The balance of nature (also known as ecological balance) is a theory that proposes that ecological systems are usually in a stable equilibrium or homeostasis, which is to say that a small change (the size of a particular population, for example) will be corrected by some negative feedback that will bring the parameter back to its original "point of balance" with the rest of the system. The balance is sometimes depicted as easily disturbed and delicate, while other times it is inversely portrayed as powerful enough to correct any imbalances by itself. The theory may apply where populations depend on each other, for example in predator-prey systems, or relationships between herbivores and their food source. It is also sometimes applied to the relationship between the Earth's ecosystem, the composition of the atmosphere, and the world's weather.
The theory that nature is permanently in balance has been largely discredited by scientists working in ecology, as it has been found that chaotic changes in population levels are common. During the later half of the twentieth century, the "balance" theory was superseded by catastrophe theory and chaos theory. Nevertheless, the idea maintains popularity amongst the general public.
The concept that nature maintains its condition is of ancient provenance; Herodotus commented on the wonderful relationship between predator and prey species, which remained in a steady proportion to one another, with predators never excessively consuming their prey populations. The "balance of nature" concept once ruled ecological research, as well as once governing the management of natural resources. This led to a doctrine popular among some conservationists that nature was best left to its own devices, and that human intervention into it was by definition unacceptable. The validity of a "balance of nature" was already questioned in the early 1900s, but the general abandonment of the theory by scientists working in ecology only happened in the last quarter of that century when studies showed that it did not match what could be observed among plant and animal populations.
Predator-prey populations tend to show chaotic behavior within limits, where the sizes of populations change in a way that may appear random but is, in fact, obeying deterministic laws based only on the relationship between a population and its food source illustrated by the Lotka-Volterra equation. An experimental example of this was shown in an eight-year study on small Baltic Sea creatures such as plankton, which were isolated from the rest of the ocean. Each member of the food web was shown to take turns multiplying and declining, even though the scientists kept the outside conditions constant. An article in the journal Nature stated: "Advanced mathematical techniques proved the indisputable presence of chaos in this food web ... short-term prediction is possible, but long-term prediction is not."
Although some conservationist organizations argue that human activity is incompatible with a balanced ecosystem, there are numerous examples in history showing that several modern-day habitats originate from human activity: some of Latin America's rain forests owe their existence to humans planting and transplanting them, while the abundance of grazing animals in the Serengeti plain of Africa is thought by some ecologists to be partly due to human-set fires that created savanna habitats.
One of the best-known and often misunderstood examples of ecosystem balance being enhanced by human activity is the Australian Aboriginal practice of "fire-stick farming". This uses low-intensity fire when there is sufficient humidity to limit its action, to reduce the quantity of ground-level combustible material, to lessen the intensity and devastation of forest fires caused by lightning at the end of the dry season. Several plant species are adapted to fire, some even requiring its extreme heat to germinate their seeds.
Despite being discredited among ecologists, the theory is widely held to be true by the general public, conservationists and environmentalists, with one author calling it an "enduring myth". Environmental and conservation organizations such as the WWF, Sierra Club and Canadian Wildlife Federation continue to promote the theory, as do animal rights organizations such as PETA.
Kim Cuddington considers the balance of nature to be a "foundational metaphor in ecology", which is still in active use by ecologists. She argues that many ecologists see nature as a "beneficent force" and that they also view the universe as being innately predictable; Cuddington asserts that the balance of nature acts as a "shorthand for the paradigm expressing this worldview".
At least in Midwestern America, the "balance of nature" idea was shown to be widely held by both science majors and the general student population. In a study at the University of Patras, educational sciences students were asked to reason about the future of ecosystems which suffered human-driven disturbances. Subjects agreed that it was very likely for the ecosystems to fully recover their initial state, referring to either a 'recovery process' which restores the initial 'balance', or specific 'recovery mechanisms' as an ecosystem's inherent characteristic. In a 2017 study, Ampatzidis and Ergazaki discuss the learning objectives and design criteria that a learning environment for non-biology major students should meet to support them challenge the "balance of nature" idea. In a 2018 study, the same authors report on the theoretical output of a design research study, which concerns the design of a learning environment for helping students challenge their beliefs regarding the balance of nature and reach an up-to-date understanding about ecosystems' contingency.
The balance of nature (referred to as "the circle of life") is a major theme of the 1994 film, The Lion King. In one scene, the character Mufasa describes to his son Simba how everything exists in a state of delicate balance.
In the 2018 film, Avengers: Infinity War, the villain Thanos' home planet Titan has been destroyed by the overexploitation of resources, leading him to seek the restoration of balance to the universe by eliminating half of all living beings.
If left unaltered by humans, the delicate balance of nature's ecosystems ensures the survival of most species. Natural predators help maintain this balance by killing only the sickest and weakest individuals.