How Loyalty Paints a Meadow

Without loyalty, a meadow would be a much less colourful place. If a bee sticks to just one species of flower, the chances are good that it’ll transfer pollen from one individual to another. If it flits between many species, its efficiency as a pollinator plummets, its flowery partners produce fewer seeds, and the meadow loses its sea of vibrant petals.

And unfortunately, this bucolic dystopia might happen very easily. By removing bees from enclosed fields, Berry Brosi and Heather Briggs from the Rocky Mountain Biological Laboratory in Colorado have shown that the loss of a single pollinating species leads to a field-wide loss of loyalty. The remaining pollinators start visiting a wider range of flowers, chauffeuring pollen to the wrong destinations. And the flowers, as a result, produce fewer seeds.

That’s bad news because pollinating insects are already disappearing around the world.  The continuing loss of honeybee colonies grabs a lot of headlines, but many species of bumblebees, hoverflies and butterflies are in trouble too. That’s bad news. The majority of wild plants, and 75 percent of our food crops, depend on insects to ferry pollen from bloom to bloom.

There’s one silver lining: most plants are visited by many pollinators, so they should be able to tolerate the loss of any single insect. Indeed, when Jane Memmott from the University of Bristol created computer-simulated networks of plants and insects, she found that things only collapse when many, even most, of the pollinators disappear.

But Brosi and Briggs saw a flaw in this logic. It assumes that insects behave in exactly the same way, regardless of which other species are around. Remove one species, and it’s business as usual for the rest. And that’s probably not true—fields and meadows are about competition as well as partnership. Some flowers offer more pollen or nectar rewards than others, and pollinators can vie for the best prizes.  If, say, a bumblebee disappears, it creates an ecological vacuum, and other species might rush in to fill the void.

To test this idea, Brosi and Briggs worked in meadows near their Colorado lab, which are frequented by 11 species of bumblebee. The duo marked out 20 plots, each a 20-metre-wide square, and removed the most abundant bumblebee species in each one using butterfly nets. “We became very adept at identifying the bumble bee species on the wing, which made it possible to remove the target species with the nets while not disturbing other species,” says Brosi.

They kept these captives safe in vials for the duration of the experiment, before releasing them. All the while, they recorded the movements of the other bumblebees as they zipped from flower to flower, either in the presence or absence of the dominant species.

The duo found that removing the leading bumblebee changed the behaviour of the other bees, which quickly broadened the range of flowers they visited. The proportion of bees that stuck to just one plant species fell from 78 percent to 66 percent. On average, they started visiting between two and three times as many plant species as before.

The meadow plants paid the price for the bees’ falling fealty. Brosi and Briggs found that the larkspur (Delphinium barbeyi), a flower that’s pollinated by most of the local bumblebees, produced just two-thirds the usual number of seeds when the main bumblebee vanished.

“It’s a clever experiment and a difficult one to do,” says Memmott. “Many of the field plots are a kilometre apart on a mountain range so I suspect that these folk are very fit ecologists!”

These results are also rather ironic. People usually think that redundancy makes for strong and resilient ecosystems; if many species play the same roles, then losing one of them won’t matter that much. But this experiment shows that redundancy also had a dark side; if many species play the same roles, the competition between them could exacerbate the loss of just one.

This might mean that the pollinator crisis is worse than we imagined. For example, three of the UK’s bumblebee species have already gone extinct (or are possibly just extremely rare).

“While policy-makers have rightfully been worried about pollinator declines, this adds even more impetus to protect pollinators,” says Brosi. At the top of her list: taking more cautious approach to approving pesticides. “While other factors certainly contribute to pollinator decline, we can’t do much to directly counter many of the other threats that pollinators face.  In terms of the careful regulation of agrochemicals, however, we can actually do something.”

Memmott welcomes the new study even though its conclusions differ from her own simulations. “Ours was a first pass at asking about species loss in pollinator communities and is an old paper now,” she says. “Theirs is the better approach in a number of ways.  Although ours probably overestimated the resilience of the community, theirs will underestimate it because they used very small plots and only have a short-term dataset. The bottom line is that we need more work like this.”

Bill Kunin, an ecologist at the University of Leeds, says the study is “ambitious and audacious”, but he’s also surprised by the results. Ecological theories, he says, would predict that when competitors decline and resources become more abundant, pollinators should become more specialised on the flowers that offer the best rewards, rather than more generalised. Brosi and Briggs only removed the top bees for a day, and Kunin suspects that given more time, the bumblebees would settle on a new favourite plant. “It’s an interesting and original paper but I’d be a bit hesitant to draw sweeping conclusions from it concerning the implications of pollinator loss,” he says.

Kunin adds, “When species are lost, it’s usually the rarest ones, not the commonest ones, that go extinct. It’s pretty likely that when that happens, the impact on the overall resource market will be less noticeable than it was here.”

But Brosi points out that common bumblebees have disappeared. The western bumblebee (Bombus occidentalis) was once the most common bumblebee species in the western US. “Now, however, its range has contracted dramatically,” says Brosi. “A large proportion of the populations are extinct, and it is found only in scattered local areas.  This dramatic decline is thought to have been caused by an introduced pathogen, so it’s not always the rarest species that go extinct first.” However, he and Briggs are now doing follow-up experiments to check what happens when you lose rare pollinators rather than common ones, or generalists versus specialists. As ever, this study is another chapter of an ongoing story, rather than its coda.

Reference: Brosi & Briggs. 2013. Single pollinator species losses reduce floral fidelity and plant reproductive function. PNAS http://dx.doi.org/10.1073/pnas.1307438110