In 1973, the renowned Canadian ecologist C. S. Holling introduced the idea of ecosystem resilience. An ecosystem’s resilience refers to its ability to withstand environmental pressures, or perturbations—such as fires, floods, storms, deforestation, fracking, pesticide spraying, or the introduction of an invasive species—and the time it takes for the ecosystem to return to a stable, functioning state. According to Holling, all ecological systems go through cycles of good and poor health. Scientists study these cycles in detail to better understand and predict the overall health of an ecosystem.
In a new paper, Beck et al. examined a shift in an ecosystem to a stable state that can occur in response to a perturbation. Such shifts are known as “critical transitions,” and they are often sudden and unpredictable—examples include an influx of nutrients, a land use change, or a change in climate. The team identified a series of early warning signals that if detected in time, can alert observers to an impending critical transition.
Detecting these signals, such as a lowered resilience or a sluggish recovery time, can be challenging. This is especially true for ecosystems with long generational life spans, such as temperate forests, some of which have trees that are thousands of years old. Knowing this, the researchers decided to use paleoecological data, which provide ecological data over long timescales.
Using paleoecological data collected from a meter-long core extracted in 2011 from Australia’s Lake Vera in Tasmania, the researchers examined the interrelated changes in vegetation, pollen, charcoal, soil, and other components of the sediments over the past 2,400 years. In particular, they investigated the impact of fire, a major driver of ecosystem changes on land, on the aquatic ecosystem. Aquatic ecosystems, generally speaking, are highly sensitive and respond rapidly to environmental pressures.
Lake Vera is home to a thriving community of a species of diatom, or alga, called Discostella stelligera. The researchers found that the diatom community experienced a critical transition about 820 years ago that was likely caused by wildfires disrupting local vegetation around the same time. They also found that an increased rate of change, as well as increasing shifts in species composition—more oligotrophic (nutrient-poor) and acidic species of diatoms began to replace existing species—had preceded this critical transition.
This study shows that a disturbance on land, such as a wildfire, can drive a critical transition in a lake within that ecosystem. It also illustrates several early warning signals, namely, rate of change and variability of species composition, that scientists can potentially use to predict critical transitions in an ecosystem. The study also highlights the important role of paleoecological data, which provide evidence for ecosystem changes due to critical transitions and for distinguishing these changes from other kinds of abrupt change. This may also help researchers to understand the parameters that can be used to identify and measure early-warning signals with greater precision. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG004135, 2018)
—Sarah Witman, Freelance Writer