Spatial turnover in host-plant availability drives host-associated divergence in a South African leafhopper (Cephalelus uncinatus).

BACKGROUND: The evolution of reproductive isolation between herbivorous insect populations is often initiated by shifts to novel host-plants, a process that underlies some of the best examples of ecological speciation. However, it is not well understood why host-shifts occur. Arguably the most common hypothesis is that host-shifts occur in response to competition, while a less frequently invoked hypothesis is that herbivores adapt locally to geographic differences in potential host-plant communities. Here we investigate whether geographic variation in host-plant availability is likely to have driven host-shifts in restio leafhoppers. We studied local adaptation of a camouflaged restio leafhopper species, Cephalelus uncinatus, to host-plants in the Restionaceae (restios); a family of plants with exceptional diversity in the anomalously species-rich Cape Floristic Region (CFR). To determine whether C. uncinatus experiences heterogeneous host communities across its range, we first quantified the degree of geographic overlap between C. uncinatus and each of its associated host-plant species. Then we quantified trait divergence (host preference, body size and colour) for three pairs of C. uncinatus populations found on different host-plant species differing in their degree of spatial overlap. Spectral reflectance was modelled in bird visual space to investigate whether body colour divergence in C. uncinatus corresponds to leaf sheath colour differences between restio species as perceived by potential predators. RESULTS: We demonstrate that C. uncinatus is forced to use different restio species in different regions because of turnover in available host species across its range. Comparisons between geographically separated populations were consistent with local adaptation: restio leafhoppers had preferences for local host-plants over alternative host-plants and matched local plants better in terms of size and colour. CONCLUSIONS: Spatial turnover in host-plant availability has likely facilitated host-shifts in C. uncinatus. Spatial turnover in host-plant availability may be an important driver of insect diversification in the CFR and globally.

Experimental evidence for fundamental, and not realized, niche partitioning in a plant-herbivore community interaction network.

Patterns of niche partitioning can result from local ecological interactions (e.g. interspecific competition) occurring within a contemporary time frame (realized niche partitioning). Alternatively, they may represent the end product of historical processes acting over long time frames (fundamental niche partitioning). Niche partitioning is often detected by analysing patterns of resource use within communities, but experiments are rarely conducted to test whether patterns of non-overlapping resource use reflect realized or fundamental niche partitioning. We studied a community of restio leafhoppers from the genus Cephalelus and their host plants, the Restionaceae (restios). We used network and experimental approaches to determine whether network modularity (a measure of niche partitioning within local communities) reflects fundamental or realized niche partitioning. Using a weighted modularity index for two party networks (e.g. insect-plant), we determined whether the network of this community is modular (i.e. consists of groups of species interacting strongly, with weak interactions between groups). We also aimed to identify specific Cephalelus - restio modules (groups). Using knowledge of module membership to design experiments, we tested whether Cephalelus species from two different modules, Cephalelus uncinatus and Cephalelus pickeri, prefer and perform better on restios from their own modules vs. restios from other modules. These experiments were performed under controlled conditions, eliminating the influences of competition and predation on host choices. The Cephalelus - restio community was modular, implying niche partitioning. Cephalelus also preferred and performed better on restios from their own modules in the absence of local contemporary factors. Most niche partitioning in the investigated Cephalelus community is not caused by local interactions, and thus, host use patterns represent fundamental niches. Our findings highlight the importance of understanding local community structure in the light of processes extrinsic to the local community context.