The Gaia HypothesisStu Crawford
The biological and physical elements of this planet exert a substantial influence on each other. Global self-regulation is a natural consequence of these interactions between the biota and the physical world. There are negative feedback loops so that the biota of the earth can stabilize its environment within an optimal range. And the biota have efficient recycling pathways for chemicals that drastically increase the availability of critical nutrients. It seems that life on earth maintains its physiochemical environment in a manner that benefits life.
This self-regulating system consists of all life on earth and the abiotic factors that this life affects. It has been called the Gaia hypothesis. Gaia is compared to a living organism because it is self-regulating and optimizes conditions for itself. It has homeostatic mechanisms that maintain its internal conditions within the optimal range for life. Several questions arise from the Gaia hypothesis. Does the biota of earth keep its environment within an optimal range? How can these homeostatic mechanisms exist? Is Gaia a living organism? And lastly, what practical use can the Gaia hypothesis have to humans?
The earth's physiochemical environment is being regulated. The temperature of the sun has increased 25% over the last 3.8 billion years while the surface temperature of the earth has remained relatively constant (Lenton, 1998). And the chemical composition of earth's atmosphere is in violation of equilibrium chemistry by more than 30 orders of magnitude (Lovelock, 1987). This lack of entropy shows some sort of active regulation. And there is evidence that this regulation is at least partially biological in origin.
Negative feedback loops are essential for homeostatic maintenance of a system. Several biologically driven negative feed back loops have been identified. There may be short term regulation of local climate by dimethylsulphide producing algae (Lovelock and Maggs, 1972; Charlson et al, 1987). Increased temperature increases algal growth which increases DMS being released into the atmosphere. This increases cloud formation and increases atmospheric albedo which decreases temperature.(1)
Marine microorganisms may be keeping the oceans at a constant salinity and keeping nitrogen-phosphorus concentrations at the Redfield ratio of 6.7 -- these are conditions which are ideal for life (Downing and Zvirinsky,1999). There seems to be a global regulation of climate by photosynthetic organisms -- atmospheric CO2 concentrations would rise to 1% without this biological sink for carbon dioxide (Charlson et al, 1987). Life seems to maintain atmospheric O2 concentrations just below the level were fires would disrupt land life (Lenton, 1998). Various other biological mechanisms may help to control atmospheric, soil, and ocean chemical composition and pH (Lovelock, 1987).
Life is also important for maintaining nutrient cycling. It provides efficient recycling pathways for poorly supplied nutrients. Biota annually consume 200 times more carbon, 500 - 1300 times more nitrogen, and 200 times more phosphorus than is supplied by external fluxes (Downing and Zvirinsky,1999). Without this biological amplification there would be less than one percent of the biomass that is present now(Downing and Zvirinsky,1999).
Providing a mechanism for Gaia's homeostatic mechanisms via natural selection acting on Gaia's component parts (individual organisms) is difficult. Gillan (2000) suggests that population self-regulation is an emergent property of individuals maximizing fitness. Lovelock (1987) suggests that selection pressure can act on populations or ecosystems. However, models of group selection are very unsound. Selfish morphs inevitably evolve and break the system. And because there is only one Gaia natural selection cannot act on it.
The Daisyworld model(2) attempts to resolve this by showing how environmental regulation can arise out of natural selection acting on only two species. Populations of daisies stabilize the planet's temperature as the sun warms up. But there are problems with Daisyworld -- it is set up in an oversimplified manner to make self-regulation inevitable. The addition of interspecific competition in Daisyworld reduces stability of the system significantly (Cohen and Rich, 2000). And if the model is expanded to account for the daisies evolving the temperature regulation disappears completely (Robertson and Robinson, 1998).
