Ecological resilience

Ecological resilience is the capacity of an ecosystem to respond to a disturbance. A disturbance is any event that causes a sudden change. This might be a natural event like a fire, flood, or windstorm. It could also be an insect population explosion. Human activities can also cause disturbances. These include deforestation, fracking for oil, or using pesticides in the soil. Resilience allows an ecosystem to resist damage. It also helps the system recover after the event. This capacity is vital for maintaining the stability of our natural world.

When a disturbance occurs, the ecosystem goes through a specific process. It attempts to absorb the shock of the event. If the disturbance is small, the system stays within its normal limits. However, a disturbance can reach a certain magnitude or duration. If it is large enough, the system may reach a threshold. A threshold is a critical limit or a bifurcation point. Crossing this point can cause a regime shift. This means the ecosystem moves into a different state with new processes and structures. Once a regime shift happens, the system may not return to its original state.

Ecologists study several distinct aspects of how these systems behave. They use four specific terms to describe resilience: latitude, resistance, precariousness, and panarchy. Latitude is the maximum amount of change a system can handle before it loses its ability to recover. Resistance describes how difficult it is to change the system. Precariousness measures how close a system is to its limit or threshold. Panarchy refers to how different levels of an ecosystem influence one another. For example, a large population can influence the structure of a small community. These factors determine how stable or vulnerable an ecosystem remains.

The concept of resilience was first introduced by a Canadian ecologist named C.S. Holling. He wanted to explain how natural systems persist despite changes in their variables. Holling identified two different ways to define resilience. The first is called engineering resilience. This describes the time required for a system to return to its steady state. The second is called ecological resilience. This describes the capacity of a system to absorb disturbance and reorganize. This reorganization allows the system to keep its essential function, structure, and identity. This second definition assumes that ecosystems can exist in multiple stable states.

There are many notable examples of these different stable states in nature. Some shallow temperate lakes can exist in a clear water regime. This state provides many helpful ecosystem services. However, these same lakes can also shift into a turbid water regime. This cloudy state provides fewer services and can cause toxic algae blooms. The state of the lake depends on phosphorus cycles. Another example is found in the Mulga woodlands of Australia. These woods can exist in a grass-rich regime that supports sheep herding. They can also shift into a shrub-dominated regime that has no value for grazing. These shifts are driven by rainfall, fire, and herbivory.

Human activities often reduce ecological resilience and cause undesirable regime shifts. In agriculture, intensive practices can damage the soil. Using herbicides and pesticides can reduce plant biodiversity. This diminishes the organic matter, such as carbon and nitrogen, needed to replenish soil nutrients. Deforestation is another major threat. A forest can usually recover from damage to up to 10 percent of its area. However, larger damage can cross a threshold and make recovery impossible. Deforestation also increases carbon emissions. This leads to more drought and climate change-induced damage.

Marine ecosystems also face significant threats to their resilience. The United Nations Food and Agriculture Organisation estimates that over 70 percent of the world's fish stocks are fully exploited or depleted. This overfishing reduces biodiversity and creates imbalances in the food chain. Pollution is another major issue. The Woods Hole Oceanographic Institution calls nutrient pollution the most widespread chronic problem in coastal oceans. Discharges of nitrogen and phosphorus can cause harmful algal blooms. These are sometimes called red tides or brown tides. These blooms can create low-oxygen conditions and pass toxins up the food chain to humans.