Latest buzz: rare bees are an integral part of diverse and resilient ecosystems

A bee of the genus Ceratina on a morning glory flower, genus Ipomoea. (Photo by Joe Zientek)

Honey bees are big business. As primary pollinators for about a third of all US agricultural production, they contribute at least $15 billion a year to the US economy. Wild bees also contribute to US agricultural yields, increasing the value of just seven major crops by $1.5 billion a year.

But while there are about 20,000 species of bees in the world, of which about 4,000 live in the United States, the bulk of crop pollination is supported by only a tiny fraction. A study found that just 2% of bee species are responsible for 80% of crop pollination. When it comes to crop pollinators, quantity seems to be more important than variety. Fewer pollinators lead to lower yields of popular foods like apples, blueberries, cherries, almonds and watermelons.

What about the other 98% of bee species? Do they have no value?

A new study published in the Journal of the Royal Society for Biological Sciences analyzed more than 20,000 bee-flower interactions in New Jersey to assess the value of bee diversity in wild plant communities. The researchers found that the number of functionally important bee species increased with the number of plant species in a community. In clearer terms, diverse ecosystems require a greater diversity of bees.

It’s unclear what would happen if bee diversity in an area plummeted, but a dramatic decrease in the number and variety of pollinators could reduce plant pollination and, in concert with climate change, destabilize ecosystems. Whole plants and animals – including of course humans – rely on.

“Until we did this paper, very little work had been done on the role of pollinator diversity in natural systems or in various plant communities,” said Dylan Simpson, the paper’s lead author. .

bee on yellow flower
The bee Halictus ligatus on a flower of a partridge pea, Chamaecrista fasciculata. (Photo by James Reilly)

Most studies of pollinator diversity to date have focused on crop pollination. “In this context,” Simpson said, “there’s certainly evidence that pollinator diversity matters, but there’s also a fair amount of evidence that abundance is really what matters most.”

This conclusion reflects the normal distribution of species. “Generally in nature there are a few really abundant species, and most species are actually relatively rare,” Simpson said. So if you have two fields and one field is less pollinated than another, there are probably just fewer of these very common bees.

“That can kind of lead to this idea that relatively few species are really doing all the work,” Simpson said. “And we need to focus on maintaining the abundance of these common species.” It may also lead researchers to conclude that species diversity has no value beyond biodiversity for the sake of biodiversity.

But food crops are only a small fraction of the plants that depend on animals like bees for pollination. Nearly 90% of flowering plants are pollinated by animals.

“On a farm, you’re only looking at one plant species, whereas in the wild plant community adjacent to the farm, there are many different plant species.” said Simpson. “Different species of bees will visit different species of plants, or different species of plants will depend on different species of bees. And that would suggest that bee diversity is going to be much higher if you have many plant species that need to be pollinated. And it looked like a very simple idea that can’t be shown, if you just look at cultures. It’s just kind of invisible.

Simpson’s dataset was gleaned from 11 sites in New Jersey, which the researchers visited multiple times over the course of a year. At each visit, the researchers – equipped with note-taking equipment and a net – systematically surveyed the site, looking for bees that had landed on the reproductive parts of a flower, caught in the act of pollination, maybe. They would then collect the bees, note the last plant they visited, and then bring them back to the lab for identification. The researchers collected over 20,000 bees, and the final dataset Simpson analyzed (once a few plants with too little bee data were excluded) consisted of 20,942 plant-bee interactions, representing 70 species of plants and 173 species of bees. (There are about 400 species of bees native to New Jersey.)

scientist in a field
Simpson co-author Molly MacLeod in the field. (Photo by Rachael Winfree)

Simpson’s first question was about the importance of bee diversity; the second was the importance of rare bee species. “When we think about biodiversity loss and fear losing species, it’s usually the rare species that we fear losing,” Simpson said. “If we are thinking about the consequences of biodiversity loss, we really should be thinking about the functional consequences of losing these rare bee species. And because of the way we asked this question primarily for crops, it generally seemed like rare bee species didn’t really do much for pollination.

Simpson’s analysis found that a quarter of community-important bee species were rare. To clarify, these were rare species among the sample he analyzed, so rare in this case doesn’t necessarily mean endangered, or even rare in the US – just numerically rare at these 11 sites from New Jersey.

“Usually endangered things are rare, but not all rare things are endangered,” Simpson explained. “In the context of the academic literature, there’s a lot of reason to think, especially in the way this pollination has been studied, that rare species weren’t really contributing to it. So whether they were endangered or not, I think that’s a good demonstration that things don’t need to be super abundant to have significant effects.

Simpson’s study goes a long way in establishing the value of bee diversity for the role it plays in the larger ecosystem. People don’t necessarily want to save bees to save bees – they need research like this to show and say that rare bees matter. Simpson’s study may not have quantified this value, but it still demonstrates that the diversity of bees has a function.

Of course, Simpson can’t say for sure what would happen if bee diversity in New Jersey or elsewhere were to collapse, because pollinator plant communities are complex and dynamic. But a dramatic decrease in the number and variety of pollinators could reduce plant pollination and destabilize ecosystems. “As species go, there are fewer interactions, and generally the systems are going to get simpler, and then they’re a little more rigid, there’s less backing up,” Simpson said. “They become less resistant to change, less resistant to disruption, and more prone to collapse.”

For example, a big concern with climate change is phenological mismatch. “It’s the idea that the flowers and the bees that pollinate them might not emerge at the same time, and then the plants will stop being pollinated and the bees will stop feeding,” Simpson said. Loss of diversity will make bee and plant communities more vulnerable to this risk. In a healthy and diverse ecosystem, there is more likely to be some redundancy, which means that more than one bee species can pollinate more than one plant species, and more than one plant species can feed more than one species of bee. But if this system is simplified and a type of plant depends on a type of bee, or vice versa, these two species will be more vulnerable to climatic changes which will desynchronize them.

This is precisely what Simpson would like to see investigated in the future. “We documented which bees were pollinating which plants when we observed; we actually didn’t collect data that would tell us what would happen if these species went extinct,” Simpson said. “If a particular species of bee disappears from an area, what happens to the community? …[And] what is the real cost of plant fitness? »

Wild bees face all kinds of threats, not just from climate change, but also from habitat loss, pesticides, and viral and fungal pathogens. And this threatens all bees, not just the rare ones.

“Really, you wouldn’t expect the bee community to stay the same except for one less species,” Simpson said. “Usually if you lose species, it’s because you lose individuals overall. And as you lose individuals overall, some populations become too small to sustain, and then you start to see species start to disappear.

This suggests that any steps taken to maintain bee abundance – from saving habitat to reducing pesticide use to diversifying cropping systems – will likely save bee diversity as well. This will benefit wild plants and crops, and the people and animals that depend on them.