Close Encounters

Below-ground plant competition alters attractiveness to pollinators

Can the presence of a competitor plant modulate neighbouring insect-pollinated plant attractiveness to pollinators and resulting fecundity?

Floral attractiveness traits are particularly sensitive to environmental factors, especially soil resource availability. Increased nutrient supply and water availability generally result in increased flower numbers, larger flower size and longer flowering time. As they influence soil nutrient availability and plant nutrient uptake ability, below-ground biotic interactions can also modulate floral attractiveness traits. Likewise, when focusing on plant–pollinator interactions, competition between plants can affect floral parameters of insect-pollinated plants. However, most studies investigating such responses have used nutrient supply as a proxy for belowground competition, rather than studying actual responses to plant-plant competition.

Experimental plots covered with nylon mesh cages in the CEREEP Ecotron-Ile de France, a French research station. Image credit: A. Hansart

In their new study published in AoBP, Flacher et al. set up a field experiment in which an insect-pollinated plant (Sinapis alba) was grown in a mixture dominated by a wind-pollinated plant (Holcus lanatus). By choosing a wind- pollinated plant competitor, this study focuses on competition between plants for abiotic resources only, excluding competition for pollinators that would have influenced pollinator response to floral traits. They show that the presence of a competitor plant can modulate neighbouring insect-pollinated plant attractiveness to pollinators through belowground competition for soil resources. Several floral traits as well as pollinator visitations were negatively influenced when there was competition between neighbouring plants. A decrease of plant fecundity was also observed but probably linked to the competition itself rather than changes in pollinator visitation induced by competition. Still, this study suggests that, in a plant mixture, the presence of any competitor plant may influence plant-pollinator interactions and future studies on pollination would benefit from considering whole plant communities.

Researcher highlight

Floriane Flacher conducted a PhD in plant ecology and pollination from 2012 to 2016 at the UPMC (now Sorbonne Université) in Paris, France. She studied the influence of belowground competition between plants on plant-pollinator interactions through modification of plant attractiveness traits. She is now a research engineer at Aix-Marseille Université, France in the Mediterranean institute of biodiversity and ecology (IMBE).  She works for a citizen science program that aims at studying the effects of climate change on the local flora. She is interested in functional ecology, especially in plant-plant as well as plant-insect interactions, and also studies plant traits (e.g. nectar production, phenology) and their response to biotic or abiotic factors.