Urbanization Influences Pollinator-Mediated Plant Reproduction

If you’ve been following this blog, you’ll know that urbanization has an impact on the ecology and evolution of species. However, we know much about how urbanization affects species interactions. For example, plant-pollinator interactions may be particularly sensitive to urbanization because both plants and their insect pollinators have been shown to respond to urbanization positively and negatively. A recent study published in Oecologia tested how plant-pollinator interactions alter along a gradient of urbanization. The study by Rivkin et al., examined two opposing hypotheses in the literature, the Urban Pollinator Enrichment Hypothesis, where plant-pollinator interactions respond positively to urbanization, and the Urban Pollinator Deficit Hypothesis, where plant-pollinator interactions respond poorly to urbanization.

(a) Map of Toronto with the 30 sampling sites and (b) bucket with the 5 plants at sitesTo test these hypotheses, the authors created 30 artificial populations of rapid-cycling Brassica rapa along an urbanization gradient in the Greater Toronto Area. Over the course of two years, they measured how pollinator-mediated plant reproduction and pollen dispersal changed with the amount of impervious surface surrounding a population and distance from the city center. They paired these measurements with pollinator observations conducted at a subset of sites, to understand if changes in plant reproduction were associated with variation in the pollinator community along the gradient.

The authors found that plants exhibited reduced reproductive effort (i.e., flower number) in urban populations, although the plants exhibited no overall reduction in fitness (i.e., seed number per plant). Urbanization was associated with decreased pollen dispersal between patches, suggesting that pollinators disperse less often between patches in urban habitats.

These results are most consistent with the Urban Pollinator Deficit Hypothesis. However, the effect of urbanization on seed production varied across the season: seed number was highest in urban populations early in the summer, but this trend reversed later in the season. This trend corresponded with greater pollen dispersal that was early in the summer. These results are most consistent with the Urban Pollinator Enrichment Hypothesis. The fine-scale environmental conditions of the cities, the extent of habitat fragmentation, and level of connectivity between plant patches likely play a role in explaining these trends. The authors recommend incorporating environmental and landscape features of cities to help inform predictions of the effects of anthropogenic disturbance on urban plant-pollinator interactions.

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Want to know more? Read the paper here:

Rivkin, L.R., Nhan, V.J., Weis, A.E. et al. Variation in pollinator-mediated plant reproduction across an urbanization gradient. Oecologia (2020). https://doi.org/10.1007/s00442-020-04621-z