|The cover of the recent issue of Proceedings B. (Photo: Marc Johnson.)|
I was not very optimistic when my M.Sc. supervisor, Dr. Marc Johnson, proposed that we study whether plants were adapting to urban environments. Looking back, with the study being recently published, it is clear that my pessimism was unwarranted. This study ended up being a very fun ‘whodunit’ with unanticipated discoveries around every corner, and one that will, it seems, keep on surprising us into the future.
During the summer of 2014, I was living at the Koffler Scientific Reserve at Joker’s Hill, the University of Toronto’s picturesque* field research station. There, I was conducting an experiment on the evolution of plant defences using white clover (Trifolium repens L.). This plant has a genetic polymorphism for the production of hydrogen cyanide (cyanogenesis), where within-population genetic variation causes some individuals to produce cyanide, and others to lack it.
|Me and my clover mane.|
|Figure 1 from our paper. In three of four cities, the frequency of cyanogenesis increased toward rural areas.|
|The three authors about to depart on a ferry crossing the Ottawa River to Oka, QC.|
When the field experiment ended, we were surprised to find that there was no change in herbivory along the urbanization gradient in Toronto. This was initially disappointing because I was left with no ideas about the causal factor, but this feeling didn’t last. At my committee meeting, the ever-insightful ecologist, Peter Kotanen, posed an alternative explanation for our findings. Peter suggested that reduced urban snow cover caused by the heat island effect could ultimately leave plants exposed to cold air temperatures, while rural plants would be kept warm by a relatively thick layer of insulating snow cover.
|Figure 3A from our paper. The 'relative urban coldness' index shows the cold island (values above 0) appearing during the winter, and then changing back into a heat island (values below 0) following snowmelt at the end of winter. Curve is 95% CI. More details in paper.|
|Sampling clover at the Washington Monument.|
While urban evolution studies may be well-suited for testing fundamental questions in evolution, they have a unique ability to motivate ecologically-minded urban design & policy. There have been many ecological studies conducted in urban environments, but it’s not always clear that the variables measured are important for the biology of organisms. The unique promise of urban evolutionary studies is to identify the ecological variables that affect biological fitness (i.e., 'reverse ecology') in cities, and in doing so can motivate urban design that mitigates such stressors. My ultimate hope for the field of urban evolutionary biology is that its discoveries are used to generate in city-dwellers a curiosity for the natural world. And who knows, maybe some theoretical advances will be made along the way.
****To our knowledge we are the first to document this phenomenon.