What causes abrupt changes in ecosystems?

Large and abrupt changes in forests, coral reefs and Arctic sea ice are driven by climate change, food production and urbanisation. Managing their causes can reduce the occurrence of these shifts around the world and ensure the well-being of local communities.

In 2016, fishers in Nueva Venecia on the Colombian Caribbean coast woke up one morning to find a thick carpet of dead fish floating in the water. The massive fish death had been caused by an overload of nutrients in the Ciénaga Grande de Santa Marta, the largest coastal lagoon and wetland system in the country.

It was the rainy season, Sandra Villardy, an ecology professor at Magdalena University in Santa Marta, explains, and when the rain finally arrived this year, the Magdalena river that feeds the wetland system, brought with it a higher concentration of nutrients and sediments than it had done during the dry season. 2016 was also an El Niño-year, and unusually high temperatures had made the water in the lagoon, rivers and on the coast warmer than normal 1. The nutrients in the river water together with unusually warm conditions in the lagoon, created conditions perfect for algal blooms. During blooms, algae grow in great numbers and consume oxygen to the point where few other organisms can survive in the water. The fish had suffocated.

For fishers this is more than an ecological tragedy. “This affects us in all respects, our economic situation, because in Nueva Venecia our livelihood is fishing, without fish there is nothing,” says Edgardo Camargo 2 from the Association of Fish Producers in Nueva Venecia.

Ciénaga Grande de Santa Marta, a 4,120 square kilometre lagoon complex, is home to, and a major source of livelihood for many fishing and agricultural communities. “81% of our city’s inhabitants live off fishing,” says Fransisco Gutierrez 3, who, as mayor of the town Pueblo Viejo in the Ciénaga, was concerned about how the fish death would impact on livelihoods. Only in Pueblo Viejo there are roughly 64,000 fishers.

In the last 50 years the Ciénaga went from producing around 30,000 tons of fish per year to only 5,000, according to fishers in the area. An estimated 20 tons of fish perished in the mass fish death of 2016. And the dead fish was not only a wasted resource; it was also a source of disease and foul odours when it was left to rot.

The drastic changes the fishers experienced in Nueva Venecia were not an isolated event. Disruptions to ecosystems occur all over the world, often with dire consequences for both the ecosystems and for the people who depend on them. Insights into what causes these changes can improve management to avoid them.

Abrupt and persistent regime shifts

Large, abrupt and persistent changes in the function and structure of ecosystems are known as regime shifts. They are very hard to predict; costly and difficult or impossible to reverse; and they often affect people’s wellbeing.

The regime shift that occurred in Ciénaga, where normal levels of oxygen dropped to oxygen-starved levels, has been documented in over 300 case studies around the world, in oceans, lakes and lagoons: some cases have been temporary events where the ecosystems could recover, others have been permanent shifts where the water became acidic and only bacteria could survive.

This is one of the over 30 different types of so called regime shifts that have been identified in the world’s ecosystems – in water, on land and in polar environments.

In 2009, a group of researchers started developing the regime shifts database at the Stockholm Resilience Centre. Today it is the world’s largest database on regime shifts that identifies the main drivers, impacts and mechanisms that underlie regime shifts. To date, the database has reviewed over 1000 scientific papers and provides a knowledge base for researchers and practitioners dealing with regime shifts.

Figure 1: Fig 2. The regime shifts database: each dot is a literature review of a generic regime shift (orange) or case study (blue). By clicking on each point you will find a link to each literature review in the database.

From reef to algae, from rainforest to savanna

Fishers in Colombia are not the only ones affected, and regime shifts are not always limited to small communities. In 2016, 21 million people in São Paulo suffered one of the worst droughts on record 4 and politicians and decision makers were worried about the risk of riots. Researchers suggested that the lack of rain was triggered by increasing deforestation in the Amazon rainforest 5. This connection was also emphasized by the state-owned water company serving most of São Paulo.

“The Amazon creates a movement of water. If you could follow a molecule of water you would see that most of the clouds that are over São Paulo have passed across the Amazon. If the forest is cut, we’ll be in trouble,” Jerson Kelman, president of the water company Sabesp said in an interview with the Guardian 6.

Trees contribute to increased humidity, and when they disappear rainfall volumes decrease as the humidity drops. Less trees also means less water vapour flowing up to the atmosphere, which in turn means less rainfall somewhere else. Ecosystems are connected across great distances, through for example flows of water and water vapour, migrations of animals, and dispersal of seeds. And in the same way that São Paulo is feeling the consequences of deforestation, couldn’t it be possible that other regions are suffering the consequences of changes in the Amazon forest as well? Researchers believe that changes in moisture recycling from the Amazon can have impacts in the mountain forests and agricultural landscapes in the Andes 7.

Continued deforestation together with climate change could eventually cause the Amazon to shift from rainforest to savannah. Such a shift would mean that all the benefits of the forests would be lost, including carbon storage and the oxygen production that has earned the Amazon the title “the world’s lungs”.

