The problems faced by a hypothetical planet with only one species strongly suggest that any functioning ecological system must have organisms from at least two major ecological guilds: autotrophs and decomposers. While conventional predators do not seem to be crucial to planetary ecologies it is likely that parasites will quickly evolve, and through density dependent processes help to regulate population sizes. Density dependence may be crucial in preventing the runaway population growth of a species, leading to it monopolizing a planet's ecology. While density independent processes (be they a cold winter on a local scale, or the impact of a large meteorite at the planetary scale) can greatly affect abundance, they cannot provide regulation; this requires the ‘thermostat’ like behaviour of density dependence. As such, both multiple guilds and the presence of parasites are likely to have positive Gaian effects in most biospheres.