Rivers can be difficult barriers for animals to cross, especially animals that cannot swim or fly. The Amazon region has many of the largest rivers in the world, which limits the movements of many animals that cannot cross them. Thus, some animal species occur on one side of a river but not on the other side. Isolation of animal species caused by rivers or other physical barriers can generate what are called centers of endemism, which are regions that have unique species not seen anywhere else. In this article, we will explain how rivers create barriers to animal movement and how centers of endemism can contribute to the fascinating biodiversity of the Amazon region.
Amazonian Rivers Are Huge!
The Amazon forest is one of the most biodiverse places on the planet. Research has shown that a large percentage of all animal and plant species in the world are found in the Amazon, including over 7,500 butterfly species (over 40% of the total number of butterfly species worldwide), over 1,500 bird species (close to 15% of the number of bird species), and more than 11,200 tree species (over 15% of tree species) [1, 2]. Moreover, the Amazon forest is called a rainforest because a lot of rain falls throughout the year. All that rain, together with the region’s flat plains and high Andes mountains where most major Amazonian rivers are born, contribute to the existence of 20 out of the 34 largest rivers in the tropics (in terms of the amount of water that flows through them) and four out of the 10 largest rivers in the world (Figure 1). Amazonian rivers can be thousands of kilometers long. For example, the Amazon River, the second-longest river in the world, extends for almost 7,000 km.
Furthermore, Amazonian rivers can be so wide that, in some parts, it is impossible to see the riverbank on the other side. This makes many rivers throughout the Amazon basin quite difficult or even impossible for some animals to cross, leading to what is called endemism. Endemism is a situation in which animals are found only in specific areas—nowhere else on Earth. In this article, we are going to talk about Amazonian rivers and learn how they influence the animals of this region.
Rivers Can Isolate Animals
During the 19th century, Alfred R. Wallace, a scientist known as the “father of biogeography”, showed that rivers are responsible for shaping the distribution of Amazonian animals . Decades later, after much research, the idea that rivers are important barriers that prevent the movement of animals has even more scientific support . Imagine that many species are separated by a wide river such that, on each riverbank, there are different combinations of animal species (Figures 2A–C). Since these species cannot cross the river to reach other areas, they are isolated in giant “land islands” (Figure 2C). Thus, each side of the river could have different animals distributed in one or a few areas. This distribution pattern is called centers of endemism . So far, 11 centers of endemism are proposed to exist in the Amazon, based on the distribution of animal species influenced by nine main rivers: Branco, Negro, Solimões, Javari, Xingu, Madeira, Tocantins, Tapajós, and Amazonas (Figure 2D).
But how did these species end up in their isolated locations on either side of the river? Maybe they got there thousands of years ago, before the rivers were in their current locations, or when the rivers were not so broad. Amazonian rivers do change a lot over time. Also, when one species of animals becomes divided—with some animals trapped on one side of the river and some on the other side—each group will begin to accumulate differences over time. Over many thousands of years, what was once one species can become two or more different species. This process is called allopatric speciation, and it can help to explain the high levels of animal biodiversity seen in the Amazon.
Trumpeter birds, which got their name due to the trumpet-like threat call of the males, are a good example. These birds are endemic to the Amazon forest and there are eight currently recognized species. Two groups of species were separated north and south of the Amazon River between 2.7 and 2.0 million years ago. Evidence shows that the emergence of certain Amazonian rivers, such as the Madeira and the Tapajó rivers, have isolated some birds on each side of the river, leading to the development of new species across time.
How Do Rivers Form Barriers?
Not all animals like water or can swim well. While some animals, such as otters and caimans, are good swimmers that spend much of their lives in the water (Figure 3), other animals, such as primates and toads, are not adapted for the water—even though they can swim for a little while, they cannot go very far and may even drown if forced to swim long distances. That is why large rivers can be important barriers to their movement .
