How to pick your pollinator

This story about the birds and the bees might be different to the one you’ve heard before.

Colour is the main tool that plants use to communicate with pollinators. The often bright, showy displays, sometimes including instructions invisible to the human eye, have evolved to attract creatures that will end up transferring pollen from one plant to another.

But what if there are certain creatures that you don’t want to attract?

The campo rupestre, a rupestrian grassland ecosystem in eastern Brazil, is home to hundreds of flowering plant species. It is also home to many species of bees, and hummingbirds, both of which feed on the nectar proferred by the flowering plants to their pollinators as a sweet temptation to encourage visitation.

The flower in the foreground is a bee-pollinated species of Lavoisiera in the Melastomataceae, an important botanical family in the campo rupestre. This species, like most of the Melastomataceae found in the campo rupestre, has a colour display typical of bee-pollinated flowers. Image courtesy of Dr Maria Gabriela Gutierrez de Camargo.

Some of the flowers of the campo rupestre have evolved in such a way that they are pollinated more efficiently by hummingbirds, than by bees. Flower colour evolved to attract pollinators but, like choosy nightclub bouncers, flowers can be selective in the pollinators that they attract. It is in the plant’s interest to be visited by the most efficient pollinators possible, and flowers pollinated by hummingbirds may use colours that are less conspicuous to bees than flowers pollinated by bees – a phenomenon known as the ‘bee avoidance’ hypothesis.

Dr Maria Gabriela Gutierrez de Camargo and colleagues tested for the existence of the bee avoidance hypothesis among the flowers of the campo rupestre by examining and recording the colour traits of flowers pollinated by bees and hummingbirds. The researchers analysed the reflectance data of 283 plant species to verify that hummingbird-pollinated flowers reflect light in longer wavelengths that be seen by hummingbirds, but not bees.

Chamaecrista ochnacea. Image courtesy of Dr Maria Gabriela Gutierrez de Camargo.

The researchers confirmed that flowers pollinated by hummingbirds take advantage of the differences in how bees and hummingbirds see the world to avoid pollination by bees. Flowers that attract bee pollinators tend to have characteristics that are well-suited to a bee’s world-view: petals that reflect light in shorter wavelengths, and a prominent, yellow, UV-absorbing centre to improve bee attraction at short distances. Hummingbird-pollinated flowers tend to lack these characteristics and reflect light in longer wavelengths that appear red, a colour that bees cannot perceive.

Camargo and colleagues have shown that the diversity of flower colour in the campo rupestre is driven by avoidance, as well as attraction, confirming the bee avoidance hypothesis at the community level for the first time.

Vochysia thyrsoidea. Image courtesy of Dr Maria Gabriela Gutierrez de Camargo.

Read the paper: Camargo, M. G., Lunau, K., Batalha, M. A., Brings, S., Brito, V. L. and Morellato, L. P. (2018) How flower colour signals allure bees and hummingbirds: a community‐level test of the bee avoidance hypothesis. New Phytologist. doi: 10.1111/nph.15594

Dr Mike Whitfield
Development Coordinator
New Phytologist Trust