Flowers let pollinating insects down
As the decline in insect numbers continues unabated, new questions are being asked about how pollinator-dependent plants adapt. How do they reproduce under these conditions? In a recent study we compared field pansy(Viola arvensis) flowers growing today in the Paris region with older plants of the same species, "resurrected" from seeds collected 20-30 years ago. We found that today's flowers are 10% smaller and produce 20% less nectar than their ancestors - important characteristics in attracting pollinators, who consequently visit them less. These changes show that the ties that bind pansies to their pollinators are breaking.
Samson Acoca-Pidolle, University of Montpellier
To highlight the evolution of today's flowers in relation to their ancestors, we used a method known as "resurrection ecology". This involves comparing individuals from the same species, but collected several years apart.
In the case of this study, published in the scientific journal New Phytologist, ancient plants were "resurrected" from seeds collected in the years 1990-2000 and conserved by the Conservatoires botaniques nationaux de Bailleul et du Bassin parisien. These old plants were compared with plants collected in 2021. The comparison between the old pansies and their descendants growing today in the same fields of the Paris Basin provides an insight into the evolution of the species over the last 20 to 30 years.
Is the decline in pollinators to blame?
In this way, we were able to study the evolution of four populations of field pansy, a messicolous plant, i.e. a wild plant found in agricultural crops, in the Paris Basin. Messicolous plants play an important role in pollination services by attracting pollinating insects and providing them with a diversified resource. A decline in the attractiveness of messicolous plants could reduce the attraction of pollinators, which are necessary for the good yields of 75% of agricultural crops.
The reduced attractiveness of flowers to pollinators is probably their response to the decline in insect numbers over the last few decades, reported by several studies across Europe. Over 75% of the biomass of flying insects, including pollinators, has disappeared from German protected areas in 30 years. Field pansies, like most flowering plants, have co-evolved with their pollinators over millions of years to achieve a mutually beneficial relationship. The plant produces nectar for insects, and the insects in turn transport pollen between flowers, ensuring their reproduction.
With the decline in pollinators, and therefore the transfer of pollen between flowers, plant reproduction is becoming more difficult. The results of this study reveal that pansies are evolving to do without pollinators for reproduction. They are increasingly practicing self-fertilization, i.e. reproducing with themselves, which is possible for hermaphroditic plants - around 90% of all flowering plants.
A similar evolution has already been observed in experiments where plants, in just a few generations and in the absence of pollinators, reproduce more by self-fertilization and produce flowers with less nectar and less attractiveness than their insect-pollinated counterparts. On the other hand, our study is the first to show that the decline in pollinators could already be responsible for an evolution towards self-fertilization in nature.
Consequences for the entire ecosystem
Self-fertilization is a reproductive strategy that may be effective in the short term, but would limit the species' ability to adapt to future environmental changes by reducing genetic diversity, thus increasing the risk of extinction.
These results are also bad news for pollinators and the rest of the food chain. Our study has indeed highlighted a vicious circle: reduced nectar production by plants means less food for insects, which in turn can contribute to threatening pollinator populations. We show that pollinator decline has not only demographic but also evolutionary consequences, which are all the more difficult to reverse.
Samson Acoca-Pidolle, PhD student in evolutionary ecology, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.