What about birds, bats, or even bees? You might think that these animals can fly across rivers for sure. Well, some species might be able to, but it is not that simple for all of them. Many Amazonian birds fly within the forest, below the trees but not above them. Animals adapted to these conditions do not take chances flying in open areas, such as deforested regions or across large rivers, because they are more exposed to predators. However, many birds and bats are known to fly long distances and even to migrate across continents—so it would seem that those species can easily cross the distances created by Amazonian rivers. It took scientists a few decades of research on the distribution of Amazonian birds and bats to show that most rivers are not important barriers for these animals [6, 7].
Animal Distributions Change Over Time
The Amazon region did not always look the way it does now, and it continues to change. Rivers can change depending on geologic formations, climate, and vegetation. For species that cannot swim or fly, when a new river appears it may divide and isolate some groups of animals but not others, as we have already explained. On the other hand, when old rivers dry out and disappear, formerly separated animal groups can come into contact. These processes drive evolution and animals’ distributions patterns, and scientists are still working to understand which events are most important for Amazonian animals. Unfortunately, human activities are speeding up changes in the environment. Climate change and habitat loss are very serious threats to the Amazon region. Thus, scientific knowledge is crucial for understanding how current and future environmental changes will affect native animals and how we can better protect them.
The Amazon is one of the least explored parts of our planet, and many areas have still not been reached by scientists. This means that more research is needed on Amazonian animals, because lack of knowledge can hinder our ability to fully understand which factors affect their distribution and evolution. This is especially true for elusive groups of species, such as bats, which are not easily researched due to their nocturnal habits and other challenges associated with studying them. Moreover, many new species of animals are still being described every year, which were previously unknown to science, and that can change our current understanding of the distribution of Amazonian animals.
Even though rivers are now known to be barriers for some types of animals, our understanding about the role of rivers and other environmental factors shaping the distribution of Amazonian animals is still far from complete! The full picture of the role of rivers as barriers to Amazonian animals may still change over time, and we always need new scientists and explorers to help us. Will you be one of them?
Biodiverse: ↑ Having a high level of species of living organisms.
Endemism: ↑ Refers to a species found only in one area, not anywhere else on the planet. These species are called endemic species.
Centers of Endemism: ↑ Areas inhabited by species not found anywhere else on the planet.
Allopatric Speciation: ↑ The mechanism by which one species separates into two (or more) due to being separated by physical barriers.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge our sons and daughters, who have always been a big source of inspiration to keep an open and curious mind about the natural world and for always instigating us to find straightforward answers to complex questions.
 ↑ Hoorn, C., and Wesselingh, F. (eds.). 2011. Amazonia: Landscape and Species Evolution: A Look Into the Past. Hoboken, John Wiley & Sons. 464 p.
 ↑ Dagosta, F. C. P., and Pinna, M. D. 2019. The fishes of the amazon: distribution and biogeographical patterns, with a comprehensive list of species. Bull. Am. Museum Nat. Hist. 2019:1–163. doi: 10.1206/0003-0090.431.1.1
 ↑ Wallace, A. 1852. On the monkeys of the Amazon. Proc. Zool. Soc. London 20:107–10.
 ↑ Cracraft, J. 1985. Historical biogeography and patterns of differentiation within the South American avifauna: areas of endemism. Ornithol. Monogr. 36:49–84. doi: 10.2307/40168278
 ↑ Boubli, J. P., Ribas, C., Alfaro, J. W. L., Alfaro, M. E., da Silva, M. N. F., Pinho, G. M., et al. 2015. Spatial and temporal patterns of diversification on the Amazon: a test of the riverine hypothesis for all diurnal primates of Rio Negro and Rio Branco in Brazil. Mol. Phylogenet. Evol. 82:400–12. doi: 10.1016/j.ympev.2014.09.005
 ↑ Silva, D. C., Oliveira, H. F. M., Zangrandi, P. L., and Domingos, F. M. C. B. 2022. Flying over amazonian waters: the role of rivers on the distribution and endemism patterns of neotropical bats. Front. Ecol. Evol. 10:774083. doi: 10.3389/fevo.2022.774083
 ↑ Oliveira, U., Vasconcelos, M. F., and Santos, A. J. 2017. Biogeography of Amazon birds: rivers limit species composition, but not areas of endemism. Sci. Rep. 7:1–11. doi: 10.1038/s41598-017-03098-w