Written by Kelly Reed and first published at Revolutionary Ecology
All of the world’s ecosystems experience small changes on an
ongoing basis, but over time have relatively stable characteristics. For
example, in a given year a prairie may receive more or less precipitation, more
or less nutrient input, or more or less grazing by herbivores, but over time it
maintains a relatively consistent suite of organisms and nutrient cycling.
However, changes can build up until a threshold is reached whereby the
ecosystem undergoes a substantial shift in the character of organisms and
functioning: this is termed an ecological tipping point. Once reached, it is
often very difficult, or even impossible, to return the ecosystem to its former
state. For example, a savannah may be increasingly grazed by cattle until the
point where the compacted, bare soil can no long retain sufficient moisture. At
a certain point, the feedback cycles cause the savannah to shift to desert with
little hope of return.
Ecological tipping points can occur naturally or through
human actions. Natural changes in aspects such as shade or diseases can cause
shifts. Many trees have a hard time taking hold in open meadows because they
thrive in shadier environments, but if a few trees do make it, other trees can
grow in their shade, and eventually the entire meadow can been turned into a
shady forest where the meadow grasses cannot compete. A recent example of
natural disease causing an ecosystem to cross an ecological threshold occurred
in parts of the Mediterranean when large amounts of sea urchins were killed by
a pathogen. The urchins feed on brown algae, and when they declined the algae
started to overgrow coral on coral reefs. At a certain point, the coral could
no longer survive and died underneath the algae. With the coral dead, the
habitat it provided is gone, leaving the coral reef fishes and recovering
urchin populations with few places to live, and making it difficult to shift
back to the coral ecosystem.
Although natural causes of crossing ecological thresholds
are not uncommon, rapid changes in ecosystems due human activities are causing
large numbers of thresholds to be crossed rapidly, most of which would never
naturally occur. The desertification of savannahs described above through
human-controlled cattle grazing has occurred over large expanses of Mongolia1,
Australia2, and the Sahel region of western Africa3,4. Another example is dead
zones throughout the world caused by nutrient inputs from farming. The most
prominent of these is in the Gulf of Mexico. Excess nutrients from agricultural
fertilizers in the Midwestern US wash into the Mississippi River and out to the
Gulf of Mexico where they cause huge blooms of phytoplankton. When the large
amount of phytoplankton dies, the decomposition process uses up most of the
oxygen in the water. The zones of very low oxygen kill most animal life that is
in them, causing more decomposition and even lower oxygen levels. Another
example occurs in coastal mangrove forests throughout the world where areas are
often cleared for shrimp farming. Mangrove forests help protect coastal areas
from storm damage and erosion. The cleared areas often experience large amounts
of erosion during storms, further destroying mangroves on the edges of these
clearings, making them more exposed and vulnerable. Even if shrimp farming is abandoned,
the mangroves often cannot reestablish because new seedlings cannot survive in
the exposed areas. Over one hundred examples of human actions causing
ecosystems to cross ecological tipping points have been documented5.
While ecological tipping points may be reached under any
economic system, capitalism’s focus on short-term gain and profits over
long-term stability lends itself to ignoring the warnings of ecological shifts.
Threats of future changes are often not considered in the short-sighted race
for growth and expansion. It is easy to imagine how climate change alone,
largely driven by the carbon emissions produced in search of profits, will
cause many destructive shifts due to crossing various ecological tipping
points.
Several small-scale ecological tipping points have already
been crossed, such as those described above, largely fueled by the search for
short-term gains. Scientists around the world are warning of major ecological
tipping points that we are quickly approaching—such as the melting of
permafrost in the arctic and the acidification of the oceans—and that will
cause substantial shifts in human and non-human livelihoods if drastic changes
are not made quickly. One of the most severe and rapidly approaching tipping
points is the drying of the Amazon rainforest.
The heart of the Amazon is a lush tropical rainforest that
hosts a plethora of biodiversity and indigenous cultures. Rapid deforestation
paired with increasing droughts due to changing climate will likely shift this
ecosystem from a tropical rainforest to a dry savannah and turn the region from
a global carbon sink into a source of carbon emissions, further exacerbating
climate change. This change would cause numerous species and human cultures to
go extinct while also degrading the regulating ecological functions of this
area.
