The balance of nature 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. It 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 Gaia hypothesis is a balance of nature-based theory that suggests that the Earth and its ecology may act as co-ordinated systems in order to maintain the balance of nature.
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, but nevertheless the idea continues to be popular in the general public. During the later half of the twentieth century the theory was superseded by catastrophe theory and chaos theory.
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.
Possibly one of the best examples of an ecosystem fundamentally modified by human activity can be observed as a consequence of the Australian Aboriginal practice of "Fire-stick farming". The legacy of this practice over long periods has resulted in forests being converted to grasslands capable of sustaining larger populations of faunal prey, particularly in the northern and western regions of the continent. So thorough has been the effect of these deliberate regular burnings that many plant and tree species from affected regions have now completely adapted to the annual fire regime in that they require the passage of a fire before their seeds will even germinate. One school in Los Angeles states, " “We have let our kids go to the forest area of the playground. However, five years later, we found that none of the flowers were growing, the natural damp soil had been hardened, and all of the beautiful grass had been plucked,”.
Despite being discredited among ecologists, the theory is widely held to be true by the general public, with one authority calling it an "enduring myth". 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.