According to the ideal free distribution (IFD: Fretwell & Lucas, 1970), animals should distribute themselves to maximize resource acquisition in a heterogeneous environment. However, empirical studies reported that some phytophagous insects aggregate more than that expected by IFD.
The secondary metabolites in plants are the ones that potentially deviate the spatial distribution of their herbivores from the IFD. Some secondary metabolites in host plants are typically utilized as a cue for host-finding by the phytophagous insect species. For example, a leaf beetle (Gastrophysa atrocyanea) senses the secondary metabolites to detect its host plant (Rumex obtusifolius). Because the metabolites are accumulated due to the interaction between host species, the beetles aggregate on specific host individuals under intensive intraspecific competition. Therefore, we predicted that phytophagous insects that have evolved a chemical preference to detect hosts, but aggregation on specific host individuals with high chemical concentrations might lead to severe competition among herbivores.
Here, we constructed a mathematical model for the evolution of chemical preference in herbivorous insects. Plant community consists of host and non-host plants in a 2-dimensional space. Host plants accumulate the chemicals if they interact with neighboring conspecifics. Herbivores choose their host individual based on the size and chemical concentrations of plants, and are suffering from resource competition among insects on the same host. Using this model, we investigated the evolutionary consequences of chemical preference and resulting spatial distribution in insects.
As a result of the evolutionary game, an evolutionarily stable strategy (ESS) of chemical preference exists in response to the plant community structure. The ESS of chemical preference was always higher than that maximized population size. Consequently, insects with ESS aggregated on some chemical-rich host plants. As the frequency of non-host plants in the plant community increased, the ESS of the chemical preference of herbivores became larger, resulting in further aggregation on specific hosts. If the spatial distribution of host plants became more heterogeneous (i.e., some individuals were clumped while others were sparse), chemical preference of insects evolved to be small, which relaxed the resource competition among insects.
Overall, the spatial aggregation of phytophagous insects was mediated by the evolution of chemical preferences, which was determined by the plant community structure in terms of both species composition and spatial distribution.