The Metabolic Theory of Ecology (MTE) is a novel approach to understanding the distribution of species at community and ecosystem levels. The theoretical models produced from MTE consider the biochemical and physiological constraints, like nitrogen availability or body size. The primary concept of the MTE is the biological process of metabolism. Metabolism is the rate by which resources are acquired by an individual and then converted into energy. This energy is then allocated to growth, reproduction, or maintenance of the individual (Enquist et al, 1999). The waste or unusable materials are then excreted back into the environment to the potential use of another individual. It is commonly understood that growth rate is limited by water or nitrogen (Brown, 2004).
The MTE demonstrates that due to constraints on metabolism from physical and chemical laws scaling of body size can be observed across species to ecosystem levels of organizaton. The allometric equation,
serves as the backbone of metabolic theory. The natural log-transformed dependent variable is represented by Y. This variable can represent a variety of biological processes including metabolic rate or photosynthesis. Y0 is a normalization or scaling constant that varies with the relationship of the processes being compared. This variable is generally omitted and the = is replaced by a ∝to indicate a proportional relationship. However, the normalization constants are important as they differ between biological processes and may serve to increase the power and predictability of those relationships. M represents the body mass of individuals and b is the allometric exponent that indicates the rate at which scaling occurs (Brown, 2004).
The unifying power of MTE lies in its recognition of the first principles of biology, chemistry, and physics. Biological body size varies by 21 orders of magnitude and vascular plants vary in size by 12 orders of magnitude (West et al., 1999). These large-scale differences are impacted by a number of factors that can differ based on the size of an individual or its extent of distribution. However, first principles recognize the physiological constraints on organisms at all sizes. These constraints include body size (mass), operating temperatures (kinetics), and chemical compositions (energy). Other recognized constraints are derived from biogeochemical cycles and transformations, stoichiometric balances, and fluid dynamics. So not only does body size constrain biological functions, but it is also constrained by more simple factors such as temperature or fluid dynamics. Models using the MTE recognize these constraints and can predict body structure or size depending on the conditions.
Models using the MTE were able to predict that woody plants would reach a maximum height of 100m. The vertical architecture of these plants is constrained by the pressure that is required in order to maintain transpiration rates which transport essential materials to the tips of branches (West et al., 1999). Prior to this prediction, a few assumptions were explored to maximize accuracy of the model. First, it was assumed that branching of tubes would occur in a fractal-like manner that would maximize space filling and pressure maintenance, while minimizing the dissipation of energy. The model also assumes that the terminal tubes do not vary with body size. In other words, there is a minimal size that can optimally maintain fluid transfer, which will limit all individuals in their growth. Due to comparable conditions in similar environments, all plants should have a similar xylem flow. Competition for light then maximizes height of the individual and minimizes tapering. Therefore, if all species experienced a similar need for maintenance or had a similar reproductive strategy, all species would tend to be the same size. This is obviously not the case because of trade-offs between energy allocation to growth, reproduction, and maintenance, but it is an interesting idea to consider in terms of constraints from first principles.
Brown, J.H. et al. 2004. Toward a Metabolic Theory of Ecology. Ecology. 85(7): 1771-1779.
Enquist, B.J. et al. 1999. Allometric scaling of production and life-history variation in vascular plants. Nature. 401: 907-911.
West, G.B. et al. 1999. A general model for the structure and allometry of plant vascular systems. Nature. 400: 664-667.