The climate changes we are experiencing today are already impacting the natural world around us. The average temperature is increasing, temperature and rainfall extremes are increasing, summer droughts are becoming commonplace. What influences will these climatic phenomena have on the relationships between plants and their pollinators? Charlotte Descamps, PhD student at the UCLouvain Earth and Life Institute, has been addressing this question for four years.
Pollination is one of the best examples of the fact that ‘nature is well made’. The pollinator feeds on pollen and nectar, then, by foraging, deposits the pollen on the pistils of other flowers, thus allowing the reproduction of plants and the production of fruit. A win-win process for both parties, which is possible if, and only if, floral resources (pollen and nectar) are available and accessible to pollinating insects. With climate change, floral resources are likely to be impacted by increases in average temperature and frequency of droughts. These resources are at the heart of the relationship between plants and pollinators. If pollen and nectar resources change, what will happen to this win-win partnership? This is the question that Charlotte Descamps, teaching assistant in the UCLouvain Faculty of Bioengineering, decided to address.
Model plants, chosen and cultivated under controlled conditions
To delve into this rarely studied subject, Ms Descamps began by selecting plant species that would serve as study models, on the basis of their attractiveness to pollinators and their high pollen nectar production. One such model is borage, a garden plant that, with its star-shaped blue flowers and very sweet nectar, attracts many bees. Each experiment requires three to five months, from seed germination and greenhouse cultivation to the actual experiment during flowering, which takes place in growth rooms under controlled conditions. Six treatments are applied, combining three temperatures and two different watering regimes. Each growth room is programmed to a fixed temperature: 21, 24, or 27° C. The plants are watered differently: one group is watered regularly and the other not, in order to put the plants in water stress to simulate summer drought conditions. These two parameters, water and temperature, were chosen because they are fundamental to plant growth and are likely to vary in the context of climate change.
Changing floral features
While Ms Descamps was planning to focus on the effects of climate change only on floral resources, she gradually widened her study focus to floral traits. She explains, ‘The results on models studied so far confirm that there is a real impact of climatic conditions on floral traits. It was expected that the amount of nectar would decrease because of stress caused by high temperature and insufficient water, but it was not expected that the flowers themselves would react to it this much. However, we have noticed a real floral plasticity that manifests itself in different ways.’ This is one of the unexpected findings: the morphology of plants and flowers changes under stress. On the one hand, the number of flowers decreases, which reduces a plant’s amount of pollen and nectar available to insects. On the other hand, flower size also decreases when the plants are at 27° C or in water stress. Flower shape is therefore affected and some petals may even disappear. Decreases in the number of flowers and their size therefore reduce their attractiveness to insects, which could lead to a change in their visiting behaviour. At the same time, another question arises: Will floral resources always be available to all pollinators? Take the example of a flower that is funnel-shaped. If the diameter of the funnel is reduced, some insects such as bumblebees will no longer be able to enter in order to collect pollen and nectar.
One of the plants used for the experiment is borage, which, with its star-shaped blue flowers and very sweet nectar, attracts many bees.
Less nectar and pollen. What about their quality?
Both stresses reduce the flower’s pollen and nectar. There remains the question of the quality of these two resources. Experiments are in progress. For insects, nectar is the main source of sugars, while pollen is the main source of protein and sterols. In the wake of stresses induced by climate change, does the quality of resources change? This could particularly affect insects that depend only on pollen and nectar for food, such as bees (honey bees, drones or solitary bees) or some dipterans such as hoverflies.
Can conclusions apply to other plant species?
Ms Descamps would now like to extend her findings to other plant species. ‘This summer, two bioengineering students will be working with me to test plant species found in “flower band pollinator” mixtures to see if the results obtained on the four plant species studied are confirmed with other species’. The establishment of flower bands (with native species) for pollinators is, for Ms Descamps, an excellent measure that individuals can implement to support pollinating insect populations. ‘Everyone, on their own scale, can also leave more wild areas in their gardens for flowers that feed insects, it costs nothing and requires no maintenance ... Even clovers (for pollen) or dandelions (for nectar) are great resources for insects’.
What is the future of the plant-pollinator partnership?
Ms Descamps continues her research with a central question: If floral traits and quantities of floral resources are impacted by climate change, what are the consequences for plant-pollinator relationships? Concerning plant breeding, the researcher is worried about rare plants that might be more intensely affected by the changes, but many plants do not rely exclusively on insects for reproduction. On the other hand, many insects, including all drones and solitary (and domestic) bees, depend exclusively on plant pollen and nectar for food.
« Because of the changes to our land and the use of pesticides and herbicides, we have drastically reduced the plant diversity of our landscapes and greatly affected pollinating insect populations. »
Meanwhile, she finds that in the field of agriculture, remedial efforts are well under way: ‘More and more farmers are becoming aware of the importance of pollinating insects and of replanting hedgerows and orchards, or engaging in more complex agri-environmental measures such as managed strips or grasslands of high biological value’. Regarding the evolution of insect populations, her words are less reassuring: ‘Many recent studies have confirmed the decline of many insects. And some countries, like China, already employ workers to pollinate fruit trees. Some research teams are also thinking about robotic bees. I hope with all my heart not to end up there at our latitudes. But to prevent it, everyone needs to take action on their own scale to strengthen the ecological network, so that insects can continue to forage and find enough resources in our landscapes’.
A glance at Charlotte Descamps's bio
Charlotte Descamps is a teaching assistant in the Faculty of Bioengineering. After completing her master's degree in bioengineering in forest and natural areas management in 2014, she earned her teaching licence in secondary education. In April 2015, she joined the faculty as a teaching assistant. Her time is divided between teaching and pursuing a PhD on plant-pollinator relationships, with Profs Anne-Laure Jacquemart and Muriel Quinet. Botany plays a central role, as well as supporting student projects; her teaching duties range from first-year bachelor’s to first-year master’s students in the Faculty of Bioengineering.