The composition of plants in a local neighborhood (relative densities and frequencies of different species or genotypes) can influence plant interactions with pollinators and consumers. Many papers have described effects of both focal plant density and the frequency of different neighboring species or genotypes (i.e. associational effects) on herbivore attack and pollination. However, little is known about whether consumers and pollinators respond similarly to neighborhoods, and thus what the net outcome of plant neighborhood effects mediated by consumers might be for a plant species. In set of preliminary studies we have been examining these issues for the common aster Erigeron speciosus, attack by tephritid fly seed predators, and pollination in sub-alpine meadows at the Rocky Mountain Biological Laboratory in Colorado. In observational studies (2014 and 2015) we characterized the effect of E. speciousus density and frequency relative to E. elatior and Heliomeris multiflora (two other asters that share some of the same tephritids) on the number and identity of flies emerging from flowers. We also recorded visits to E. elatiorflower heads with different levels of fly damage and conducted an experiment manipulating fly and pollinator access to plants to determine whether access to pollination influences fly emergence or size.
We found evidence of effects of both local conspecific flower density (resource dilution) and local flower frequency (an associational effect) for E. speciousus. The number of flies reared from flowers generally decreased with increasing densities of flowers of the focal species and when the focal plant species was at lower frequency. Due to the relatively long handling times required for flies to oviposit, the functional responses of the flies are likely to be strongly saturating, consistent with the observed resource dilution effects. Since different fly species are prevalent in different asters, the presence of large numbers of non-host flowers may make it harder for flies to locate their preferred hosts, consistent with the observed associational resistance. Pollinator observations suggest that flowers with fly damage (which causes brown spots on the flower disk) are visited less often than undamaged flowers, suggesting that the neighborhood effects observed for fly attack would in part be exacerbated by lower pollen receipt. Future studies will also quantify effects of the plant neighborhood on pollination directly, and net seed set in different neighborhoods.