How Pollinator Decline Impacts Entire Ecosystems

As pollinators like bees and butterflies decline around the world, their loss reverberates across entire ecosystems, a Princeton-led team of researchers report in the journal Nature. They found that when plants have to compete to court pollinators, it poses a significant threat to biodiversity.

“Our study identifies an unexpected and insidious way in which the loss of some species in an ecological network can trigger the loss of yet others,” said lead author Jonathan Levine, a professor at Princeton University and holder of the chair of ecology and evolutionary biology. “This suggests that the continued decline of pollinators may be unraveling the very fabric that keeps plant diversity stable.”

He and his colleagues found that a reduced number and variety of pollinators shifts the competitive playing field in favor of plants that are better able to compete for pollinators. This in turn can negatively affect organisms that rely on lost plants. These ripple effects could be devastating to ecosystem health.

Conservationists know that many flowering plants compete for pollinators – which are essential for reproduction – in the same way that they compete for resources such as sunlight, water and soil nutrients. In a world where pollinators are in decline, we would expect competition for pollinators to increase, but the implications of increased competition are unclear.

If plants of the same species can monopolize pollinators when together – for example, by putting on showy flower displays as a group – this would disadvantage rarer plant species and jeopardize their long-term coexistence.

First author Christopher Johnson, now a researcher at the University of Basel, led the project as a postdoctoral researcher at Princeton in Levine’s research group. He, Levine and co-author Proneet Dutt from ETH Zürich combined tightly controlled field experiments with mathematical models.

“This study builds on years of work in our group designing field experiments to inform mathematical models that predict species coexistence or exclusion,” Levine said. “Here, these methods pay off by revealing the hidden impact of plant competition for pollinators on plant diversity.”

To experimentally eliminate competition for pollinators, the researchers used brushes to manually pollinate certain plants. Other plants were left to the whims of the local pollinator community, which consists mainly of bees and bumblebees, along with a few solitary bees and hoverflies.

“Pollinating insects are much better pollinators than us, but they wouldn’t necessarily visit all plants,” Johnson said. “We were extremely faithful pollinators – we would pollinate every flower, not just those that were abundant and showy, but also flowers that were buried by their competitors.”

At the end of the summer, the study team collected all the seeds produced by each plant, brought them back to the lab, and “counted and germinated” them to determine how many viable seeds actually produced a seedling. .

Their results suggest that a global decline in pollinators could affect some plant species more severely than others.

“Perhaps unsurprisingly, the reduction in single-species pollinator diversity at very low densities resulted in a drastic drop in pollinator visits to flowers as well as seed production per plant,” Johnson said.

“It’s a bit of a cautionary tale that declining pollinators can have serious consequences for plants,” he said. “As someone who has been in the field and pretended to be a pollinator and tried to pollinate these plants, I can say that the ecosystem service they provide is enormous. Pollinating insects cannot be easily replaced, and we should really focus on conserving pollinator diversity.

“Competition for Pollinators Destabilizes Plant Coexistence,” by Christopher A. Johnson, Proneet Dutt, and Jonathan M. Levine, published July 20 in Nature (DOI: 10.1038/s41586-022-04973-x) was supported by Princeton University, ETH Zürich Center for Adaptation to Changing Environments and National Science Foundation (DEB 2022213).