Gene flow and pollen limitation on experimental green roofs

Background/Question/Methods

Green roofs are habitats for wildlife and providers of ecosystem services such as mitigation of urban heat-island effects and storm-water runoff. As isolated, island-like systems, green roofs can serve as experimental models for examining the effects of habitat fragmentation on important ecological phenomena such as pollen limitation and gene flow. Pollen limitation is particularly likely in isolated habitats, so we expect that pollen limitation may occur on green roofs. We also expect restricted gene flow from ground to roof sites, which would reduce genetic diversity and self-sustainability of roof populations. We assessed the extent of pollen limitation on rooftop plants by comparing seed set of open-pollinated with hand-pollinated plants. We used two self-incompatible species, Brassica rapa and Helianthus agrestis, planted in pots placed on roofs at the Fordham University’s Rose Hill campus in the Bronx and at the Calder Research Center in Armonk, NY. We also evaluated the extent of gene flow between rooftop plants and those located at the paired ground-level sites using two species, Brassica rapa and Pisum sativum. Both species have visible polymorphisms, and phenotypes of offspring were used to determine the extent of gene flow. 

Results/Conclusions

Results thus far have not found any evidence of pollen limitation, since there is not yet any statistically significant difference in seed set between open-pollinated and hand-pollinated plants at either site. This finding accords with previous studies showing a large number and diversity of pollinators on these green roofs. We also have not found any evidence for gene flow between ground and roof sites, since no offspring show phenotypes indicating transfer of pollen between the ground and roof. These findings suggest that although gene flow from the ground to the roof is highly limited, there is a sufficient number of pollinators and enough movement of pollen within the roof site to avoid pollen limitation. Thus pollen supplementation would be necessary only to increase genetic diversity and not to reduce pollen limitation with these species at these green roof sites. The results of this work have important implications for managing green roofs to be self-sustaining, and for understanding the connections between these habitat islands and the surrounding environment.