A better model was recently developed by Downing and Zvirinsky (1999)(3). In this model biochemical guilds of different groups of bacteria evolved to utilize different nutrient niches. Then the ratio of chemicals in the environment stabilized at whatever the optimal ratio for growth was set to be. Competition between bacteria created diversification, increased efficiency of nutrient use, and increased the biomass that could be supported. High transfer fluxes of nutrients between bacteria reduced the communities sensitivity to outside change. Competition resulted in frequency dependent selection which maintained the global nutrient balance.
Stability in the Downing and Zvirinsky model arises from the diversity of interactions. Similar theoretical results were shown in food webs by McCann et al (1998). Interactions in food webs are dominated by many weak and a few strong interactions. The weak links dampen oscillations and increase stability. The presence of a large number of interactions might help to explain why negative feedback loops (stabilizing) seem to evolve instead of positive feedback loops (destabilizing). If the feedback is destabilizing it will increase in size, affecting more and more things. If there are enough interactions one of them will be negative and a stabilizing feedback will result and increase until a new equilibrium is reached (Kump, 1996). This implies that there is a possibility of brief periods of positive feedback. These have been seen in the past: the snowball earth of 0.7 and 2.2 billion years ago (Lenton, 1998) and CO2 starvation from the plant explosion 400 million years ago.
It seems that Gaia has homeostatic mechanisms that evolved from natural selection acting on individual organisms, and that these mechanisms maintain the physiochemical environment in conditions optimal for life. Does this mean Gaia is a living entity? The best definition of life presented so far is that life is something which evolves by natural selection (Maynard-Smith and Szathmary, 1998). Natural selection does not act directly on an organism, it acts on the organism's germ line replicators(4). It is only because of the emergent properties of the group of germ line replicators that make up the organism that natural selection ends making organisms evolve to optimize their fitness. The situation is similar with Gaia. Natural selection is acting on Gaia's component parts (organisms) and the emergent properties of these components end up making Gaia evolve to maximize Gaia's fitness. There is a slight different in the exact mechanism, Gaia does not evolve by less fit Gaias dying and thus killing their inferior components (this is how organisms evolve), but rather by the mechanisms outlined above. But Gaia does seem to fit the definition of life close enough to make comparisons between Gaia and an organism legitimate.
The holistic approach of viewing Gaia as one entity and comparing Gaia to a living organism seems to be legitimate and it has a practical use. Humans have an inherent inability to comprehend the complicated interactions between organisms on this planet and the emergent properties of complex living systems when considering the effects of their actions. This is evidenced globally by the lack of any real environmental concern in governments and industry. The effects of human manipulations of specific conditions may become more readily apparent to decision-makers if they view the planet as Gaia rather than considering it in a more reductionist light. Widespread acceptance of the Gaia hypothesis may be our only hope to reduce human impact on the environment.
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1. There may be evidence against algal-mediated DMS regulation of temperature. DMS concentration in sea water was found to remain constant with El Nino-induced swings in temperature and cloud cover in the Pacific (Holden, 1997).
2. A grey planet has only two species on it: black daisies and white daisies. Both species have the same optimal temperature range in which they can survive. The planet starts out to cold but the sun warms up until some daisies can sprout. At first the black daisies do better because they heat up more in the sun. The black daisies eventually cover the entire planet. They lower the albedo of the planet which makes it heat up faster. As the planet gets too hot the white daisies do better because they don't get as hot. They increase in number and cool the planet. This negative feedback look stabilizes the planet's temperature as the sun heats up until it just gets too hot and then everything dies. But the presence of the daisies caused the planet to stay within the optimal range for growth for much longer than a dead planet.
3. This model is a genetic algorithm modelling bacteria living in an environment with n number of nutrients. The bacteria take in one nutrient as food and give off a different nutrient as waste. Which nutrients each bacteria takes in and gives off is dependent on its genes and thus subject to natural selection. An initial population of bacteria are then left to evolve.
4. A germ line replicator is a little piece of DNA or RNA that gets replicated over time and is subject to evolution by natural selection (Dawkins, 1990). This is the basic unit of life.