Flowers attract pollinating insects
As the decline in insect populations continues unabated, new questions are arising about how plants, which depend on pollinators, are adapting. How do they reproduce under these conditions? In a recent study we conducted, we compared field pansies (Viola arvensis) growing today in the Paris region with older plants of the same species, "resurrected" from seeds collected 20 to 30 years ago. We found that today's flowers are 10% smaller and produce 20% less nectar than their ancestors, characteristics that are important for attracting pollinators, which consequently visit them less often. These changes show that the links between pansies and their pollinators are breaking down.
Samson Acoca-Pidolle, University of Montpellier
To highlight the evolution of today's flowers compared to their ancestors, we used a method called "resurrection ecology." This practice 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, the ancient plants were "resurrected" from seeds collected in the 1990s and 2000s and preserved by the National Botanical Conservatories of Bailleul and the Paris Basin. These ancient plants were compared to plants collected in 2021. The comparison between the ancient pansies and their descendants growing today in the same fields in the Paris Basin provides insight into the evolution of the species over the last 20 to 30 years.
The decline of pollinators: the designated culprit?
This enabled us to study the evolution of four populations of field pansies, a messicole plant, i.e., a wild plant found in agricultural crops, in the Paris Basin. Messicole plants play an important role in pollination services by attracting pollinating insects and providing them with a diverse resource. The decline in the attractiveness of messicole 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 likely their response to the decline in insect populations over recent decades, as reported by several studies across Europe. More than 75% of the biomass of flying insects, including pollinators, has disappeared from German protected areas in 30 years. Field pansies, like most flowering plants, are the result of millions of years of co-evolution with their pollinators, leading to a mutually beneficial relationship. The plant produces nectar for the insects, and in return, the insects transport pollen between flowers, ensuring their reproduction.
With the decline of pollinators, and therefore pollen transfer between flowers, plant reproduction is becoming more difficult. The results of this study reveal that pansies are evolving to reproduce without pollinators. They are increasingly practicing self-pollination, which consists of reproducing with themselves, something that is possible for hermaphroditic plants, which account for approximately 90% of flowering plants.
A similar trend 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 attractive than their insect-pollinated counterparts. However, our study is the first to show that the decline in pollinators could already be responsible for a shift 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, thereby increasing the risk of extinction.
These findings are also bad news for pollinators and the rest of the food chain. Our study has highlighted a vicious circle: a reduction in nectar production by plants means less food for insects, which in turn can contribute to threatening pollinator populations. We show that the decline in pollinators has not only demographic consequences but also evolutionary ones, which are even 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. Readthe original article.