When scientists discovered an outbreak of Crown-of-Thorns Starfish (COTS) among the World Heritage-listed reefs off Western Australia's Pilbara last year, there was cause for great concern. The starfish, which feed on coral, have a history of doing serious damage to coral reefs—outbreaks of COTS are one of the main reasons the Great Barrier Reef has lost half of its coral cover in the last 30 years.
But that isn't the worst of it. The outbreak comes at a time when those reefs are recovering from a series of severe coral bleaching events, caused by marine heatwaves, which have reduced live coral cover to an average of just over five per cent. The reefs could probably cope with one of these threats but the combination of stressors could seriously impact on long term coral cover and reef health.
This double-whammy of threats, caused by climatic changes and local issues, is common to many of the world’s ecosystems. And it is that very double-whammy that scientists are now saying holds the key to preserving iconic ecosystems as the climate continues to change. By managing the local stressors, we can take some pressure off the ecosystem, thereby increasing its resilience to climate change.
A safe operating space
Many ecosystems react to changing conditions in non-linear ways; they show little response until a threshold or tipping point is reached where even a small perturbation can trigger collapse into a different state that is hard to recover from.
An international team of researchers this month concludes, in a paper published in Science, that by managing the local stressors, we can give an ecosystem more ‘room to breathe’, in effect increasing its tolerance to climate change.
Lead author Marten Scheffer, chair of the Department of Aquatic Ecology and Water Quality Management at the Netherlands’ Wageningen University, calls this extra breathing space an expansion of the ecosystem’s ‘safe operating space’.
“The tolerance to climate change can be enhanced through local management,” says Scheffer. “For example, we can increase the resilience of tropical rainforests to drought by maintaining a closed canopy.”
However, while local interventions can provide much needed breathing room, the authors stress they are no panacea for the threats of climatic change to ecosystems. For example, melting of the arctic sea ice, with its far-reaching ecological consequences, is not something that can be arrested by local management. Tackling the causes of climate change, through reducing greenhouse gas emissions, will still be necessary to deal with those kinds of threats.
But ways of building climate resilience are emerging for a broad range of ecosystems, ranging from control of local sources of nutrient runoff into the ocean to management of grazing pressure on rangeland ecosystems.
“There is sound science showing how we can make wetlands, coral reefs and rainforests more resilient to climate change,” says Scheffer.
Iconic ecosystems under threat
The research team examined the climatic threats and local stressors for three of the planet’s iconic ecosystems, all of which are UNESCO World Heritage Areas—the Great Barrier Reef, the Doñana wetlands and the Amazon rainforest.
For each of these ecosystems, they propose local actions for maintaining a safe operating space.
The Great Barrier Reef in Australia
The Great Barrier Reef is the largest coral system in the world and has been listed as a World Heritage Area since 1981. Despite this protection, about half of the coral cover on the Great Barrier Reef has been lost in recent decades, and the outlook is poor, and declining, according to the Australian Government’s report card, the Great Barrier Reef Outlook Report 2014 (see Note 2).
The resilience of coral reefs to ocean acidification and coral bleaching is highly dependent on locally manageable stressors such as overfishing, nutrient runoff and dredging.
Since 2004, fishing has been prohibited within zones that cover 33% of the Great Barrier Reef Marine Park, and work has begun to reduce runoff of nutrients, pesticides, herbicides and sediments from the land. But as climate pressures mount, more may need to be done to improve the Reef's resilience.
The research team see climate change, fishing pressure, coastal development and unprecedented amounts of port dredging as the major threats to the Reef.
“The critical finding of our paper is that climate change is exacerbated by local stressors like fishing pressure and by pollution,” says Terry Hughes, Director of the Australian Research Council Centre of Excellence for Coral Reef Studies. “So we can actually make the impacts of climate change worse by not taking local steps to manage the rate of fishing and the level of pollution.”
In an experiment conducted after a coral bleaching event on the Great Barrier Reef in 1998, scientists excluded herbivorous fish from some sections of reef while it was recovering. They observed that coral was much slower to recover where there were no herbivorous fish to control seaweeds and algae. This demonstrated that local conservation efforts, such as management of fish stocks, can greatly help in maintaining and enhancing resilience, and in limiting the longer-term damage from bleaching and other climate-related impacts.
The Doñana wetlands in Spain
The Doñana wetlands in the south of Spain are Europe’s most important wintering site for waterfowl. They also contain Europe’s largest temporary pond complex, with a unique diversity of amphibians and invertebrates.
The wetlands are threatened by nutrient runoff from the use of agricultural fertilisers and urban wastewater and reduced flows of incoming streams. This promotes toxic blooms of Blue-green algae and dominance of invasive floating plants that deplete the water of oxygen. Groundwater is also being extracted here to irrigate strawberry crops and to supply the tourism industry, and this too is having major impacts.
