Ecological threshold can be described as the point at which a relatively small change in external conditions causes a rapid change in an ecosystem

Ecosystem

An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

. When an ecological threshold has been passed, the ecosystem may no longer be able to return to its state. The trespassing of ecological threshold often leads to rapid change of ecosystem health.

Definitions

There are several definitions for the concept of ecological threshold. The common feature of most definitions is the non-linearity of the responses in ecological or biological systems to pressures caused by human activities or natural processes.

Groffman and others define ecological thresholds as the points at which there is an abrupt change in an ecosystem quality, property or phenomenon, or where small changes in an environmental driver produce large responses in the ecosystem. On a general level, ecological thresholds are the breaking points of ecosystems at which the pressures lead to abrupt changes in the ecosystem.

Critical load

Critical load

In the study of air pollution, a critical load is defined as ”A quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge”...

, tipping point

Tipping point

In sociology, a tipping point is the event of a previously rare phenomenon becoming rapidly and dramatically more common. The phrase was coined in its sociological use by Morton Grodzins, by analogy with the fact in physics that adding a small amount of weight to a balanced object can cause it to...

 and regime shift

Regime shift

Regime shifts is defined as rapid reorganizations of ecosystems from one relatively stable state to another. The idea of a regime shift was first used in investigations of freshwater and terrestrial systems. It is now been used for marine and estuarine systems to describe rapid shifts between two...

 are examples of other closely related terms.

Characteristics

Thresholds can be characterised as points or as zones. Zone-type thresholds imply a gradual shift or transition from one state to another rather than an abrupt change at a specific point. Ecological thresholds have caught attention because many cases of catastrophic worsening of conditions have proved to be difficult or nearly impossible to remedy (also known as points of no return). Ecological extinction

Ecological extinction

Ecological extinction is defined as “the reduction of a species to such low abundance that, although it is still present in the community, it no longer interacts significantly with other species.”...

 is an example of a definitive point of no return.

Ecological thresholds are often characterised by hysteresis

Hysteresis

Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

, which means the dependence of the state of a system on the history of its state. Even when the change is not irreversible

Irreversible

Irreversible may refer to:*Irreversible process, in thermodynamics, a process that is not reversible*Irréversible, a 2002 film*Irréversible , soundtrack to the film Irréversible...

, the return path from altered to original state can be drastically different from the development leading to the altered state.

Another related concept is panarchy

Panarchy

Panarchy is a conceptual term first coined by the Belgian botanist and economist Paul Emile de Puydt in 1860, referring to a specific form of governance that would encompass all others. The Oxford English Dictionary lists the noun as "chiefly poetic" with the meaning "a universal realm," citing...

. Panarchy views coupled human-natural systems as a cross-scale set of adaptive cycles that reflect the dynamic nature of human and natural structures across time and space. Sudden shifts in ecosystem state can induce changes in human understanding of the way the systems need to be managed. These changes, in turn, may alter the institutions that carry out that management and as a result, some new changes occur in ecosystems.

Detection

There are many different types of thresholds and detecting the occurrence of a threshold is not always straightforward. One approach is to process time series which are thought to display a shift in order to identify a possible jump. Methods have been developed to enhance and localize the jumps.

Examples

Some examples of ecological thresholds, such as clear lakes turning into turbid ones, are well documented but many more are probable to exist. The thresholds database by Resilience Alliance and Santa Fe Institute includes over one hundred examples.

External links

• Resilience Alliance A multidisciplinary research group that explores the dynamics of complex adaptive systems
• Thresholds of environmental sustainability A research project focusing on ecological thresholds