The Amazon’s Growth into the Richest Area of Organisms in the World

The Amazon rainforest and the Amazon River support an abundant forest full of biodiversity. Yet, it is still a mystery as to how this region became so rich as it is today. Hoorn et al. (2010) pull together resources from around the scientific community to piece together the history of Amazonia. Their research focuses on the effect the development of the Andes had on the entire Amazonian region. The Andes changed its climate, redirected the water flow, distributed soil and nutrients, and brought a great influx of diverse species from North America down to the Amazon. Over a hundred million years the Amazon rainforest slowly developed into what it is today, but there are still many questions on how exactly this happened. Hoorn et al. attempt to answer some of these questions, while also raising more. What becomes clear is that the development of a large ecosystem, such as Amazonia, is not a simple process, but rather a long, complicated process dependent on many factors. —Mathew Harreld
Hoorn, C., Wesselingh, F.P., ter Steege, H., Bermudez, M.A., Mora, A., Sevink, J., Sanmartín, I., Sanchez-Meseguer, A., Anderson, C.L., Figueiredo, J.P., Jaramillo, C., Riff, D., Negri, F.P., Hooghiemstra, H., Lundberg, J., Stadler, T., Särkinen, T., Antoneli, A., 2010. Amazonia Through Time: Andean Uplift, Climate Change, Landscape Evolution, and Biodiversity. Science 330, 927–931.

          Protecting the Amazonian region of South America is critical in preserving regional and global biodiversity. The Amazon rainforest is home to what may be the most diverse and unique terrestrial species. The authors believe that the uplift of the developing Andes largely influenced the ecological development of the Amazonian region, and examine this thesis using a variety of new models and data in the fields of geology, paleontology, ecology, and molecular phylogenies.
In order to understand the effect the growth of the Andes had on the Amazonian rainforest, Hoorn et al. first studied the pre-Andean Amazonian region from 10 million years ago (MYA) to 135 MYA. Over the course of continental breakup (135 to 100 MYA) the Amazonian region developed the initial basins that would become home to the modern rainforest, as well as the beginnings of the mouth of the Amazon River. The tectonic plate shifts during this time also began the initial formations of the Andes Mountains. This entire region is known as pan-Amazonia, which existed up to 10 MYA, and extended past the present area of the Amazon into Orinoco, Magdalena, and sometimes into the northern Paraná region. Over the next few million years, pan-Amazonia became home to a variety of mammalian species, freshwater fish species, and even at some points saltwater fish species.
          Sixty-five to 34 MYA the movement of tectonic plates southward began creation of the Central Andes, and then about 23 MYA additional plate movement began the creation of the Northern Andes. This development also saw the first of modern plant and animal mountainous species rise in this region. About 12 MYA the region underwent its most intense mountain building. During this period the gap between South America and Panama was closed, giving way to the Great American Biotic Interchange. This brought a great number of new species to the Amazonian region from North America, furthering the diversity that was already taking place. During this time the land continued to advance, as mountains began to surpass 2000 meters in height and basins grew out of the mountainous regions. These changes in geological environment caused rainfall to increase in the southeast. The continued uplift and increased rainfall resulted in erosion and sediment dispersal. Over the next millions of years the sediments made their way westward.
          The development of the mountainous regions coincided with the development of a large wetland. The wetland, along with warmer temperatures, brought the rise of many large invertebrates, now extinct. Evidence of seasonal monsoons also gives evidence of a rapidly developing environment, as a water influx becomes a seasonal norm. Around this time there is also evidence of a rise of salinity, giving way to more marine species inland. The overall influence of these marine conditions on Amazonia is still in debate, however. The continued growth of the mountains resulted in the creation of valleys and advanced water systems. Around 10 MYA when the sea level dropped, the Amazon River became fully established by reaching the Atlantic Ocean. This also coincided with the change of the Western Amazon basin from a megawetland to more river conditions, as seen today. This change brought the end for many endemic species, and the rise of grasslands that would later give rise to the forests of today. In the last 3.5 million years the Andes have continued to rise, readjusting river patterns, and fully closing the gap between South America and Panama. The influx of North American plant and animal species, as well as African plant species played large roles in diversifying the Amazonian region during this time, creating the forest and animal species we know today.
          What these results mean for the current and future Amazonian region is still unclear, as further specific studies need to be done. However, Hoorn et al. show that tree diversity is dependent on wetter, less seasonal areas, and that animal diversity is affected by many factors that need further studying. The younger, western region of Amazonia is home to more species than the older, eastern half. This raises many questions over how diversification occurs, and what drives it. What these data do suggest is that wetter, less seasonal western Amazonia might play a key role in sustaining climates faced with change, while sustaining, and perhaps driving, diversity. What is clear is that there is no simple answer to how Amazonia became so diverse and abundant. There is no one large event in global history that single-handily affected the region, but rather it has been a mix of many events from the formation of the continents onwards. 

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