By reflecting the heat away from the planet, the white daisies cool the planetary atmosphere. Over a long period of time on Daisyworld, the temperature of the planet is automatically regulated by the fluctuations in population of the black and white daisies, competing according to the laws of classical Darwinian natural selection. Critics point to the over-simplicity of the Daisyworld model, which they say can't tell us much about the vastly more complex real world. To answer these criticisms, it's worth taking a look at the way carbon dioxide concentrations in our atmosphere are regulated, and at the way clouds are formed. The only natural way in which it is taken out of the atmosphere in any significant quantity is through the weathering of certain kinds of rock.
Water reacts with the rocks in the presence of carbon dioxide.
In the reaction, carbon dioxide is taken out of the atmosphere and locked up with calcium to form calcium carbonate, which is eventually washed into the sea. Life isn't needed for the reaction to happen, but bacteria and plant roots in soils greatly increase the rate of the process by actively pumping carbon dioxide out of the atmosphere and into the soil, where much more of the gas comes into contact with rock and water than would be the case without the intervention of living organisms. These rain down to the bottom of the sea when their tiny occupants die, thereby safely locking away the carbon dioxide as massive deposits of chalk and limestone. The presence of these tiny living creatures means that the chemicals which made up the atmospheric carbon dioxide are 'processed' by a living being and turn back into rock. Coccolithophores also play a part in the formation of clouds.