By Kelly Reed
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.
1Zhao, 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.
2Ludwig, 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.
4Weber, Keith T and Shannon Horst. 2011. Desertification and livestock grazing: The roles of sedentariazation, mobility and rest. Pastoralism: Research, Policy and Practice. 1:19.
6Victoria, 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.
7Vergara, 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.
8Lewis, 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