Featured Image Caption: This bumblebee visits the flowers of a field mustard plant to collect pollen (Source: Rasbak via Wikimedia Commons).
Reference: Rivkin, L. R., Nhan, V. J., Weis, A. E., & Johnson, M. T. J. (2020). Variation in pollinator-mediated plant reproduction across an urbanization gradient. Oecologia, 192, 1073-1083. https://doi.org/10.1007/s00442-020-04621-z
Plants and Pollinators
You might have noticed that gardens are often visited by lots of bees and butterflies. Butterflies and flowers are certainly beautiful additions to any garden, but they serve another important purpose. Plants and pollinators – including bees, wasps, butterflies, and moths – have a special relationship where pollen and nectar feed pollinators and pollinators move pollen from one plant to another. This process, called pollination, allows plants to reproduce by forming fruits that contain seeds. These fruits are often sources of food for wildlife and humans. Thus, pollinators are vital for the production of our crops.
Even though our survival depends on sustained plant-pollinator relationships, we are also their biggest threat. The chemicals we spray on plants to deter pests and air pollution make cites especially challenging habitats for a lot of plants and animals. On top of these conditions, cities are highly fragmented habitats. This means that a bee may have to travel a long way to reach another garden with unfavorable conditions (for example, a busy road) between patches. As cities continue to grow, these conditions could have increasing and variable effects on the interactions between plants and pollinators.
Life in the City
L. Ruth Rivkin and her fellow researchers at the University of Toronto in Canada wanted to determine how cities affect plant-pollinator interactions by measuring plant reproduction and observing pollinators. To accomplish this, the researchers needed a plant that flowered quickly and in any season. The field mustard plant, Brassica rapa L., is the perfect plant for the job! Field mustard produces yellow, pollinator-loving flowers in just 14 short days. Although a single plant produces both egg and sperm, it can only be fertilized by sperm (pollen) from a different plant.
The researchers put five field mustard plants into a bucket and distributed a bucket to each of 30 sites all over urban and rural Toronto. They left the buckets for 1 or 2 weeks and counted the number of seeds and flowers produced by each plant. They performed these experiments for two years (2017 and 2018) in early and late summer. The researchers compared seed and flower numbers of the plants by their distance to the center of the city and by how much of the land is covered by pavement and buildings. These characteristics can be used to determine where a site falls on the urban-to-rural gradient. For example, more urban sites are closer to the city center and have higher coverage of pavement and buildings.
Seed production depended on the year, and both seed and flower number depended on the season. The seed number was higher in more urban sites in 2017 but did not significantly differ between urban and rural sites in 2018. Additionally, urban plants produced fewer seeds and flowers in early summer than late summer. Plants at more rural sites produced more flowers than urban areas. Even though there appeared to be more pollinators in urban areas, the researchers found that urban sites had lower pollen dispersal. Altogether, environmental conditions in cities affect plant-pollinator relationships, and these relationships change throughout the growing season and across years.
Save the Pollinators
Urban plants bloomed fewer flowers than rural plants. Pollution in cities from car emissions could be detrimental to the health of the plant, causing urban plants to produce fewer flowers and seeds. Differences in seed and flower numbers between early and late summer could reflect turnover in pollinators. The presence of different types of pollinators changes throughout the growing season. Certain pollinators may be more resilient to urban disturbances than others. Pollinators, if more abundant, might be less efficient in cities where flower and seed numbers were generally lower. Difficulty in travel between gardens may be the source of this inefficiency. Overall, plant-pollinator responses to urbanization varied between years and growing seasons, which exemplifies the delicate relationship between plants and pollinators.
Pollinators and plants rely on each other for survival and reproduction, and we rely on this relationship for our food supply. Cities pose many challenges for both plant and pollinator survival. Most notably, fragmented habitats make travel between gardens difficult for pollinators, and pesticides and air pollution can be detrimental to plant health. Cities are not going to de-densify anytime soon, but there is something we can do to help! By planting more native plants that attract pollinators to our yards, we can increase floral resources for pollinators. Visit this guide to learn more about what species you can plant to attract different types of pollinators. Happy gardening!