Less buzz, less variety: fewer pollinators lead to lower plant diversity

In an unexpected twist on Darwin’s survival of the fittest, a research collaboration from Princeton University, US, and the Swiss Federal Institute of Technology (ETH), Zürich, has found that as plants compete for a declining population of visiting pollinators (bees, butterflies, and other insects), they experience population decline and an overall decrease in species diversity.

This flies in the face of ecological theory, which suggests that environmental pressures such as pollinator competition would drive adaptation among the plants, making the species more diverse, not less.

The research team planted five annual flowering plant species (including poppies, cornflowers, and wild fennel) in a meadow, gradually changing density of the planting – that is the number of plants nearby to each other. Some sections were exposed to normal levels of pollination – by bees (honeybees, bumble bees, solitary bees), hoverflies and butterflies, in this case. Other sections received an extra helping hand (literally), with humans physically transferring extra pollen.

The researchers discovered that in situations where plants competed for the attention of bees, flies, and butterflies (without human assistance), there was greater decline in population growth and diversity.

To live as a successful population, plants develop niche differences that interact with the unique adaptations of other plants in a kind of system balance, or equilibrium. When that balance is disrupted – when pollinator numbers decline, for instance – the imbalance tends to favour the common species at the expense of the rare species. The reasons for this are many and complicated but include factors such as the ease with which pollen is picked up and dispersed by pollinators, the already inflated numbers of more common varieties, and the increased likelihood of receiving common pollen from a visiting pollinator.

“Climate change, habitat modification and pesticides are thought to be the major drivers of pollinator decline,” says Dr Christopher Johnson, a research scientist and engineer at the University of Washington, US, and one of the authors of this study, and this may “change the competitive playing field for plants, causing some plant species to go extinct”.

There are some important implications; not only for how we understand ecological evolution in this setting, but also potentially for feeding a growing global population. “Food security is critical for a growing world population – which also puts increasing pressures on plant and pollinator communities – so it is important to better understand plant-pollinator communities and work to conserve them worldwide,” explains Johnson.

In the future, we may have engineered pollinators, such as “RoboBees”, that can help plants maintain diversity and population, but for now, it is worth focussing on conserving natural pollinators. “What I can say from being out in the field, hand-painting plants with a paint brush is that insect pollinators are really, really good at their jobs, so there is no substitute for healthy and diverse plant-pollinator communities,” says Johnson.

For those lucky enough to have a backyard veggie patch or a flowering garden, the message from Johnson is simple: “Grow a wide range of plant species and enjoy the diversity of pollinators that visit your garden.”