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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