When we think of biology, we normally think of individual organisms and how they work, or perhaps molecular biology and genetics. We rarely go up a scale to ecology - the study of the distribution and interactions of many organisms, and few people at all think of biology on a planetary scale, and how the interactions of the trillions or quadrillions of organisms on Earth build up.

Those few people, however, did include atmospheric chemist James Lovelock and evolutionary biology Lynn Margulis. Together, these two scientists conceived the Gaia Hypothesis, that the interactions of not only all the organisms on Earth but also all of the environments of Earth put together makes the Earth 'alive'.

This bold claim is explained by saying that these interactions can control the average temperature of the surface, the salinity of the oceans, and the carbon dioxide content of the atmosphere and keep them constant via a intricate network of negative feedback cycles.

Most scientists have rejected what is now called the 'strong' Gaia Hypothesis, that the Earth is in actuality an organism that, in its regulation of the atmosphere and all other elements of the environment, is undergoing homeostasis (in biological terms, homeostasis is the regulation of a constant internal state of an organism). This has unfortunate religious connotations and many people do not like to feel as if there is some kind of 'purpose' behind Gaia ('Gaia' is used to represent the Earth in totality, including all of its organisms).

The moderate Gaia hypothesis is that the Earth behaves as if it were a living organism, but it isn't actually one.

Most favored is the weak Gaia Hypothesis, which says that all of this 'homeostasis' where the temperature is kept constant and so on is merely a simple processes that is governed by negative feedback - negative feedback essentially means that when something gets too large or too high (like temperature) it is automatically regulated down by another process that 'kicks in' due to the high temperature.

An effective way to envision the Gaia Hypothesis is through the famous Daisyworld model. In this model, an imaginary world completely covered with the seeds of two types of flowers, white and black. At first, this world's sun is too cold to allow the seeds to germinate but gradually it warms up and all the seeds germinate, sprouting flowers (Figure 1). The black flowers are more successful than their white counterparts because they are better at absorbing light from the sun (black absorbs more light than white). Consequently, the black flowers reproduce in greater numbers while the number of white flowers decrease (Figure 2). (You can play around with this Flash animation of Daisyworld or watch this video.)

However, as these black flowers grow larger and larger in number, they begin to absorb more and more heat from the sun, and this has the resultant effect of increasing the planet's temperature. With an increased temperature, the white flowers begin to outcompete the black flowers since their white petals are better at reflecting light than the black petals, and so they can avoid overheating.

So it goes in the opposite direction - the white flowers proliferate and the black flowers decrease in number (Figure 3). Eventually, the planet's temperature cools down since all the white flowers are reflecting more and more heat from the sun and not absorbing it. Things get cold, and the black flowers again have the upper hand because they can absorb more light than the white flowers and so have more offspring.

While Daisyworld is criticized for oversimplification, it does serve to demonstrate an important principle--that both the abiotic components (non-living materials like rocks and atmosphere) and biotic components (living organisms) of an ecosystem regulate each other. As white flowers increase, they can lower the temperature; the lowered temperature in turn causes the white flowers to die off and black flowers to increase, increasing the temperature.

Although the real world may not be quite this simple, the concept remains essentially the same. The amount of carbon dioxide and oxygen in the atmosphere is, broadly speaking, regulated by plants, animals, and outgassings from volcanoes so that they stay at a constant level.