restricting the proportion of the environment explored by a randomly moving organism. This limitation in movement may alter intra- and interspecific interaction rates such that population dynamics are changed (i.e., diffusion-limited interaction rates). We tested this theory using isogenic peas (Pisum sativum) with differing morphologies, pea aphids (Acyrthosiphon pisum (Harris)), and ladybug predators (Harmonia axyridis). 

Results/Conclusions: We looked for alterations to intraspecific aphid interaction rates in laboratory experiments with peas that have compact or highly branched and tendrilled forms. We found that pea morphology does not have a measurable impact on pea aphid reproductive rates nor aphid crowding. However, we did find that more branching plant morphology may increase pea aphid spread rates by about 1cm/day in greenhouse experiments. We also examined effects of pea morphology on interspecific interactions between pea aphids and their predators. Predation success was affected by large scale differences in the spatial structure of the foraging environment, but it is not clear if the smaller structural differences between different plant morphologies have a significant effect. Finally, we tested for effects of plant morphology on aphid population dynamics in greenhouse experiments.  We find a difference in aphid population size and dynamics between greenhouse plots containing predators and plants with structures that are more or less branching.  Therefore, in this system, it seems likely that the branching structure of plants can alter predator-prey interactions, but probably not intraspecific interactions. However, these effects are not yet well-described by theory.