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The numbers of flying insects are decreasing. There are now fewer bees, flies, moths, beetles and butterflies around to visit flowers and move pollen between plants. Now, Pierre-Olivier Cheptou, an evolutionary ecologist from the Centre of Functional and Evolutionary Ecology in Montpellier, France, explores what fewer insects might mean for plants in a recent correspondence article published in Botany Letters.
Because less pollen movement can result in fewer fruits and seeds, plants may need to adapt rapidly to future conditions with paltry pollen movement. One way that plants might adapt is by increasing the ability and prevalence of pollinating themselves, a process also known as self-fertilization. Cheptou reasons that, unless plants evolve rapidly to reproduce without animal-mediated pollen movement, fewer pollinators will result in plant community change — a shift in the number and abundance of plant species in an area. Because of the close relationship between plants and insects, “the question of sustainable maintenance of pollinators and plants has to be considered together,” says Cheptou.
It’s important to note that pollinator decline is much larger than the die-off of honeybees. Pollinator decline often gets lumped together with honeybee decline and worker honeybees abandoning their colony, a phenomenon known as colony collapse disorder. However, honeybees are only one of the more than 16,000 known species of bees that may act as pollinators. And we can’t forget that non-insect animals, including birds, bats, and even large mammals like lemurs, can be important pollinators too.
Flying insects have gone down in number by more than 75% in the past 30 years because of habitat decline, agricultural chemicals (including pesticides), infection by parasites and pathogens, and climate change. You may have personally observed some of this decline via the “windshield phenomenon” – fewer bugs on your windshield in recent years. With these reports and observations, it’s only natural to wonder how flowers will fare with fewer insect helpers. After all, about 87% of plant species depend on pollinators. That’s over 300,000 plant species.
So what happens to all of these plants when there are fewer bees, flies and butterflies?
One of the first impacts of pollinator decline could be an increase in the phenomena known as “pollen limitation.” This means that a plant is limited in their ability to produce fruits and seeds by the amount of pollen deposited on their flowers.
Take blueberries for example. A group of researchers from Rutgers University published a paper in 2014 sharing their results on blueberry flowers receiving insufficient amounts of pollen. They found that pollen limited blueberry bushes produce fewer and smaller berries. This can happen when flower partners are scarce, but can also occur when there are fewer pollinators. Pollen limitation seems to be quite common among plants, but pollinator decline is predicted to increase the occurrence and severity.
Data on how pollinator decline will influence pollen limitation is still insufficient to make any conclusions.
“[S]tudies are scarce, and, to date, we do not have a clear picture of the effect of pollinator decline on plants,” Cheptou concludes in his paper.
This lack of data, particularly long-term trends in plant communities where pollinators have declined, reveals a critical research gap. We simply need more data on how fewer pollinators translates to pollen transfer and seed set.
“Given the few empirical results we have, I fear that pollinator decline may lead to the evolution of a breakdown between plants and pollinators,” says Cheptou.
The full impact of fewer pollinators on plants is still uncertain and seems to vary depending on the taxonomic group. The key to understanding part of this mystery may lie in plants that have already died. Herbaria are now being used to observe trends in pollen transfer and pollen limitation.
In 2010, a group of scientists from Stellenbosch University in South Africa, examined 1220 specimens of 15 oil-secreting orchid species stored in three herbaria collected over 180 years. Researchers counted the number of pollinia — specialized pollen packets — in the pressed flowers. Bees of the species Rediviva peringuey visit these orchids for their oil but normally end up with pollinia stuck to their legs. Because pollinia can only be removed by the insertion of a small object, like a bee leg, into a slit on the flower, any pollen removed from a flower indicates a bee visit. Thus, by counting the number of pollen packets that bees had removed from the flowers, researchers could figure out often bees are visiting each plant. They found that less pollen has been removed from flowers in years since 1950 when compared with previous years. This suggests that less pollination is happening, likely due to the decline of Rediviva peringuey.
On the other hand, other studies show that pollinator decline might not necessarily lead to pollen limitation.
Scientists from several Chinese research institutions examined 4637 legume specimens from two different herbaria. They found that only a few species have been producing fewer seeds, while others seem to have actually increased seed production in recent years. The authors write in their paper that “legume pollination in China shows no sign of disruption and the effects of plant-pollinator disruption may be more complicated than simplistic predictions have allowed.”
With plant specimens held in herbaria across the world, there is a huge potential for more investigations like this. This only goes to show that the data pressed between sheets of cardboard in herbaria are invaluable, and likely hold the answers to questions we don’t yet know to ask.
Rachael Olliff-Yang is a plant ecologist based in Northern California. Her research focuses on climate change impacts on plants and pollinators, and conservation strategies to respond to these impacts. She is also the communications specialist for The California Botanical Society and is passionate about making science accessible to all. You can find her on Twitter @rlolliffyang and at https://olliffyang.com/.