Another worrying example of a regime shift in the making is the coral die-off around the world. In Australia, a nine-month marine heatwave in 2016 caused 30% of the corals on the Great Barrier Reef to die 8. Coral reefs provide important ecosystem services such as storm and wave erosion protection in coastal areas. They are homes to many marine species that fishers depend upon and a hotspot of biodiversity that fuels an important tourism industry. A combination of climate change, fishing pressure, pollution and nutrient run-off from land is gradually eroding the reefs’ capacity to deal with shocks and disturbances, such as storms or heatwaves – in other words reducing their resilience, and putting these ecosystems at risk 9 of a regime shift.

Resilience turns out to be key in understanding regime shifts. In fact, all the examples of regime shifts from across the globe show that ecosystems tip from one state into another when their resilience is eroded. It often happens because of a shock, such as a heatwave, hurricane or heavy pollution, or a slow change that makes it less likely that the ecosystem can recover, for example deforestation, or a combination of both. Research has shown, over and over again, that ecosystems have thresholds, critical points, where they can flip and turn into something different, changing their structure, function, and ability to provide services that are important for society.

In Ciénaga Grande de Santa Marta it is the low level of oxygen that cause the death of the fish. In the Amazon it is the level of deforestation that reduces moisture to feed the rain that is supposed to fall in São Paulo and leads to drought. And in Australia it is the increase in heatwaves and the number of days with higher-than-normal temperatures that is causing corals to die. The level of oxygen that would be tolerable, the minimum humidity that would still create rain, or the number of days that a coral can stand heat - these tipping points are moving targets.

The exact value of the tipping point is different from place to place, from species to species, and might even differ dramatically within the same place, from one situation to another depending on a number of other conditions. To compare, today it is well known that smoking can cause lung cancer. However, the exact number of cigarettes that will trigger the effect varies from person to person, and the risk of getting cancer can be influenced by other factors such as air pollution, exposure to asbestos and a person’s genes 10. Knowledge about potential causes can shed light on what actions are necessary to avoid tipping point where cancer forms. This is also true for regime shifts in the environment.

To stave off drastic changes

So what are the main causes of regime shifts globally? Researchers have used the regime shifts database to identify what the main causes, or drivers, of regime shifts are, and to study patterns of how different drivers occur together. Understanding the drivers behind regime shifts can generate insights around how to prevent ecosystems from tipping into undesired states, or how to reverse changes. They found that climate change, food production and urbanisation are the three main groups of drivers of regime shifts 11. Managing these will be crucial for avoiding regime shifts, however, they cannot be managed at the same scale. International efforts and a global approach are needed to tackle climate change. Countries need to cooperate with one another and reduce their emissions to fulfil the Paris agreement. But food production and urbanisation are different – they can be managed at the regional level and sometimes even at the community level.

Managing these drivers on a local level can build resilience - increase the probability that an ecosystem will withstand disturbances - and delay the effect of global drivers, buying some time before regime shifts occur. For example, researchers have found that by managing local scale drivers, such as pollution or fishing pressure, one can delay the impacts of climate change on coral reefs, buying up to a decade of time to tackle climate 12. However, to ensure that ecosystems can continue to provide critical services on which people depend, the global challenges cannot be neglected.

Lack of information and insufficient data collection make it harder to study regime shifts in developing countries. Traditional methods to study these shifts are not always applicable; one typically needs long time series and consistent data collection programmes in place. Understanding true drivers, finding tipping points, or anticipating potential shifts is harder when data is scarce.

So in the absence of solid science, what could managers or policy makers do to avoid undesirable regime shifts, or create changes that might lead to more desirable regime shifts? Researchers working with the regime shifts database have compared case studies to better understand different types of regime shifts and documented ways to manage the causes 13. In this way, lessons learned can be useful in places that are vulnerable to the same types of regime shifts. For example, managing drivers related to coastal development, fishing and pollution can prevent regime shifts in coastal ecosystems. Research shows that the drivers of change on coral reefs are in many ways the same across the world – overfishing, pollution, and climate change for example. Successful management of pollution in one place on the globe can offer insights that are useful somewhere else.

Not all regime shifts are negative for society. Although the database focuses on regime shifts that negatively affect people, some regime shifts are considered more desirable. In the Arctic, for example, biological productivity is expected to increase the capture of nutrients and carbon in the ocean as climate change continues. This can be beneficial for algae and for other organisms that feed on algae and could benefit fisheries and people who depend on fish for protein, as fish populations may grow when more food becomes available for them. Another example is the boreal forest expanding north in North America, Scandinavia and Russia. The forest could speed up the development of new timber industries.

Both of these scenarios are uncertain, with hypothetical effects of a shift, and it is important to keep in mind that when regime shifts occur there are always winners and losers. That’s why it is important to prepare for the unexpected.

Resilience thinking offers a new way of approaching these challenges, and stresses the need to understand how different parts of a system are connected to and affect each other. It is about acknowledging that society and nature impact and are dependent on each other, and that they can change in abrupt and non-linear ways. Implementing this non-linear thinking in the way we plan policies and collectively decide how to manage ecosystems will make it possible for nature and people to benefit, and to avoid undesirable states, such as the changes the fishers were experiencing in Nueva Venecia.


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