Commuting to… pollinate? How bees are choosing fragmented sites to forage

Featured Image Caption: Like many bee species, alfalfa leafcutter bees are solitary bees. They don’t live in hives, but are still important pollinators. (This photo by Claire O’Neill is licensed by CC BY-NC 4.0.)

Source Article: Fragoso, F. P., & Brunet, J. (2023). The decision-making process of leafcutting bees when selecting patches. Biology Letters, 19(2).

Habitat for hiveless bees

What comes to mind when you think about bees? Honey? Summertime stings? Watching pollen-dusted bumblebees whiz from flower to flower? Many people think about bees’ social lives — in the case of honeybees, working alongside tens of thousands of their hivemates — but in reality, over 85% of bee species are solitary! They don’t live in hives, though some species still nest in groups. Some build their own nests out of materials like soil, pieces of leaves, or even small rocks stuck together with plant resin.

Like other bees, solitary bees are important pollinators, so it is important to understand how to protect them. One current threat to solitary bees (as well as non-solitary bees and many other species) is habitat fragmentation. This is when a once-continuous piece of habitat is broken up into smaller pieces, usually because of land development. This makes it harder for animals to find resources, and may cause conflict between animals and humans due to increased contact between the two.

Habitat fragmentation occurs due to land development, including clearing of land for construction or agriculture. It can happen to any habitat, including forests and prairies. (“Fragmentation of forests due to plantations 2” by Harikrishnan S is licensed through CC BY-SA 4.0.)
The buzz about alfalfa bees

Scientists are trying to figure out the best way to manage land to conserve biodiversity, especially in locations where fragmentation is inevitable. For example, is it more effective to protect many small, disconnected pieces of habitat, or protect a few large pieces? If we create ‘corridors’ connecting habitat fragments, how can we encourage animals to use them? Recently, scientists were interested in how solitary bees traveled between habitat patches. They were especially curious about how patch size and distance from the bees’ starting location impacted the bees’ choice. Would they prioritize closer patches, even if they were very small, or would they travel far distances to reach larger patches with more flowers?

The experiment used the alfalfa leafcutting bee (Megachile rotundata), a solitary bee which builds nests out of leaf pieces. Alfalfa leafcutter bees originated in Eurasia, but became important pollinators in North America after their introduction in the early 1900s.

Leafcutter bees use pieces of leaf to make nests. In each nest, they lay an egg and leave a supply of nectar and pollen for the larva to use once it has hatched. (“Megachile rotundata” by Jodelet / Lépinay is licensed by CC BY-SA 2.0 FR.)

These bees’ economic impact is second only to the honeybee, and they are especially known for improving alfalfa seed production because of their pollination. As a result, scientists studying the effects of habitat fragmentation on solitary bees chose to focus on this species and set up four patches of alfalfa: two large and two small. One patch of each size was located closer than the other to an additional center patch, where the bees were released. The scientists recorded when bees traveled from the center patch to any of the other patches in order to see which ones the bees preferred.

Figuring out flight paths

So how do bees choose where to commute?

The scientists found that the leafcutting bees preferred to visit close patches (both large and small ones). In contrast, a previous experiment found that the common eastern bumblebee (Bombus impatiens) strongly preferred patches which were both large and close. This potentially suggests that social and solitary bees have different ways of deciding where to forage, or that the common eastern honeybee — which is larger and likely has a larger brain than the alfalfa leafcutting bee — may be better at figuring out where resources are.

Scientists were interested in how alfalfa leafcutter bees (left) decided which plots to forage at. They compared their findings with results from a similar study about the common eastern bumblebee (right). (The left photo by matthew_wills is licensed through CC BY-NC 4.0 and the right photo by Elena is licensed through CC BY-NC 4.0.)

These results suggest that when designing habitats for bee conservation, having larger, relatively close patches would best encourage bees to travel between them. This is beneficial because it would give the bees access to more resources (instead of restricting them to the resources within one patch). Large patches may also attract a greater number of bees. This setup not only helps the bees, but gives the plants access to pollinators: a win-win!

Bees are important ecologically and economically, but they are currently threatened by habitat loss and other human impacts. Here are some ways you can help them! And if you’re interested in reading more about bee research, take a look at previous Envirobites articles here.

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Lauren Otolski

Lauren Otolski

Hello! I am a third-year PhD student at the University of Illinois Urbana-Champaign, studying tropical ecology. I'm specifically interested in decomposition, and how factors like wood and soil nutrients, fungal communities, and wood chemistry interact! I also love writing, playing tabletop and video games, and spending time outside.

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