For this ecosystem, the main climatic risk is warming, which can have the same effects as an increase in nutrients. With warming and nutrient loading at play, there is a tipping point at which floating plants invade or Blue-green algae takes over aquatic ecosystems.
But within this potential double-whammy, the science reveals an opportunity. To prevent Blue-green algae, reducing the nutrient load can almost entirely compensate for the effects of modest levels of warming. And to keep floating plants at bay, reducing nutrient concentrations by one third may be enough to compensate for the effect of a one degree Celsius increase in water temperature.
Andy Green, a professor at the Doñana Biological Station, suggests that nutrient control measures could include reducing fertiliser use, improving water treatment plants and closing illegal wells that are decreasing flows of clean water to the wetlands.
The Amazon rainforest in Brazil
In the Amazon rainforest, one of the world’s greatest biological treasures and a vital component of the functioning of the Earth’s climate system, rising temperatures and severe dry spells, exacerbated by logging, could turn the ecosystem into a drier, fire-prone and species-poor woodland.
When trees are removed, whether through logging or drought, the forest becomes more fire-prone, increasing the risk of collapse of the forest during dry years. As the canopy cover shrinks or is opened up, flammable grasses can invade, elevating the fire risk. Less forest cover also means less rainfall because around 80% of the rain that falls in the Amazon is water recycled via transpiration in the forest.
Keeping a critical mass of forested areas and preventing openings in the forest canopy can enhance the safe operating space of tropical forest in the face of rising drought risks.
“A combination of bold policy interventions and voluntary agreements has slowed deforestation in the Brazilian Amazon to one fourth of its historical rate,” says Daniel Nepstad, Executive Director of Earth Innovation Institute. “The stage is now set to build on this success by ramping up efforts to tame logging and inhibit fire.”
Implications for Australia’s landscapes
While tropical forests in Australia are no longer logged, they share some of the Amazon rainforest’s local issues.
The sharp distinction between the complex rainforest with its abundance of species and the less diverse and more fire-prone eucalypt forest all comes down to fire, according to CSIRO’s Brian Walker.
“Fire opens up the canopy, then there is a period of drying out afterwards when different species—eucalypts—come in. Eventually the canopy closes but the forest is no longer a rainforest,” he says.
Gaps in the canopy also encourage flammable grasses to grow, which in turn can catch fire through lightning strikes. Just as a rainforest can flip to become a eucalypt forest, the reverse can also happen, given enough rain, he says.
“To maintain Australia’s tropical rainforests, we need to manage forest fires and stop opening the canopy.”
Across Australia’s rangelands, however, Walker says the key will be to retain fire in the landscape, as higher temperatures are likely to bring about a woody weed problem for graziers.
“As carbon dioxide and the temperature increase, this favours the woody plants, so the rangelands are likely to become woodier,” he says. “You can flip from a grassy rangeland to a shrubby thicket as a result of sustained heavy grazing that prevents occasional fires, and that’s not good for grazing animals. If we continue as we are, we can expect to see more flips to woody weeds.”
Fire is the answer, he says. “You’ve got to have fire in semi-arid zone rangelands. Fire favours grass but kills off shrubs. And if you don’t have enough grass to carry the fire, you end up with more woody weeds.”
Increased variability in rainfall will also have a bearing, he says, and managing grazing pressure is the key.
“The big pressure that we have any control over is grazing. We need to monitor that we are not overgrazing, which means not allowing sustained, continuous high levels of grazing. As the rainfall becomes even more variable it will become harder to keep a constant number of animals without overgrazing, so graziers will need to think on a bigger scale about how they move animals around to match the variability in rainfall.”
Advantages of the local approach
Controlling local stressors to build a safe operating system for ecosystems is more conducive to local action than trying to reduce global greenhouse gas emissions, say the authors. They offer three reasons:
- Local benefits: Adaptation can be done unilaterally, with the benefits accruing to the country doing the adaptation. And sometimes the benefits spill over to the global level.
- Less uncertainty: There is less uncertainty at the ecosystem level than at the global level when it comes to effects of management options. Perceived uncertainty has often paralysed policy.
- Positive message: Expanding an ecosystem’s safe operating space frames the problem in a positive way whereas gloom-and-doom perceptions can block people from taking action.
“Local management options are well understood and not too expensive,” says Scheffer. “So there is really no excuse for countries to let this slip away, especially when it comes to ecosystems that are of vital importance for maintaining global biodiversity.”
NOTES:
- Marten Scheffer, Scott Barrett, Stephen R. Carpenter, Carl Folke, Andy J. Green, Milena Holmgren, Terry P. Hughes, Sarian Kosten, Ingrid A. van de Leemput, Daniel C. Nepstad, Egbert H. van Nes, Edwin T.H.M. Peeters and Brian Walker (2015). Creating a safe operating space for iconic ecosystems, Science, 347: 1317-1319.
- Great Barrier Reef Marine Park Authority (2014). Great Barrier Reef Outlook Report 2014, GBRMPA, Townsville.