The Concrete Landscape

                Though they may appear only to consist of monotonous hues of gray, the concrete jungles of urban areas in fact provide a multitude of minute habitats open to those plants who are able to survive in the some of the harshest of conditions. These small habitats – unmaintained gardens, vacant lots, and even small cracks in sidewalks – are all open to colonization by the most opportunistic of pioneers – weeds. Populations of plants that do get established within these urban habitats then have to cope with being trodden on by commuters, heat radiating off of nearby surfaces, and higher levels of pollution, in addition to dealing with simply existing in fragmented habitats.

                Environments that consist of many smaller habitat fragments inherently provide limits to plant population growth. Space itself may be a limiting factor, as large trees and woody shrubs take up a large amount of space that is simply not available in individual small fragments. Separation between fragments may lead to less effective pollination, particularly if pollinators have trouble moving from one habitat fragment to another. Even seed dispersal – the process by which these weeds arrived in this environment in the first place – is limited by many seeds falling outside of suitable patches and not becoming established.  

Figure 1. Crepis sancta. From http://flore.la.rochelle.free.fr

                Crepis sancta, commonly known as hawksbeard, is one such plant that has successfully colonized many fragmented habitats within urban environments, and it has been the target of multiple studies investigating plants in these fragmented, human-dominated environments. C. sancta produces two types of seeds – those designed to be dispersed by the wind, and those designed to simply fall from the flower to adjacent patches of soil. Pierre-Olivier Cheptou and colleagues have studied populations in southern France to look for evidence of evolutionary response of C. sancta to fragmented, urban environments. In one study (Cheptou et al. 2008), they hypothesized that in largely concrete environments, dispersal-type seeds are unlikely to land in new, suitable areas, so producing more non-dispersal-type seeds would be an evolutionary adaptive trait in these fragmented habitats.

                Cheptou et al. first counted the overall proportion of C. sancta seeds that are likely to land in a patch of suitable habitat, and found that in fragmented habitats, fewer seeds ended up in viable areas, meaning that plants in fragmented habitats have higher costs of dispersal. In these fragmented, urban habitats, the authors found that populations of C. sancta produce a higher proportion of non-dispersing seeds compared to contiguous, rural populations. By creating mathematical models of the evolutionary processes that would allow for this shift in reproductive strategy, the authors estimated that the observed difference in non-dispersing seed proportion represents around 12 years of evolutionary divergence, a number that is consistent with construction records for concrete areas near their study populations.

Figure 2. Proportion of non-dispersing seeds of the study populations from Cheptou et al. (2008), showing how fragmented populations generally produce higher proportions of non-dispersing seeds than contiguous populations.

This study (Cheptou et al. 2008) was the first to quantify evolutionary changes in dispersal strategies – toward an increased proportion of non-dispersal-type seeds – showing that C. sancta in fragmented habitats have adapted to better persist in these largely urban areas. Interestingly, as the authors point out, this decrease in production of dispersal-type seeds implies that it is now harder for these plants to colonize new areas, which may exacerbate the impact of continued habitat fragmentation and urban development. If plants in urban environments become less able to disperse to new areas, these plants are likely to have increasingly patchy distributions and be less able to react to ongoing changes in climate. 

                In an ongoing research project here on the Grinnell College campus, my lab group is working to characterize the distribution of multiple plant species within a largely concrete patio environment. Our study area consists of a large courtyard enclosed on all sides by the Noyce Science Center, which serves to isolate this courtyard from much of the surrounding college campus. This courtyard is further divided into many small (less than half of a centimeter wide) cracks between bricks, and seeds that colonized this area were likely carried into the courtyard from elsewhere on the Grinnell College campus by the wind. If the plant populations in our study area have been subject to fragmented environments for multiple generations, dispersal should be very limiting to the distribution of these plants within the courtyard, resulting in patchy populations of these weed species. Alternatively, if the influx of new seeds from outside the courtyard is greater than the production of seeds within it, the distribution of plants will reflect small-scale habitat features that promote seedling germination and establishment, such as the size of the gap between paving stones. By measuring the distribution of plant species within this courtyard, my group aims to determine how isolated these plant populations are from those elsewhere on campus, and how the micro-habitats in the courtyard differ in their ability to support plant growth.

References:

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