Flowers “give up” on pollinators and evolve to be less attractive to them as insect numbers decline, researchers said.
A study found the flowers of field pansies growing near Paris are 10% smaller and produce 20% less nectar than flowers grown in the same fields 20 to 30 years ago. They are also less frequently visited by insects.
“Our study shows that pansies evolve to give up on their pollinators,” says Pierre-Olivier Cheptou, one of the study’s authors and a researcher at the French National Center for Scientific Research. “They are evolving towards self-pollination, where each plant reproduces with itself, which works in the short term but may limit their ability to adapt to future environmental changes.”
Plants produce nectar for insects, and in return insects transport pollen between plants. This mutually beneficial relationship has formed over millions of years of co-evolution. But pansies and pollinators may now be caught in a vicious cycle: plants produce less nectar and this means that there will be less food available for insects, which in turn accelerate decline.
“Our results show that the ancient interactions linking pansies with their pollinators are rapidly disappearing,” said lead author Samson Acoca-Pidolle, a doctoral researcher at the University of Montpellier. “We were surprised to find that these plants develop so quickly.”
Insect declines have been reported by studies across Europe. One study on German nature reserves found that from 1989 to 2016 the overall weight of insects caught in traps dropped by 75%. Acoca-Pidolle added: “Our results show that the effects of pollinator decline are not easily reversible, because plants have already started to change. Conservation measures are therefore urgently needed to halt and reverse pollinator decline.”
The method used in the study is called “resurrection ecology”. This involved germinating ancestral pansy plants from seeds collected in the 1990s and 2000s, which were stored in the national botanical conservatories. The team compared how four populations of field pansies (Viola arvensis) changed during this period.
Apart from changes to the flowers, they found no other changes between the populations, such as leaf size or total plant size, according to the newspaperpublished in the journal New Phytologist.
If flowers are unlikely to attract insects, a plant wastes energy making them large and rich in nectar. Previous research showed that the percentage of field pansies that rely on self-pollination has increased by 25% over the past 20 years.
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“This is a particularly exciting finding as it shows that evolution is happening in real time,” said Dr Philip Donkersley, of Lancaster University, who was not involved in the study.
“The fact that these flowers are changing their strategy in response to the declining abundance of pollinators is quite mind-boggling. This research shows a plant undoing thousands of years of evolution in response to a phenomenon that has only been around for 50 years.
“Although most research has been done in Europe and North America, we know that the decline of pollinators is a global phenomenon. These results may be just the tip of the iceberg: areas with much greater plant diversity are likely to have many more examples of wild plants changing their pollination strategies in response to a lack of pollinators.
Similar processes can be seen in invasive populations that must adapt to new ecological niches. Populations of foxes have evolved to be pollinated by bumblebees in Europe. However, 200 years ago they were introduced to Costa Rica and Colombia, and since then they have changed the shape of their flowers so that they can be pollinated by hummingbirds. researchers found.
Other research show plants incapable of self-pollination go the other way and produce more pollen when pollinators are scarce. Unable to resort to other methods, they have to outplant other plants to attract a dwindling number of pollinators.
Prof Phil Stevenson, of the Royal Botanic Gardens, Kew, who was also not involved in the research, said it made sense that traits that guide or reward pollinators are likely to change when pollinator numbers decline, particularly among species that have the option to self pollinating.
“This is especially so for reproduction,” he said, “which is probably the most important living function of organisms and probably the most adaptive trait of all.”
