Complexity_ A Guided Tour - Melanie Mitchell [128]
The group also believes that the theory explains metabolic scaling in plants, many of which use fractal-like vascular networks to transport water and other nutrients. They further claim that the theory explains the quarter-power scaling laws for tree trunk circumference, plant growth rates, and several other aspects of animal and plant organisms alike. A more general form of the metabolic scaling theory that includes body temperature was proposed to explain metabolic rates in reptiles and fish.
Moving to the microscopic realm, the group has postulated that their theory applies at the cellular level, asserting that 3/4 power metabolic scaling predicts the metabolic rate of single-celled organisms as well as of metabolic-like, molecule-sized distribution processes inside the cell itself, and even to metabolic-like processes inside components of cells such as mitochondria. The group also proposed that the theory explains the rate of DNA changes in organisms, and thus is highly relevant to both genetics and evolutionary biology. Others have reported that the theory explains the scaling of mass versus growth rate in cancerous tumors.
In the realm of the very large, metabolic scaling theory and its extensions have been applied to entire ecosystems. Brown, Enquist, and West believe that their theory explains the observed −3/4 scaling of species population density with body size in certain ecosystems.
In fact, because metabolism is so central to all aspects of life, it’s hard to find an area of biology that this theory doesn’t touch on. As you can imagine, this has got many scientists very excited and looking for new places to apply the theory. Metabolic scaling theory has been said to have “the potential to unify all of biology” and to be “as potentially important to biology as Newton’s contributions are to physics.” In one of their papers, the group themselves commented, “We see the prospects for the emergence of a general theory of metabolism that will play a role in biology similar to the theory of genetics.”
Controversy
As to be expected for a relatively new, high-profile theory that claims to explain so much, while some scientists are bursting with enthusiasm for metabolic scaling theory, others are roiling with criticism. Here are the two main criticisms that are currently being published in some of the top scientific journals:
Quarter-power scaling laws are not as universal as the theory claims. As a rule, given any proposed general property of living systems, biology exhibits exceptions to the rule. (And maybe even exceptions to this rule itself.) Metabolic scaling theory is no exception, so to speak. Although most biologists agree that a large number of species seem to follow the various quarter-power scaling laws, there are also many exceptions, and sometimes there is considerable variation in metabolic rate even within a single species. One familiar example is dogs, in which smaller breeds tend to live at least as long as larger breeds. It has been argued that, while Kleiber’s law represents a statistical average, the variations from this average can be quite large, and metabolic theory does not explain this because it takes into account only body mass and temperature. Others have argued that there are laws predicted by the theory that real-world data strongly contradict. Still others argue that Kleiber was wrong all along, and the best fit to the data is actually a power law with exponent 2/3, as proposed over one hundred years ago by Rubner in his surface hypothesis. In most cases, this is an argument about how to correctly interpret data on metabolic scaling and about what constitutes a “fit” to the theory. The metabolic scaling group stands by its theory, and has diligently replied to many of these arguments, which become increasingly technical and obscure as the authors discuss the intricacies of advanced statistics and biological functions.