A long long time ago, 25 million years, to be exact, the
Andes Mountains in South America began to rise, and as they rose, all of the water in what is now Peru and that used to flow northwest had to change directions and head east. This rainwater carried with it lots of dirt, creating a big mud-pie full of cations (i.e. plant nutrients) under a lake on the edge of the ocean. But, the Ecuador Andes kept rising and this mud-pie (called the Pebas Formation) drained of water as it was lifted up out of the ocean. The Amazon River then started to flow east, carrying with it lots of nutrient-poor sand that covered up the Pebas formation. But, the Andes kept rising, and now, instead of depositing sandy dirt on top of the ground, in the western Amazon the rivers carry dirt away, exposing the older Pebas dirt. Today, the ground under the Amazon rainforest is like a half-eaten cracker. In the west, crumby islands of nutrient-poor soil occur on top of the exposed nutrient-rich Pebas layer, but in the east, where erosion has not yet carried it away, the uneaten nutrient-poor soil reigns unmolested.
A human looking down from space upon the
would see a vast expanse of green and would probably wonder why this post just spent the last 200 words talking about dirt when the dominant feature of the Amazon is clearly trees. The reason is, a special type of satellite imagery called Landsat, can look down from space upon that same vista and say- yup, those are trees, but here’s what kind they are. Amazon Basin
That’s where the geology becomes interesting to a biologist. An international group of scientists, headed by Mark Higgins, has discovered that Landsat images of the western
show abrupt boundaries between one group of plants and another, and that these boundaries correspond exactly to the underlying geologic border between where the Pebas formation has been exposed and where it hasn’t. By collecting soil and plant samples from either side of the satellite-predicted boundary, the researchers were able to show that different types of plants grow on the cation-rich soil of the Pebas than grow on the overlying cation-poor soils. Amazon Basin
It may not seem like much of a leap to say that geology influences soils, which influence plants. But, consider the fact that the geology of the Amazon has not remained constant through time. Higgins and colleagues suggest that when rivers began to erode away the nutrient-poor soil and expose the nutrient-rich soil 5 million years ago, this created a new type of lifestyle for plants to adapt to and may have spurred the evolution of some of the biodiversity that is the signature of the Amazon. Evidence for this includes the diversification during this time of a groups of plants called Inga that specialize on rich soil, as well as the fact that closely related bird and mammal species from eastern and western parts of the Amazon have only recently become separate species.
You can find this article at:
M.A. Higgins, K. Ruokolainen, H. Tuomisto, N. Llerena, G. Cardenas, O. L. Phillips, R. Vasquez, & M. Rasanen (2011). Geological control of floristic composition in Amazonian forests Journal of Biogeography, 38, 2136-2149 DOI: 10.1111/j.1365-2699.2011.02585.x