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Damaged Ecosystems Not Lost Forever

Marina Litvinsky

WASHINGTON - Most polluted or damaged ecosystems worldwide could recover within a single lifetime if societies commit to their cleanup or restoration, according to researchers at the Yale School of Forestry & Environmental Studies.

The report, ‘Rapid Recovery of Ecosystems,' states that biotic and biophysical conditions of ecosystems become degraded from exploitation by humans to meet rising demands for resources and environmental services, or from accidents.

In theory, ecosystems could recover gradually at a rate proportional to the degree to which the problem is abated. However, it is speculated that such recovery would take centuries if not millennia given the scales of current human impact.

Researchers found that forest ecosystems recovered in 42 years on average, while ocean bottoms recovered in less than 10 years. When examined by disturbance type, ecosystems undergoing multiple, interacting disturbances recovered in 56 years, and those affected by either invasive species, mining, oil spills or trawling recovered in as little as five years.

Most ecosystems took longer to recover from human-induced disturbances than from natural events, such as hurricanes.

"The damages to these ecosystems are pretty serious," said Oswald Schmitz, an ecology professor at the Yale School of Forestry & Environmental Studies and co-author of the meta-analysis with Yale Ph.D. student Holly Jones. "But the message is that if societies choose to become sustainable, ecosystems will recover. It isn't hopeless."

The Yale analysis focused on seven ecosystem types, including marine, forest, terrestrial, freshwater and brackish, and addresses recovery from major anthropogenic disturbances: agriculture, deforestation, eutrophication, invasive species, logging, mining, oil spills, overfishing, power plants and trawling and from the interactions of those disturbances.

Major natural disturbances, including hurricanes and cyclones, are also accounted for in the analysis.

"We recognise that humankind has and will continue to actively domesticate nature to meet its own needs," said Jones. "The message of our paper is that recovery is possible and can be rapid for many ecosystems, giving much hope for a transition to sustainable management of global ecosystems."

The researchers analysed data derived from 240 peer-reviewed studies conducted over the past century that examined the recovery of large ecosystems following the cessation of a disturbance. The studies measured 94 variables that were grouped into three categories: ecosystem function, animal community and plant community.

The recovery of each of the variables was quantified in terms of the time it took for them to return to their pre-disturbance state as determined by the expert judgment of each study's author. The findings "show that there may be much hope to restore even heavily degraded ecosystems."

The analysis found that 83 studies demonstrated recovery for all variables; 90 reported a mixture of recovered and non-recovered variables; and 67 reported no recovery for any variable.

Schmitz said 15 percent of all the ecosystems in the analysis are beyond recovery. Also, it was determined that 54 percent of the studies that reported no recovery likely did not run long enough to draw definitive conclusions.

In addition, the analysis suggests that an ecosystems recovery may be independent of its degraded condition. Aquatic systems, the researchers noted, may recover more quickly because species and organisms that inhabit them turn over more rapidly than, for example, forests whose habitats take longer to regenerate after logging or clear-cutting.

Recovery following agricultural activities and multiple perturbations was significantly slower than all other perturbation types

The researchers pointed out that a potential pitfall of the analysis is that the ecosystems may have already been in a disturbed state when they were originally examined. Many ecosystems across the globe that have experienced extinctions and other fundamental changes as a result of human activities, combined with the ongoing effects of climate change and pollution, are far removed from their historical, natural pristine state. Thus, ecologists measured recovery on the basis of an ecosystem's more recent condition.

Because historical reference sites are often not representative of ecosystem states that humans aspire to restore, many restoration projects have moved away from the idea of restoring back to ‘natural' or pre-human states and instead use contemporaneous reference systems as restoration targets.

Three explanations could account for lack of recovery in almost half of the systems and response variables. First, a particular study may not have been conducted over a long enough time scale to detect recovery. To assess this possibility, the researchers compared the average recovery times for those ecosystems that were found to be fully recovered with the duration of those studies reporting that variables had not yet recovered.

Second, systems may have entered into alternative states, thereby precluding recovery. Five percent of the total studies conclusively reported that the ecosystems were irreversibly entrained into alternative states.

Third, while some studies did rely on either a pre-perturbation or undisturbed control as an objective benchmark, this was not universally so. Of the 240 studies, only 20 percent used pre-perturbation data and 58 percent used undisturbed reference sites.

The Yale analysis points out the need for the development of objective criteria to decide when a system has fully recovered.

The analysis rebuts speculation that it will take centuries or millennia for degraded ecosystems to recover and justifies an increased effort to restore degraded areas for the benefit of future generations.

"Restoration could become a more important tool in the management portfolio of conservation organisations that are entrusted to protect habitats on landscapes," said Schmitz.

"Our results are not intended to give license to exploit ecosystems without regard to sustainability," said the report. "But, with even the best sustainable practices unforeseen outcomes and damages can happen accidentally. The message of our paper is that recovery is possible and can be rapid for many ecosystems, giving much hope for humankind to transition to sustainable management of global ecosystems."

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