Brazil is one of the most biodiverse countries on the planet. Some of its regions, taken individually, are home to a greater variety of species of animals and plants than the entire European continent! But why is this?
To answer this question, researchers are piecing together fragments of the evolutionary history of Brazilian biomes, especially in Amazonia, and they're making new discoveries! Amazonia's geological history directly influences the movement of species in the region over time. How the species sense the environment, and how they move within it, influences how populations either come into contact with one another or become isolated.
Isolation and contact are what ultimately lead to diversification and the formation of diversity. Geographic isolation of animal and plant populations generates new species by means of the accumulation of random genetic changes, i. e.
mutations. For years, researchers understood that the enormous Amazonian rivers, some of them spanning 20km from bank to bank, were responsible for the emergence of new species in that biome, seeing as they act as natural barriers against animals and plants. But recent studies show that the rivers' width isn't the only thing that favors the isolation of certain populations.
We must also factor in floodplains: the igapós and the várzeas. They are formed by sediments carried by the river: sand and mud, basically. Some areas of the river valley are more susceptible to the accumulation of sediments and hence create várzeas.
In other areas, the sheer speed of the river's current prevents the accumulation of sand and mud in a particular location. When this happens, the várzeas start to shrink, to decrease in terms of area. The expansion and retraction of várzeas have significant effects on a timescale of thousands, or dozens of thousands of years.
During periods in which várzeas stretched farther out than they do today, terrestrial animals became isolated. Whereas during periods in which the rivers carried more sediments and caused an increased erosion of floodplains, the rivers ceased to act as barriers against terrestrial animals, while isolating species that were adapted to the várzea. Climate cycles which lead to the increase and decrease of sediments carried by rivers directly affect the availability of flooded habitats.
And the availability of flooded habitats, in turn, affects the distribution of species which inhabit them. The current configuration of várzeas and igapós in Amazonia seems to have taken shape rather recently, in the past 11 or 12 thousand years. What we observe when we study the várzea's and igapó's genetic variety is that they reflect a past landscape in which the várzeas and igapós were less connected than they are today.
The várzeas' and igapós' expansion and retraction lead to the creation of successive and interspersed natural barriers, leading populations to become isolated and to come into contact with one another countless times. It is precisely this evolutionary history that makes Amazonia so great at generating new species of animals and plants; it helps us understand the emergence of such rich biodiversity in the region. We still know very little about Amazonian biodiversity.
Birds, which we know more about, show us that Amzonian biodiversity is extremely complex. So the birds that currently inhabit Amazonia have different links to different regions of the neotropical realm, especially in Central and South America. And, over time, there were countless exchanges between species in different periods when the connection between these environments were either stronger or weaker – the mutual influence is extremely powerful.
So Amazonia, given its enormous size, certainly and decisively influenced the diversity of the Atlantic Forest during periods in which both environments were in closer contact with one another. But the opposite is also true, and Amazonia owes a great part of its incredible biodiversity to species that come from elsewhere, which are equally important to the constitution of the incredible biodiversity we see today.
