Posted by: oikosasa | October 15, 2013

Like a missile attack on the ecosystem

The resilience of eco systems is studied in the new Early View paper “A new approach for rapid detection of nearby thresholds in ecosystem time series” by Stephen R. Carpenter and co-workers, in Oikos. Below is Stephen’s summary of the study:

When is the disappearance of a fish population like a missile attack? During the Cold War, scientists developed sensitive methods for detecting the radar signature of incoming missiles. More recently, ecologists have discovered that ecosystems display statistical signatures of changing resilience. The evidence of changing resilience is found in detailed observations that can be automated, like the signals from a radar installation.

Changing climate, land use, or chemical pollution can be as harmful to ecosystems as a missile impact. Gradual changes in climate  or other factors can erode resilience and lead to catastrophic changes. Conversion of a rangeland to a desert, collapse of a fishery, or explosion of toxic algae in a lake are accompanied by loss of resilience as an ecosystem is driven past a critical threshold.

When a complex system approaches a critical threshold, its behavior becomes more variable. Close to the threshold, resilience is low and variability is high. Therefore it might be possible to infer changes in resilience from changes in variability.

Research on lakes has shown that water chemistry, concentrations of algae, and even movements of animals become more variable as resilience declines. Some of these changes can be measured by new technology, such as the instruments mounted on the buoy shown in the photo

Buoy_PeterLake_2012-07-12

Our research team adapted the missile-detection methods to data from a lake that was manipulated to drive it slowly over a threshold. We gradually added largemouth bass to the lake to erode the resilience of minnows and other small fish that are prey to the bass. We found that variability increased in spatial pattern of minnows, abundance of small grazing animals in the water, concentration of algae, concentration of oxygen, and acidity of the water. In the Oikos paper, we applied the method to time series of chlorophyll, which is related only indirectly to the change in the fish. The growing variability of chlorophyll was the equivalent of a missile image on a radar screen.

About a year after the rising variability was detected, the old food chain of the lake collapsed and was replaced by a new food chain. The new food chain had no minnows, abundant grazers and very low concentrations of algae.

Although largemouth bass and missiles are quite different, both of them can completely transform their targets. The research shows how insights from one area of science can be applied in a new way. Perhaps missile-detection methods will one day monitor the resilience of lakes and other ecosystems in a changing world.

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