Study assesses role of rivers in creating Amazon’s rich biodiversity

  • In the 19th century, the British naturalist Alfred Russel Wallace, who developed the theory of natural selection independently of Charles Darwin, hypothesized that the great rivers of the Amazon basin could be natural barriers influencing the diversity of life in the Forest.
  • While Wallace’s theory has been proven by studies on vertebrates, a new study now shows how it applies to plant species as well.
  • The study found that the wide variety of flora in the Amazon is not the result of just one factor, but rather the combination of many different factors.
  • For some plants, the large rivers were an important barrier to being able to create new species; for others, the dispersal of seeds by wind, water and animals was the determining factor.

Ever since the first European naturalists arrived in the Amazon rainforest in the late 18th century to catalog its abundant natural wonders, one of the most intriguing questions has been what created such a rich biodiversity of flora and fauna. wildlife in the area. The Amazon is home to the greatest concentration of biodiversity on Earth, with 10% of the species described in the world within just 0.5% of the planet’s total surface.

Among the scientists drawn to this region was British naturalist Alfred Russel Wallace, who developed his theory of natural selection around the same time Charles Darwin had his own eureka moment in the Galapagos. Wallace explored the Amazon Basin from 1848 to 1852, hypothesizing that large rivers could serve as natural barriers and influence the geographic distribution and generation of the splendid array of living things in a tropical rainforest – a biome in which a single hectare of land contains as much variety of plant species as all of Europe.

While studying primates, Wallace noticed that some species found on one bank of the Negro River, a tributary of the Amazon, were not on the opposite bank of the same river. He concluded that the immensely wide rivers of the Amazon, some of which can reach widths of 50 kilometers (30 miles) during the rainy season, may have limited the dispersal of animals between the banks and blocked gene flow, migration. genes between populations. The concept can be applied to different organisms to assess the level of genetic isolation between populations located on opposite banks of rivers.

Concretely, Wallace’s hypothesis postulated that, in addition to the separation of populations of the same species, over millions of years, these geographical barriers would have led to an accumulation of genetic differences between the groups located along the different banks, leading to to the formation of entirely new species. . It is a phenomenon that occurs all over the world, especially in mountainous regions like the Andes, where the uplift of the mountain range has generated the great biodiversity that is found on either side of the range today. .

Wallace’s hypothesis has been proven in vertebrate studies, but has yet to be tested in terms of plant species. Now, more than a century and a half after Wallace’s foray into the Amazon, a group of Brazilian and American researchers set out to answer this question in a to study recently published in the journal Frontiers of plant science.

“We reinterpreted Wallace’s hypothesis in genetic terms, because the original version deals with species distribution patterns according to biogeographic barriers,” says lead author Alison Nazareno, professor at the Institute of Biological Sciences of the Federal University of Minas Gerais (UFMG). “Our study attempted to answer the question of what to expect in these terms if a large river separated populations of the same species.”

Researcher Lúcia Lohmann collects samples for the study. Image by Leonardo Ramos Chaves.

The study aimed to broaden the thesis by focusing on patterns of genetic structuring in plants. The field expedition that started the study lasted seven days and generated three years of laboratory work. Aboard an Amazonian river boat, the researchers sailed the Negro River, which in some places can reach a width of over 20 km (12 mi), to the mouth of the Branco River, one of its narrowest tributaries, which is still 1 to 4 km (0.6-2.5 mi) wide in the study area. The objective was to test the hypothesis of rivers as barriers by examining rivers of different widths.

The researchers extracted DNA from leaf samples collected to estimate the level of gene flow between representative populations of four botanical families: Bignoniaceae, Passifloraceae, Rubiaceae and Violaceae.

“We measured genetic variation within a single plant species, but in populations located on opposite banks of the river,” says Nazareno. “We used a genetic differentiation index that goes from zero to one. The scale closest to one indicates populations which, even if they were to hypothetically interbreed in the future, would not succeed in leaving fertile offspring.

The results showed that the great variety of Amazonian flora is not the result of a single factor but rather a combination of many historical and ecological factors.

Researchers store collected plant samples in diary. Image by Leonardo Ramos Chaves.

For some plant species, large rivers constitute an important barrier to genetic connection, thus allowing the process of speciation, where distinct populations evolve along different trajectories. For others, ecological aspects such as the dispersal of seeds by wind, water or animals as well as pollination and adaptation to the soil are more relevant to their evolutionary history than geography. “The Branco River did not reflect any obstacle to the gene flow of any of the plant species we analyzed in this study,” says study co-author Lúcia Lohmann from the Institute of Biosciences at the University of São Paulo (IB-USP). “On the other hand, the Negro river constituted an important barrier for Amphirrhox longifolia, a species of the Violaceae family.

According to Lohmann, A. longifolia is spread by fish living in small niches and occasionally reaching the opposite bank of the river. This contributes to the genetic isolation of populations of the species in a process that will likely lead to the emergence of new plant species in the future.

The same process of genetic isolation is evident in the species Buchenavia oxycarpa, from the Combretaceae family. Scattered by primates that cannot cross the Negro River, populations of this species found on opposite banks of the river have their own strong genetic structures, indicating limited gene flow – an important part of the speciation process.

A sample of flowers collected for the study. Image by Leonardo Ramos Chaves.

Lohmann has coordinated studies on the origin and evolution of Amazon biota since 2003, with Joel Cracraft of the American Museum of Natural History. She says this kind of data is quite important because it allows for the creation of a more effective public policy that maximizes the conservation of key regions and lineages of the Amazon.

“Our knowledge of the Amazon is still quite fragmented. It is only through management based on high quality scientific knowledge and a strong familiarity with the ecological and evolutionary processes responsible for generating and maintaining the high level of biodiversity found in this region that we will be able to establish effective strategies. for the conservation of great biodiversity. found in the Amazon today, ”says Lohmann.

“It’s detective work that integrates different types of data, both biological and geological, to discover the origin of the forest and how it was formed,” she adds. “A solid knowledge of the Amazon is essential for its preservation, because humans only keep what they know, love and value. “


Nazareno, AG, Knowles, LL, Dick, CW and Lohmann, LG (2021). By animals, water or wind: can the mode of dispersal predict genetic connectivity in riparian plant species? Frontiers of plant science, 12. do I:10.3389 / fpls.2021.626405

Banner image: Researchers collect plant samples on a boat in the Amazon. Image by Leonardo Ramos Chaves.

This story was reported by the Brazilian Mongabay team and first published here on our Brazil site October 15, 2021.


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