A new study led by the University of Bonn, Germany and Xi’an Jiaotong-Liverpool University, China has investigated why some plant species changed pollinators over the course of their evolution. By studying the reproductive system of three sister species pairs, where one species was pollinated by insects and one by hummingbirds, the scientists found that birds are much more effective than bees in pollinating plants that are not capable of self-pollination, and are thus dependent on cross-pollination.
Insects – especially bees – are the most common pollinators worldwide. However, bees have quite small activity ranges, while other pollinator species, such as hummingbirds, can fly much longer distances.
“It was previously assumed that plants switch their pollinator group from bees to hummingbirds when the activity and thus the pollination efficiency of bees is too low or too unpredictable, for example in the high mountains,” said study lead author Dr. Stefan Abrahamczyk, a researcher at the Nees Institute for Plant Biodiversity at the University of Bonn.
Surprisingly though, many plants in regions with high bee diversity and abundance have nevertheless switched to hummingbirds, bats, or even ground-dwelling mammals such as mice, honey possums, or lemurs as their pollinators.
In order to understand this phenomenon, Dr. Abrahamczyk and his colleagues analyzed three sister species pairs (new species arising from a common ancestor, but evolving into two different directions because their distribution range was divided by factors such as mountain folding or an ice age). All the examined species emerged from bee-pollinated ancestors and had their habitats in areas characterized by a high diversity and abundance of bees. However, while some were pollinated by bees, their sister species switched to hummingbirds at one point of their evolutionary history.
By using a series of pollination experiments, the scientists found that all the hummingbird-pollinated species had a significantly higher seed set. Moreover, their seeds had much higher germination rates when they resulted from pollination with pollen from another plant individual from the same species.
“From these results, we can conclude that hummingbird pollination evolved in populations of bee-pollinated species that are particularly dependent on cross-pollination, i.e., cannot self-pollinate,” said Dr. Abrahmamczyk.
Since bees often visit all open flowers on one plant before moving to another plant, they mainly encourage self-pollination. By contrast, due to their larger radius of activity compared to bees and their frequent movement between different plants from the same species, hummingbirds are highly efficient in pollinating plants that cannot self-pollinate. Moreover, bees groom extensively during flight and deposit much of the pollen taken from plants in their pollen baskets to feed it to their larvae. Hummingbirds, on the other hand, are not interested in pollen, so they use the entire pollen they collect to fertilize plants.
“These newly gained insights can also be applied to the evolution of other pollination systems, such as bat or moth pollination, in terms of their frequency and efficiency,” concluded Dr. Abrahamczyk.
The study is published in the journal Ecology and Evolution.
By Andrei Ionescu, Earth.com Staff Writer