This column originally appeared in the July 22, 2007 issue of the Poughkeepsie Journal.

In 1991, a silent and largely invisible change began unfolding in the Hudson River. The zebra mussel had arrived. By the summer of 1992, a new population of over 600 billion mussels had established in the river, transforming the ecosystem. Today, plankton, plants, fish, and bottom-dwelling invertebrates have all been impacted by the dramatic increase in mussels.

The Hudson River passed a tipping point when zebra mussels took up residence. Their invasion was one of the most rapid changes ever to hit the River. The sudden change also appears irreversible. To date, there is no remedy to eliminate zebra mussels.

Most environmental change is gradual and reversible. Pollution can damage ecosystems, but when pollutants are controlled ecosystems often recover. In the past, municipalities dumped untreated sewage into the Hudson. When this practice was eliminated, the river recovered. Oxygen levels increased, nutrient concentrations declined, and fish moved back into old habitats.

If most change is gradual and reversible— what causes sudden environmental changes that are irreversible or very difficult to reverse? Alien species that spread rapidly and reproduce quickly, such as zebra mussels, are often harbingers of sudden environmental change. Like the evils within Pandora's Box, these species are hard to contain and control once they are released into the environment.

Shallow lakes provide another example of sudden environmental change. When aquatic plants decline in these systems, they can rapidly shift from a desirable clear-water state to murky water choked with algae. The switch from clear to cloudy water is caused by increases in nutrients, changes in water levels, loss of the animals that eat algae, or the build-up of fish populations.

Shallow lakes reach a tipping point when one or several of these factors promote a sudden shift in the balance between slow-growing submerged aquatic plants and the algae that covers the water's surface. Once a lake becomes over-run with algae, recovery is a costly and time-consuming process that can involve drawing down water levels or removing certain fish species.

Sudden environmental change has the ability to undermine decades of progress in environmental clean-up. These changes can also diminish our ability to manage important ecosystem services, such as fisheries, timber production, and drinking water supplies. Rapid changes in the environment are compounded by escalating human impacts and a limited understanding of ecological tipping points.

Are there early warning indicators of sudden change? Theoretical studies from economics, oceanography, and ecology suggest that variability increases before massive shifts. For example, stock prices tend to oscillate wildly before major market changes.

Can variability help us identify the signs that precede environmental collapse? In an effort to find out, IES scientists and their collaborators will be manipulating a lake ecosystem in a large-scale project that is funded by the National Science Foundation.

When big fish are stocked in a lake that is dominated by small fish, they can drive the lake from a murky state to a clear-water state. This happens when the larger fish eat small fish that normally keep the grazers of algae under control. By slowly adding large fish to a lake, this research project will explore the threshold at which rapid change occurs. Frequent measurements of lake conditions, such as algal and oxygen concentrations, will test if variability can help forecast the onset of sudden environmental change.

Our goal is to make sudden environmental change more predictable and, ultimately, more avoidable. This will help environmental managers avoid costly restoration efforts while preserving the natural ecosystems that support our quality of life.

Dr. Michael L. Pace is an aquatic ecologist and Assistant Director at the Institute of Ecosystem Studies in Millbrook, NY.