Gaia theory is the name for a group of hypotheses including some aspects of the ideas that the creatures on a planet modify the nature of the planet, that all creatures on a planet are regulated to the benefit of the whole, and that the entire life biomass and the non-living things which compose the total mass of the earth form a type of regulatory system which in itself would be considered a living being, which is referred to as Gaia.
Whilst some would argue that forms of Gaia theory are embedded in many religions and cultures, it was first proposed as a more formal theory by James Lovelock, a U.K. chemist in the late 1960s or early 1970s (check date) .
He illustrated the theory with Daisy World. Daisy World is a hypothetical world orbiting a sun whose temperature is slowly increasing in the simulation. There are two species on Daisy World, black daisies and white daisies. Black daisies have more tolerance for cold than white daisies. Black daises absorb the sun's heat, while white daises reflect the heat back into space.
At the beginning of the simulation, Daisy World is so cold that only a few black daisies, and almost no white daises, can survive. The black daises absorb a little heat from the sun, which causes the planet's temerature to rise, allowing a greater proliferation of black daisies, more absorption of heat, and so on. As the planet became hotter white daisies begin to breed as well, and eventually the planet reaches a point of temperature equilibrium. Any increase in temperature is combated by a greater proportion of white daises; any decrease leads to more black daises. Eventually the external temperature becomes too hot for the daises to oppose, and heat overwhelms the planet.
When the simulation is run without the daisies, the planet's temperature proceeds in lockstep with that of the sun. With the daisies, at the beginning of the simulation there is enhanced warming, and at the end of the simulation enhanced cooling, resulting in a close to equilibrium temperature for most of the simulation. In this way the daisies are modifying the climate to make conditions more hospitable for themseleves.
Lovelock's theory is then that the biomass modifies the condtions on the planet to make conditions on the planet more hospitible. There is very 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. A simple example is large trees having the effect of reducing wind energy, alowing organisms to live underneath the canopy; there's an inverse relation between the amount of light an organism receives as against the degree to which it must absorb wind energy for the system. (I forget the paper where I read this forest example - I don't think it was Lovelock who said it though. Needs checking)
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 an organism.
Gaia theory is a spectrum of hypotheses, from undeniable to radical. At one end, is the almost undeniable statement that the organisms on the earth have radically altered its composition; the manufacture of oxygen being a key example. Near the middle is the idea that the Earth is a self organizing system which works in such a way as to keep its systems in some kind of equilibrium; a step beyond this is that it seeks an equilibrium conducive to life. At the far radical end, some would hypothesize that this system is consciously manipulating the climate in order to make conditions more conducive to life. Lovelock appears to sit somewhere near the middle.
Unfortunately many supporters of the theory do not state exacly where they sit on this spectrum when putting forward some argument, which can make discussion and criticism difficult. This is probably on account of the theory being a comparatively recent development, and much effort on behalf of those analysing the theory currently is an attempt to clarify what these different hypotheses are.
Gaia theory other than Lovelock