Potential Effects of the Urban Environment on Plant Adaptation
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In this except from Urban Evolutionary Biology authors Amanda J. Gorton, Liana T. Burghardt, and Peter Tiffin explore how some important urban–rural differences might be expected to alter the evolution of plant life.
Decades of research on life-history adaptation in non-urban settings has shown that selection acting on life-history traits can be strongly dependent on the environment. As such, studies in non-urban systems provide guidance for identifying traits that are likely to be subject to differential selection in urban versus rural environments, as well as specific environmental factors that may be driving that selection.
Urban environments differ from rural environments in a myriad of ways, only some of which are well characterized. In the next paragraphs, we provide a brief overview of how some important urban–rural differences might be expected to alter evolution of plant life histories. The list of environmental factors we discuss is by no means exhaustive.
One of the most evident differences between urban and rural environments is the patchiness of urban environments. Areas suitable for plant growth are interspersed among buildings, roads, pavements, etc. This patchiness can directly alter the selection acting on dispersal-related traits, which we discuss below. Habitat fragmentation is likely to affect non-adaptive evolution, also discussed below, by breaking populations into subpopulations. The evolution of these subpopulations will be affected by gene flow from other populations as well as genetic drift, which is greater in small than large populations and can reduce the efficacy of selection. While fragmentation caused by buildings, roads, and pavements can directly alter selection, the concentration of asphalt and concrete in urban areas affects the temperature and water availability. Thus, fragmentation has important indirect effects on plant growth, fitness, and selection.
One of the most evident differences between urban and rural environments is the patchiness of urban environments. Areas suitable for plant growth are interspersed among buildings, roads, pavements, etc.
The effects of urbanization on temperature are two-fold: daily temperatures are expected to be higher in cities than surrounding non-urban areas, and, at least in northern latitudes, urban areas are likely to have longer potential growing seasons than surrounding rural areas. By contrast, in lower latitudes and warmer climates, the higher temperatures in urban areas might shorten the length of the growing season due to combined effects of heat and water shortage. On the other hand, lawn watering might increase water availability in urban areas, or at least parts of urban areas, relative to surrounding rural areas.
Changes in the length of the growing season have the potential to greatly alter tradeoffs between plant growth and reproduction. Given that plants need to germinate, grow, flower, and produce seeds before the end of the growing season, a longer growing season may allow plants to grow much larger before they initiate reproduction. Similarly, longer growing seasons in urban areas may make it selectively advantageous for perennials to accelerate the transition from dormancy to growth and/or delay the transition from growth to dormancy. Warmer daily temperatures along with high concentrations of impervious surfaces have the potential to strongly affect plant water demands in urban areas.
Ample evidence from non-urban systems indicates that changes in the timing or amount of water availability can alter growing season length and selection on germination timing and flowering time and/or cause changes in seedling or adult survival that shift selection from favouring annuals to perennials. In colder climates, in which water is not limiting, warmer daily temperatures may be advantageous for genotypes that are able to extend their growing periods before transitioning to reproduction. However, when water is limiting, hotter day and night temperatures, particularly during the peak of the summer, are likely to produce stressful conditions that favour genotypes that are able to tolerate or avoid stress, possibly through phenological shifts. These shifts could take the form of favouring individuals that are able to complete their cycles — allowing plants to reproduce before the hottest times of the year, as has been shown in populations of plants that experience seasonal droughts.
Changes in the length of the growing season have the potential to greatly alter tradeoffs between plant growth and reproduction.
Alternatively, when coupled with the longer growing seasons in urban areas, selection might favour delays in reproduction, especially if plants are able to avoid or tolerate drought stress during the midst of the growing season. Warmer temperatures in urban environments might alter selection acting on plant populations in unpredictable ways that are not directly related to life history. For example, Thompson et al. found that the frequency of cyanogenesis (the ability to produce hydrogen cyanide (HCN)) producing genotypes in the perennial plant white clover increased with the distance from the urban centre in three of four cities. HCN production has been extensively studied for its role in protecting plants against herbivores, and thus the higher frequency of HCN in rural than urban areas seemed likely due to greater herbivore pressure in rural areas.
A field experiment, however, revealed no evidence for HCN concentrations having differential consequences for herbivore damage in rural compared to urban environments. A more likely driver of the putative adaptive clines in the frequency of HCN producing genotypes is that cyanogenesis can have negative fitness consequences if freezing temperatures occur when there is no snow cover. Despite the urban heat island, minimum ground temperature during the winter, and the location where plants are overwintering, it is actually colder in some urban areas than rural areas because snow acts as an insulator during the cold winter months. Since snow cover can be less persistent in urban environments, selection could be acting on winter survival, with adaptation occurring through a reduction in HCN production.
The lesson here is that although we might expect life histories to evolve in response to shifts in temperature or water availability, populations might adapt to these changes through less obvious mechanisms. Of course, it is not only the abiotic, but also the biotic environment that shapes selection, and biotic communities can be profoundly different in urban and rural areas. From the perspective of life histories, one of the most important shifts in biotic community is likely to be shifts in the herbivore and pollinator communities. Changes in the population sizes or identity of herbivores and pollinators can strongly shape selection on life histories.
Marta Szulkin, Associate Professor, Centre of New Technologies, University of Warsaw
Jason Munshi-South, Associate Professor, Department of Biological Sciences, Fordham University
Anne Charmantier, Director of Research, CEFE, University of Montpellier
