Plants can go to extraordinary lengths to entice pollinators to do their bidding. Of put on female insect disguises (with pheromones!) to lure lustful males, oozing the stench of decaying flesh to temper hungry flies, or, more often than not, offering bribes of sweet rewards to all who visit – no length is too coarse or expensive to spread their precious pollen.
Most pollinators visit multiple plant species, which is usually not a problem when there are lots of bees, flies, and other pollen-dusted tourists. But research now reveals that when pollinators become scarce, those lengths could include choosing what passes for violence between plants.
Ecological theory predicts that competition due to pollinator decline could lead plants to even more varied habitat use and interaction strategies to attract pollinators, which would increase plant diversity.
But another debunked theory that as the number of pollinators decreases, the chances of the same pollinator visiting the same type of rarer plants also decreases, meaning that more common species will outcompete them, which would reduce biodiversity.
As this is a scenario the world is heading towards – in the face of worrying declines in insects and disease affecting our closest pollinator allyEuropean bee – a team of researchers led by Princeton University ecologist Christopher Johnson put these competing theories to the test in field experiments.
Use 80, 2.25m2 plots of paired annual plant species native to Switzerland, the researchers monitored by hand the amount of pollination that occurred in certain plots. The rest had to rely on normal environmental levels of pollination. Johnson and his team compared population and fitness metrics for each.
The five plant species used – field mustard (Sinapis arvensis), corn grits (Buglossoids arvensis), the common poppy (papaver rhoeas), blueberry (Knapweed cyanus) and wild fennel (Nigella arvensis) – relied on general insect pollination, but the corn bean could also self-pollinate.
A further 22 replicate plots were closed, with half exposed to only one pollinator species and the remainder at environmental background levels to simulate pollinator decline. For these plots, the researchers also measured floral visits by pollinators.
“For nine out of ten species pairs, competition for pollinators weakened stabilizing niche differences between competitors,” Johnson and colleagues said. write in their paper, meaning that the paired species did not find a new balance of interactions with each other in the shared pockets of the environment when pollination was reduced.
“These results support the hypothesis that pollinators destabilize plant competition by favoring more common plant species over their rarer competitors,” the team said. concludes.
It seems that when our amazing assortment of pollinating insects becomes rare, each plant is there for itself, attracting the attention of all the remaining pollinators at the expense of their neighbors.
Reproductive ability was three times greater for poppy, wild fennel, and blueberry when hand-pollinated than those that relied on background pollination, showing that these species are naturally limited in themselves in order to find a balance with their neighbours.
But these plants are only considerate up to a point: the team has identified the weakening of this intra-species competition as a major factor in destabilizing multi-species communities.
The result created competitive imbalances and exacerbated mean differences in fitness between plant species: it reduced the ability of all pairs of plant species to coexist.
This could lead to the most common plant groups exiling the rarest of their once thriving communities.
The researchers did not expect this to happen so uniformly across all species pairs. Since the study was carried out over a small area and over a short period of time, the researchers say that over time a new balance of coexistence could be established with some of the species.
Johnson and colleagues, however, checked whether the pairwise interaction they measured differed when plants were grown in more specious communities (three, four and five species) and found that this was not the case in the together, further supporting their findings.
These results are of concern if they prove true over larger areas. If plant communities become so easily unstable as pollinators decline, the broader interactions between species and their implications will be difficult to predict. At least some understanding of these will be crucial in our attempts to maintain as much biodiversity as possible as conditions worsen.
In the meantime, we can all help reduce other insect declines growing native plants, turning lawns into natural habitats, avoiding pesticides, herbicides and other toxic chemicals, limiting outdoor lighting, and supporting groups and leaders who take these tiny cogs into consideration but essential to our living world.
This research was published in Nature.