Gaia hypothesis
Gaia theory (biology) is a broadly inclusive name for a group of ideas that living organisms on a planet modify the nature of the planet to make it more suitable for life. This set of theories holds that all organisms on a planet regulate the biosphere to the benefit of the whole.
All the life forms of the planet would be part of that planetary being, called Gaia. The atmosphere, the seas, the terrestrial crust would be the result of interventions carried out by Gaia, through the coevolving diversity of living organisms.
While there were a number of precursors to Gaia theory, the first scientific form of this idea was proposed as the Gaia Hypothesis by James Lovelock, a U.K. chemist, in 1970. While controversial at first, various forms of this idea became accepted to some degree by many within the scientific community.
Gaia theory is a spectrum of hypotheses, from undeniable to radical. At one end is the undeniable statement that the organisms on the Earth have radically altered its composition. In this view, the Earth's biosphere effectively acts as if it is a self-organizing system which works in such a way as to keep its systems in some kind of equilibrium that is conducive to life. Biologists usually view this activity as an undirected emergent property of the ecosystem; as each individual species pursues its own self-interest, their combined actions tend to have counterbalancing effects on environmental change. Proponents of this view sometimes point to examples of life's actions in the past that have resulted in dramatic change rather than stable equilibrium, such as the conversion of the Earth's atmosphere from a reducing environment to an oxygen-rich one.
Today a very small number of scientists, and a larger number of environmental activists, see the Earth's ecosystem as a single unified organism, and claims that Earth's biosphere is consciously manipulating the climate in order to make conditions more conducive to life. Scientists contend that there is no evidence at all to support this point of view, and it has come about because many people do not understand the concept of homeostasis. Non-scientists instinctively see homeostatis as an activity that requires conscious control, although this is not so.
Lovelock's Gaia hypothesis
Lovelock's core hypothesis was that the biomass modifies the conditions on the planet to make conditions on the planet more hospitable - the Gaia Hypothesis proper defined this "hospitality" as a full homeostasis.
Lovelock suggested that life on Earth provides a cybernetic, homeostatic feedback system operated automatically and unconsciously by the biota, leading to stabilization of global temperature and chemical composition.
A simple model that is often used to illustrate the original Gaia hypothesis is the so-called Daisyworld simulation.
A central point of Lovelock theory concerns the difference between a planetary environment which might only be an aggregation of living organisms coevolving and sharing the same host, and a planetary environment created by living organisms to accomplish the purpose of the larger being.
Margulis's Gaia Theory
A version of Gaia theory was developed by Lynn Margulis, a microbiologist, in 1979. Her model is more limited in scope than the one that Lovelock proposed. In particular, that only homeorhetic and not homeostatic balances are involved, and that there is no special tendency of biospheres to preserve their current inhabitants, and certainly not to make them comfortable.
A system in homeostasis tends to move towards constant values for its parameters, whereas a system in homeorhesis will always exhibit similar dynamic behavior, without necessarily converging to a constant state. There is strong evidence that plants are selected for the microclimate effects which they can have locally to themselves, and good evidence that these patterns also exist on some wider scales, with symbiotic relationships existing for larger scale climate modification.
Semantic debate
The argument is that these symbiotic organisms, being unable to survive apart from each other and their climate and local conditions, form an organism in their own right, under a wider conception of the term organism than is conventionally used. It is a matter for often heated debate whether this is a valid usage of the term, but ultimately it appears to be a semantic dispute. In this sense of the word organism, it is argued under the theory that the entire biomass of the Earth "is a single organism".
Lack of clarity
Unfortunately, many supporters of the various Gaia theories do not state exactly where they sit on this spectrum; this makes discussion and criticism difficult.
Much effort on behalf of those analyzing the theory currently is an attempt to clarify what these different hypotheses are, and whether they are proposals to 'test' or 'manipulate' outcomes. Both Lovelock's and Margulis's understanding of Gaia are considered valid scientific theories, and are now a part of biology proper.
More speculative versions of Gaia, including all versions in which it is held that the Earth is actually conscious or part of some universe-wide evolution, are currently held to be outside the bounds of science. These are discussed in the Gaia theory article.
See also
keystone species, climate engineering, gardening, urban ecology, industrial ecology, arcology