Much of the rain that falls on the Amazon basin comes from
the trees themselves. Water is pulled up through the roots and transpired
through the leaves during photosynthesis; the process of water being released
from plants during photosynthesis is termed evapotranspiration. This water then
condenses into clouds and falls as rain on the forest. The trees in the Amazon
are highly adapted to this ultra-moist environment and depend on this constant
cycling, having very little tolerance for drought. More than half of the rain
falling on the Amazon comes directly from evapotranspiration of the forest6. As
trees continue to be felled to make room for agriculture and mining and are
killed by flooding from new hydroelectric projects, the remaining trees are
starved of the water the dead trees would have produced through
evapotranspiration. Once enough trees are removed, the forest will reach a
tipping point where cascading tree death will cause the entire ecosystem to
collapse. Recent analyses have predicted this tipping point could be a low as
20% deforestation7. The Amazon is currently ~18% deforested.
The profound long-term destructiveness of this process is
overwhelmingly recognized by local residents, governments, scientists, and most
people who have taken a few minutes to lean about the situation; however,
deforestation of the Amazon continues at an alarming rate because the current
global economic system focuses on short-term gains and profit.
To add to the urgency, droughts have become more frequent in
this region due to changes in climate, and major droughts in 2008 and 2010
killed large numbers of trees in the Amazon. When trees are alive they act as a
carbon sink by taking in carbon dioxide through photosynthesis and
incorporating it into their woody structure. However, dead trees either burn or
decompose, releasing the carbon that has been stored for decades. This release
is not trivial. The amount of carbon released in the Amazon due to dead trees
during the 2010 drought (8 billion tonnes of COs) was greater than the total
annual release of carbon from China, the largest carbon emitting country in the
world8. The nearing ecological shift is turning the Amazon rainforest from a
carbon sink into a source of carbon emissions.
Although local groups, non-profits, and regulations are
working hard to protect small pieces of the Amazon rainforest, it is unlikely
that these efforts will be sufficient to avoid the tipping point. They are
making progress in the right direction, but the current efforts are progressing
too slowly to reverse the trend before we lose the Amazon rainforest forever
and produce another great source of carbon emissions.
For this reason, it is necessary to look beyond viewing
capitalism as a given, and to work towards a system that recognizes the
reliance on healthy land-bases and long-term, stable-state planning that allows
for continued use and coexistence.
References:
1 Zhao, H-L, X-Y Zhao, R-L Zhou, T-H Zhang, and S Drake.
2005. Desertification processes due to heavy grazing in sandy rangeland, Inner
Mongolia. Journal of Arid Environments. 62(2): 309-319.
2 Ludwig, John A and David J Tongway. 1995. Desertification
in Australia: An eye to grass roots and landscapes. Environmental Monitoring
and Assessment. 37: 231-237.
3 Kandji, Serigne Tacko; Verchot, Louis; and Mackensen,
Jens. 2006. Climate Change and Variability in the Sahel Region: Impacts and
Adaptation Strategies in the Agricultural Sector. United Nations Environmental
Programme and World Agroforestry Centre.
4 Weber, Keith T and Shannon Horst. 2011. Desertification and
livestock grazing: The roles of sedentariazation, mobility and rest.
Pastoralism: Research, Policy and Practice. 1:19.
6 Victoria, Reynaldo L, Luiz A Martinelli, Jefferson
Mortatti, and Jeffrey Richey. 1991. Mechanisms of water recycling in the Amazon
Basin: Isotopic insights. Ambio 20(8): 384-387.
7 Vergara, Walter and Sebastian M Sholz (eds). 2011.
Assessment of the Risk of Amazon Dieback. The International Bank for
Reconstruction and Development/The World Bank. Washington, DC.
8 Lewis, Simon L., Paulo M Brando, Oliver L Phillips, Geertje
M F van der Heijden, and Daniel Nepstad. 2011. The 2010 Amazon drought. Science
331(6017): 554.
Photo credit: World Wildlife Fund
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