PARENT SESSION
Oral Session # 34: Plant Ecology II: Pollination and Dispersal.
Presiding: A McEuen
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 202.
Coexistence among plants that share pollinators: Facilitation by conspecifics and heterospecifics in Clarkia.
Moeller, David1, Geber, Monica1, 1 Cornell University, Ithaca, NY, USA
ABSTRACT- Understanding the mechanisms of species coexistence has been a challenging problem for ecologically similar species that share resources. While the underlying assumption for most models of coexistence is that competition for limiting resources is the dominant force regulating diversity, a growing number of studies on plant communities have found evidence that direct plant-plant interactions may vary from competitive to facilitative. Relatively little is known of indirect positive interactions among plant species, such as through the attraction or maintenance of shared pollinators. Although it is clear that plants can compete for (or through) pollinators, co-flowering of plants that share pollinators is surprisingly common. We examined the nature of reproductive interactions among Clarkia species pollinated by a group of pollen specialist bees. For intensive studies, we focused on the effect of community, population, and local factors on the reproductive ecology of a target species, Clarkia xantiana. We found that congeners frequently coexist with C. xantiana across its geographic range and that the frequency of coexistence declines sharply at the species range margin. A comparison of observed patterns of flowering phenology to a null model revealed that communities range from aggregated to segregated. The effect of congeners on pollinator abundance and pollen limitation of reproduction in C. xantiana was positive to neutral where pollen limitation declined with increasing numbers of coexisting Clarkia species. Further, we found strong evidence of Allee effects suggesting that both the abundance of conspecifics and heterospecifics positively affected reproductive success through increased mate and pollinator availability, respectively. Detailed surveys of bee visitor communities confirmed that the availability of specialists was much greater to C. xantiana in communities with multiple Clarkia species. Both the patterns and processes examined in this study indicate that positive interactions between conspecifics and heterospecifics may influence the population dynamics and distribution of Clarkia xantiana.
Key words: allee effects, coexistence, facilitation, pollination