Global insect population decline may cause plants to battle with each other over pollinators, study warns

Competition for pollinators amid the observed decline of insect populations across the world may reduce the ability of plant species to coexist, a new study has warned.

Naturalists have documented over the years the lengths to which plants go to attract pollinators, including special visual cues like showy petals and sepals, as well as special scents called pheromones to attract some insects.

Although many pollinators such as bees and butterflies visit multiple plant species, new research suggests the population decline of these insects could destabilise co-existence among plants.

The study, published last week in the journal Nature, warns that the global “insect apocalypse” aggravated by climate change may impact plant diversity as the number of pollinators decreases across the world.

Researchers say this die-off of pollinating insects has incited the need to better understand their role in maintaining plant diversity.

Previous research has theorised that the increased competition for pollinators may increase plant diversity, by providing a context that forces plants to adapt and establish their own, unique niches.

However, studies also suggest competition may instead result in some species being excluded from their niches, decreasing diversity in a community.

Scientists attempted to address these contradicting beliefs in the new study by manipulating the pollination environment of five annual flowering plant species – field mustard, corn gromwell, common poppy, cornflowers, and wild fennel – grown in a field whereby plant density was made to vary along a gradient.

In the study, researchers assessed pollination and plant growth in 80 small 2sqm plots of paired plants native to Switzerland.

They controlled how much pollination happened in some species by hand and the rest had to rely on the normal environmental levels of pollination.

The supplementary hand pollination reduced the competition for pollinators between plant species.

The five plant species used in the study relied on general insect pollination, except corn gromwell which could also self pollinate.

“For nine of ten species pairs, competition for pollinators weakened stabilising niche differences between competitors,” scientists wrote in the study.

This indicated that when pollination was reduced, the paired species did not arrive at a new balance of their interactions with each other within their environment.

It suggests competition for pollinators destabilised interactions between plant species, resulting in fewer species coexisting together.

“These results support the hypothesis that pollinators destabilise plant competition by favoring more common plant species at the expense of their rarer competitors,” researchers wrote.

They say the plant-pollinator interactions contributed toward “competitive imbalances” between plant species – an effect they expect will be further exacerbated by declining pollinator levels amid the ongoing global sixth mass extinction sparked by climate change.

The experimental stimulation of pollinator decline further revealed that plant species with a sharper drop in pollinator visits to their flowers also experienced larger declines in population growth rate.

Based on the results, scientists conclude that these findings reveal how interactions between plant species can be weakened and competitive imbalances intensified as plants compete for pollinator attention.

They believe this information will be “invaluable” for determining the ecological consequences of pollinator decline.

“Our results reveal that competition for pollinators may weaken plant coexistence by destabilizing interactions and contributing to competitive imbalances, information critical for interpreting the impacts of pollinator decline,” researchers concluded.