Cities represent one of the most rapidly expanding environments on the planet, and are characterized by intensive land-use changes. The resulting habitat loss can alter the pollinator community which may affect plant fitness. Because pollination success depends on both the quantity and quality of pollen deposited on the stigma, plants which experience an altered pollination environment may experience reduced fitness. Plant fitness is often lower in cities, but whether this is due to reduced pollination success, or smaller plant population sizes and inbreeding depression, is unclear.
Here, we sought to experimentally test how pollination success varies with urbanization, and whether pollinators in urban habitats move less frequently between plant patches than in nonurban habitats. We created 30 experimental arrays of self-incompatible, rapid-cycling Brassica rapa in urban, suburban, and rural yards in Toronto, Canada. Within each array, we created two patches: a hermaphroditic and a male-sterile patch. The male-sterile patch was used to assess how often pollinators move between patches because male-sterile plants can only set seed if a pollinator first visits the hermaphroditic patch then travels to the male-sterile patch. Finally, we replicated the experiment in early, mid, and late summer 2017.
Results/Conclusions
We found that plant fitness significantly decreased with distance from the city in June, but this effect disappeared later in the season, suggesting that plants experience different pollination environments due to seasonal changes in the pollinator community. We also found that while male-sterile plants had significantly lower fitness than hermaphroditic plants, this trend did not vary with distance from the city, suggesting that pollinators move between patches with equal frequency in urban and nonurban populations.
Overall, our results suggest that cities can reduce pollination success, but this effect depends on season. Pollinator communities experience species turnover across the flowering season, and a pollinator which is rare early in the season may become common later in the season. Consequently, pollination success in urban environments may be low in the spring due to decreased pollinator abundances, but increase as pollinators become more abundant later in the summer and fall. Our research highlights the importance of considering plant-pollinators interactions when studying the role of urbanization on plant evolution.