Low bee visitation rates explain pollinator shifts to vertebrates in tropical mountains, New Phytologist 2021
Bimodal Pollination Systems in Andean Melastomataceae Involving Birds, Bats, and Rodents, The American Naturalist 2019
For their reproduction, a large proportion of flowering plants are dependent on animals for pollination. While in Europe this task is performed by insects such as bees and moths, in the tropics the pollen is distributed by a variety of creatures, including hummingbirds, bats and mice. The appearance and shape of a plant species can be strongly impacted by its evolutionary adaptation to a specific pollinator group. The shape and color of the flowers vary, as do attractants and rewards for pollinators. One hypothesis, which is not entirely uncontroversial, assumes that certain combinations of characteristics apply – for example, flowers approached by birds are often red, non-fragrant and rich in nectar, while flowers visited by moths are white, fragrant and poor in nectar. Researchers refer to these correlations as pollination syndromes.
In a project supported by the Austrian Science Fund FWF, Jürg Schönenberger and Agnes Dellinger from the University of Vienna investigated this adaptation of flowers to their pollinators and possible pollinator shifts prompted by changing environmental conditions. The botanists focused on a specific group of tropical plants. “In the field of pollination biology, many research groups work in Europe and North America. We, on the other hand, focused on a purely tropical group of plants about which little is known and which lends itself well for studying the evolutionary mechanisms,” explains Schönenberger.
The scientific name of the plant tribe they investigate is Merianieae. It is part of the large family of Melastomataceae or Melastomes, which numbers almost 6000 species worldwide. Taking their name from the German naturalist Maria Sibylla Merian, who was born in 1647, the Merianieae tribe is native to South America only. “The roughly 300 species in this group include shrubs, small trees and lianas native to the rainforests of the Amazon, but even more so to the cool Andean cloud forests,” Dellinger notes. “The different species have very diverse flowers. At low elevations, the flowers are mostly small and white, while they tend to be large and red or pink in the mountains.”
These differences are in line with the different pollinators these plants favor . “Most species do use bees and pollination by vibration – also widespread in Europe – which involves bees releasing pollen from the plant by means of a vibrating movement. When it comes to the Merianieae, however, evolution has resulted in some of the species switching to birds, mice and bats as pollinators,” Schönenberger explains. “That's what makes this group so interesting for our research.”
Field research in Ecuador and Costa Rica constituted a large part of the project. “It was all about finding, studying and observing the different plant species,” reports Dellinger. “In the course of the expeditions we were able to document 30 species in the field.” Including loans of preserved flowers from existing collections, the researchers were able to collect morphological data, i.e. data on the shape and structure of the flowers, from 140 species. In some cases, the researchers extracted plant DNA to determine the relationships between plants. In individual experiments, specimens were even pollinated by hand.
According to existing theories, a plant species will adapt to the most efficient pollinator group in the course of its evolution. Analysis of the data collected on the Merianieae shows that this adaptation is strongly influenced by the prevailing climate. In many cases, the decisive factor is altitude: “Bee pollination is predominant in the warm and dry climates of the lowlands, but also in sunny and wind-protected locations higher up. In the mountain forests and cloud forests, on the other hand, the pollinators are very often hummingbirds, and also other vertebrates,” Dellinger notes. “In areas with high humidity one finds few bee-pollinated species.” While the results also suggest that climate change has an influence on this system, this is a question that requires more research.
In essence, the Vienna-based botanists were able to confirm the existence of pollination syndromes – i.e. the correlation of pollination groups with certain plant traits. “However, the plant traits that characterize the syndromes in the Merianieae are very specific. We found, for example, that in this case red flowers are not associated with hummingbird pollination, as one might assume, and white flowers are not associated with moth pollination,” Dellinger explains. “On the contrary, color does not seem to be a relevant trait in all Melastomes. Instead, an important criterion turned out to be the reward that the animals receive when visiting the plant. Bees, for instance, are rewarded with pollen, hummingbirds, bats, and mice with nectar, and passerine birds with feed contained in something resembling small balloons.” In addition, the floral organs can adapt quite quickly to a new pollinator group – they do so faster than the evolutionary development of the plant as a whole would suggest.
In the future, the botanists want to expand their research even further and focus on the entire Melastome family. As Dellinger notes: “The family is found throughout the tropics. We want to learn if the findings on pollination mechanisms and pollinator shifts dependent on climate and altitude found in the Merianieae are replicated in populations in the rest of South America, Africa and Southeast Asia.”
Jürg Schönenberger is a Professor of Structural Botany at the Department of Botany and Biodiversity Research at the University of Vienna. He specializes, inter alia, in the analysis of floral structure and function. Earlier research stations included the University of Zurich, the Swedish Museum of Natural History in Stockholm, Stockholm University, and the University of Wisconsin-Madison in the United States.
Agnes Dellinger is an assistant professor and focuses on pollinator shifts, floral evolution and floral diversity. Prior to her current engagement at the University of Vienna, she was a postdoctoral researcher at the University of Colorado in the USA. The project “Pollinator change and floral evolution in the Merianieae” was awarded EUR 300,000 in funding from the Austrian Science Fund FWF.