New insights into plants coexistence in species-rich communities: the pollination interaction perspective

Questions: In animal-mediated pollination, pollinators can be regarded as a limiting resource for which entomophilous plant species might interact to assure pollination, an event pivotal for their reproduction and population maintenance. At community level, spatially aggregated co-flowering species can thus be expected to exhibit suitable suites of traits to avoid competition and ensure pollination. We explored the problem by answering the following questions: (i) are co-flowering species specialized on different guilds of pollinators? (ii) do co-flowering pollinator-sharing species segregate spatially? (iii) do co-flowering pollinator-sharing species that diverge in anther position spatially aggregate more than those that converge in anther position? Study site: Euganean Hills (NE Italy). Methods: Plant composition, flowering phenology and interactions between each entomophilous plant species and pollinating insects were monitored every fifteen days in 40 permanent plots placed in an area of 16 ha. We quantified the degree of flowering synchrony, pollinator-sharing and spatial aggregation between each pair of entomophilous species. We then tested the relationship between the degree of co-flowering, pollinator-sharing and spatial aggregation, and between spatial aggregation and anther position. Results: Entomophilous species converged, at least partially in flowering time, and the phenological synchronization of flowering was significantly associated with the sharing of pollinator guilds. Coflowering pollinator-sharing species segregated spatially. Furthermore, co-flowering pollinatorsharing species that diverged in anther position aggregated more than those that converged in anther position. Conclusions: Reproductive traits that facilitate the coexistence of co-flowering species include specialization on different pollinator guilds and a phenological displacement of the flowering time. Furthermore, in circumstances of increased competition due to phenological synchronization, pollinator sharing and spatial aggregation, the chance of an effective pollination might depend on differences in anther position, resulting in a divergent pollen placement on pollinators’ body. One of the most interesting results we obtained is that the presence of one mechanism does not preclude the operation of others and each plant species can simultaneously exhibit different strategies. Although more studies are needed, our results can provide additional information about plant-plant interactions and add new insights into mechanisms allowing the coexistence of a high number of plant species into local communities.