A study of Cistus albidus, Salvia rosmarinus and Thymus vulgaris has revealed that reduced rainfall can alter a flower’s scent and impact its ability to attract pollinators. The paper by Coline Jaworski and colleagues in the Journal of Ecology indicates that drought could have cascading consequences on plant-pollinator interaction.
The study was an experiment to see how the plant-pollinator relationship was affected when plants suffered from drought. In one set of plots, a mesh of gutters removed 30% of the rainfall, while in the other, rain fell as usual. The scientists found that drought had little effect on much of the plants’ reproductive organs, though the chemical composition of the flowers’ scent changed consistently with a moderate drought. Jaworski and colleagues attribute this to the production of plant defences in response to drought, which causes the plants to make volatile stress molecules.
The change in floral scent might explain a change in the visitors the biologists observed visiting the flowers. “We found that drought altered the relative number of visits by different pollinator functional groups. Workers of Apis mellifera and Bombus gr. terrestris visited more S. rosmarinus flowers in control than in drought plots, while the species-rich group of small wild bees visited more S. rosmarinus flowers in drought than control plots. The same trend, although not significant, was found in C. albidus.” write Jaworski and colleagues.
The ecologists caution that the difference might not be due to floral scent alone, nor that the small bees found the stressed plants more attractive. It might be that the bigger bees are using the scent to identify the flowers in better condition and concentrating their efforts there. As a result, the smaller bees are left foraging among the stressed plants, a situation the scientists call “resource partitioning due to interspecific competition”.
Jaworski and colleagues note their experiment has some limitations, not least that the bees had a choice between visiting control and drought plots. As climate changes, there will not be such a convenient choice for pollinators.
Jaworski and colleagues conclude: “Reduced floral resources and altered pollination functions may result in population declines in both pollinator and plant communities, reducing the effectiveness of pollination functions and ecosystem productivity in biodiversity-rich but also already fragile Mediterranean ecosystems. Under predicted climate change, those ecosystems will also likely endure a combination of extreme events such as intense drought episodes and heat waves of higher frequency and intensity. This is likely to exacerbate the effects we observed on flower attractiveness, plant–pollinator interactions and plant reproduction.”
Jaworski, C.C., Geslin, B., Zakardjian, M., Lecareux, C., Caillault, P., Nève, G., Meunier, J.-Y., Dupouyet, S., Sweeney, A.C.T., Lewis, O.T., Dicks, L.V. and Fernandez, C. (2022) “Long‐term experimental drought alters floral scent and pollinator visits in a Mediterranean plant community despite overall limited impacts on plant phenotype and reproduction,” The Journal of Ecology. https://doi.org/10.1111/1365-2745.13974