Weaker resource diffusion effect at coarser spatial scales observed for egg distribution of cabbage white butterflies

Marc Hasenbank, Stephen Hartley

Mobile organisms frequently forage for patchy resources; e.g. herbivorous insects searching for host plants. The resource diffusion hypothesis predicts that insect herbivores, such as Pieris rapae butterflies, are disproportionally attracted to more isolated, or 'diffused', host plants. Surprisingly little is known about how this response to variation in resource density manifests itself at different spatial scales. We measured the outcome of oviposition by P. rapae butterflies foraging among groups of host plants, with plant density experimentally varied to achieve comparability between three nested scales: fine (1 × 1 m), medium (6 × 6 m), and coarse (36 × 36 m). Hierarchical linear models were used to measure density-dependent responses in the number of eggs laid per plant, with plant density measured at nested spatial scales. At a fine scale, isolated plants received significantly more eggs, while at medium and coarse scales the differences were less pronounced, and tended towards a neutral distribution of eggs across plants. Larger plants also tended to receive more eggs. Since multiple processes, acting at multiple scales, are likely to be the rule rather than the exception in ecology, methods for detecting and characterising multi-scale responses are important to ensure a robust transfer of ecological models from one situation to another.