How do seeds travel when they're on an (urban) island?

     As urban areas become increasingly prevalent, ecologists are turning to study the (previously unrecognized) natural aspects of these environments. Urban areas offer different resource limitations and selective pressures than rural or natural areas, so studying other communities in these environments can illuminate interesting ecological phenomena. One of the challenges that plant communities face in these conditions is seed dispersal. In a city-like environment, resources are more likely to be fragmented by concrete or asphalt, leading to isolated patches of growth. The question is, then, if this island effect changes the efficacy of a species seed dispersal mechanism, and if this is putting selective pressure on plant communities. Cheptou et al. (2008) investigated this phenomenon in their research on the evolutionary consequences of seed dispersal in the weedy, allogamous plant Cresis sancta. 

 Crepis sancta, http://www.freenatureimages.eu/plants/flora%20c/Crepis%20sancta/index.html   

 Since C. sancta produces both dispersing and non-dispersing seeds, the researchers were able to plant C. sancta in artificial patches throughout a city south of France, and measure how many dispersing and non-dispersing seeds were produced in a given season. Then, they were able to germinate the seeds from this first experiment in the field in the lab, and measure how urban plants compared in their seed dispersal profile to populations in “country-side-like” conditions. What the researchers found was that dispersing seeds had, on average, a 55% lower chance of landing in suitable habitat, which makes sense when considering the likelihood of a seed landing on concrete in a city. Non-dispersing seeds were, therefore, more likely to land on suitable ground. In a comparison of urban and rural populations’ progeny, the researchers also found that urban populations produced a significantly higher ratio of non-dispersing seeds. Genetic modeling showed that this shift in the ratio of seed dispersal was exceptionally quick, demonstrating that fragmented landscapes such as those found in cities put a lot of selective pressure on communities to produce more non-dispersing seeds. 

    Cheptou et al.’s research highlights the impact that fragmentation has on plant communities, even down to their ability to spread to other areas. Riana, Catherine and I are similarly interested in studying fragmentation of plant communities, but we are studying patches under spruce trees on Grinnell’s campus. The microhabitats under spruce trees may be impacted by the island effect, especially if the plants in question cannot disperse their seeds in the same way that they might in a non-fragmented area, as Cheptou et al.’s research suggests. We suspect that spruce trees in proximity to each other may have similar plant species profiles, while spatially segregated spruces will be fairly different. For example, a disturbance-tolerant plant like Plantago major that sends out wind-dispersed seeds might not be present in disturbed areas that we would expect it to be because of these added difficulties in seed dispersal. Granted, the conservation of P. major is not a concern among ecologists because it is weedy, but this research is relevant in understanding these patterns of dispersal in the plant community. Microhabitats under trees may prove to be a vital source of biodiversity in our increasingly urban human habitats. 

References

Cheptou, P.O., Carrue, O., Rouifed, S. & Cantarel, A. (2008). Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta. PNAS, 105(10), 3796-3799.