Pollinators such as bees, butterflies, bats and birds have contributed significantly to the creation of the great diversity of flowering plants. Closely related plant species can often have different pollination systems. For example, one plant species can be pollinated by bees, while a closely related species can be pollinated by hummingbirds. How such a change occurs over the course of evolution is still unclear.
Over five expeditions, researchers examined 11 species in the Merianieae tribe in the lowland and mountain rainforests of Costa Rica, Ecuador and Colombia. 'We found that bees were frequent visitors to flowers in the lowlands, where the examined plant group presumably has its evolutionary origin,' explained lead author Agnes Dellinger, from the Department of Botany and Biodiversity Research at the University of Vienna.
Photo © Francisco Sornoza.
Over the last five million years, these plants spread into newly created mountain habitats as the Andes rose. 'At this altitude of around 2,400 to around 3,300 metres, bees are very rare visitors to flowers, while vertebrates such as bats, hummingbirds and passerines can often be found on flowers,' said Dellinger.
How do the pollinators of a plant species change over the course of evolution? 'Theoretical concepts say that it is above all extrinsic factors such as changes in the climate that lead to this,' said Dellinger. These theories assume that climatic changes reduce the frequency of the original pollinator (e.g. a species of bee) and that flowers can adapt to another pollinator over longer evolutionary periods. To date, however, empirical evidence for these theories has been lacking. Groups of plants that are common both in low-lying areas and in mountains are particularly suitable for testing these theories.
Researchers from the University of Vienna examined eleven plant species in the lowland and mountain rainforests of Costa Rica, Ecuador and Colombia (© Francisco Sornoza).
Compared to bees in the tropical mountain rainforests, vertebrates like birds and bats transport larger amounts of pollen between the flowers. This more efficient pollen transfer is very likely an important factor in the evolution of pollinator changes. 'The switch from bee to vertebrate pollination has only taken place in the Merianieae tribe in mountain forests, where the climate is generally more humid, cooler and windier than in the lowland rainforest,' said Dellinger. The flowers have also adapted to the new vertebrate pollinators in terms of their functioning and their morphology over long evolutionary periods.
Will climate change also lead to pollinator changes and the corresponding adaptations? 'There will probably be changes in the frequency of different flower visitors,' said Dellinger. It is questionable, however, whether the adaptation of the flowers to new pollinators can keep up with the currently rapidly changing climate.
Read the paper: Dellinger, A.S., et al. (2021) Low bee visitation rates explain pollinator shifts to vertebrates in tropical mountains. New Phytologist 231: 864-877. doi: 10.1111/nph.17390
Edited from a University of Vienna press release.
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