Wild bees thrive among diverse flower communities, study shows

Spring is approaching in the northern hemisphere, and with it come the perennial considerations about which kinds of flowers to plant and where. These questions go far beyond mere aesthetics. Flowers are an essential source of food for birds and insects, including the dozens upon dozens of species of wild bees found in most cities. For them, what's on the menu is a matter of life or death.

The relationship between flower species and pollinator populations matters deeply to urban biodiversity. In a study published in the journal Ecological Applications, Concordia researchers have looked at how certain floral characteristics interplay with urban bee populations.

They found that a diversity of size and shape of the corolla—the tube-like structure formed by a ring of petals around the flower's center, where nectar is typically stored—in the urban flower community has a positive relationship with wild bee biodiversity. The more diverse the urban flower population, the richer the wild bee population.

"We know that flowers are an important source of nutrition for bees, but not all flowers are created equal," says the study's corresponding author Carly Ziter, an associate professor in the Department of Biology in the Faculty of Arts and Science.

"Having a high diversity of flowers is a key to supporting bee populations, but our research goes one step further to show a possible mechanism: that having a diversity of floral species is important because they have different shapes and sizes.

"This follows the idea of size-matching, where corollas are the right size for a certain type of bee's proboscis or tongue, allowing it to effectively connect to the flower's nectar. So, whether a bee has a short or longer proboscis, they will be able to access the nutrition they need if there are multiple kinds of flower morphology nearby."

A food web buzz

The data used for this study was collected by Ziter and colleagues in July 2020. They collected almost 2,000 bee samples from 16 sites around the Island of Montreal, each site visited twice. The sites included community gardens, nature parks and cemeteries, all at least one kilometer apart from each other.

At each site, the researchers identified plots that measured two by two meters and identified every flower species found within them. Floral density was calculated, and the corolla of each species was measured using digital calipers. They also collected nectar samples from each flower but were unable to establish a firm connection between sugar content and bee diversity for this study.

In total, they identified 94 different species of bees and almost 150 different species of flowers.

"We wanted to see if there was an optimal value for corolla traits," Ziter says. "Was it better to have small corollas, medium ones or a diversity of them? The results showed quite strongly that having a diversity of corolla sizes was important."

Ziter says the study supports the belief that a diversity of flowers impacts the next trophic level, which contributes to an ecosystem's food web in a way that goes far beyond bees. It can also inform anyone with an interest in promoting healthy landscapes, from city planners to amateur gardeners.

"If you are planting, try to think about what makes these species different. Are they different colors? Are they different sizes? Do they bloom at different times? The more you can think about these different axes of variety in your own garden, the more you are likely to be better off in terms of supporting biodiversity."

Serena Sinno, MSc; former Concordia MITACS postdoctoral fellow Gail MacInnis; and Jean-Philippe Lessard, a professor in the Department of Biology, contributed to this study.