Metabolic theory of ecology
The metabolic theory of ecology (MTE)[1] is the ecological component of the more general Metabolic Scaling Theory[2] and Kleiber's law. It posits that the metabolic rate of organisms is the fundamental biological rate that governs most observed patterns in ecology. MTE is part of a larger set of theory known as metabolic scaling theory that attempts to provide a unified theory for the importance of metabolism in driving pattern and process in biology from the level of cells all the way to the biosphere.[2][3][4]
MTE is based on an interpretation of the relationships between body size, body temperature, and metabolic rate across all organisms. Small-bodied organisms tend to have higher mass-specific metabolic rates than larger-bodied organisms. Furthermore, organisms that operate at warm temperatures through endothermy or by living in warm environments tend towards higher metabolic rates than organisms that operate at colder temperatures. This pattern is consistent from the unicellular level up to the level of the largest animals and plants on the planet.
In MTE, this relationship is considered to be the primary constraint that influences biological processes (via their rates and times) at all levels of organization (from individual up to ecosystem level). MTE is a macroecological theory that aims to be universal in scope and application.[1][5]
Fundamental concepts in MTE
Metabolism
Metabolic pathways consist of complex networks, which are responsible for the processing of both energy and material. The metabolic rate of a heterotroph is defined as the rate of respiration in which energy is obtained by oxidation of a carbon compound. The rate of photosynthesis on the other hand, indicates the metabolic rate of an autotroph.[6] According to MTE, both body size and temperature affect the metabolic rate of an organism. Metabolic rate scales as 3/4 power of body size, and its relationship with temperature is described by the Van’t Hoff-Arrhenius equation over the range of 0 to 40 °C.[7]
Stoichiometry
From the ecological perspective, stoichiometry is concerned with the proportion of elements in both living organisms and their environment.[8] In order to survive and maintain metabolism, an organism must be able to obtain crucial elements and excrete waste products. As a result, the elemental composition of an organism would be different from the exterior environment.[9] Through metabolism, body size can affect stoichiometry. For example, small organism tend to store most of their phosphorus in rRNA due to their high metabolic rate,[10][11][12] whereas large organisms mostly invest this element inside the skeletal structure. Thus, concentration of elements to some extent can limit the rate of biological processes. Inside an ecosystem, the rate of flux and turn over of elements by inhabitants, combined with the influence of abiotic factors, determine the concentration of elements.[1]
Theoretical background
Metabolic rate scales with the mass of an organism of a given species according to Kleiber's law where B is whole organism metabolic rate (in watts or other unit of power), M is organism mass (in kg), and Bo is a mass-independent normalization constant (given in a unit of power divided by a unit of mass. In this case, watts per kilogram):
At increased temperatures, chemical reactions proceed faster. This relationship is described by the
While Bo in the previous equation is mass-independent, it is not explicitly independent of temperature. To explain the relationship between body mass and temperature, building on earlier work [13] showing that the effects of both body mass and temperature could be combined multiplicatively in a single equation, the two equations above can be combined to produce the primary equation of the MTE, where bo is a normalization constant that is independent of body size or temperature:
According to this relationship, metabolic rate is a function of an organism's body mass and body temperature. By this equation, large organisms have higher metabolic rates (in watts) than small organisms, and organisms at high body temperatures have higher metabolic rates than those that exist at low body temperatures. However, specific metabolic rate (SMR, in watts/kg) is given by
Hence SMR for large organisms are lower than small organisms.
Past debate over mechanisms and the allometric exponent
Researchers have debated two main aspects of this theory, the pattern and the mechanism. Past debated have focused on the question whether metabolic rate scales to the power of 3⁄4 or 2⁄3w, or whether either of these can even be considered a universal exponent.
Much of past debate have focused on two particular types of mechanisms.
In contrast, the arguments for a 3⁄4
Despite past debates over the value of the exponent, the implications of metabolic scaling theory and the extensions of the theory to ecology (metabolic theory of ecology) the theory might remain true regardless of its precise numerical value.
Implications of the theory
The metabolic theory of ecology's main implication is that
Organism level
Small animals tend to grow fast, breed early, and die young.
Population and community level
MTE has profound implications for the interpretation of population growth and community diversity.[27] Classically, species are thought of as being either r selected (where populations tend to grow exponentially, and are ultimately limited by extrinsic factors) or K selected (where population size is limited by density-dependence and carrying capacity). MTE explains this diversity of reproductive strategies as a consequence of the metabolic constraints of organisms. Small organisms and organisms that exist at high body temperatures tend to be r selected, which fits with the prediction that r selection is a consequence of metabolic rate.[1] Conversely, larger and cooler bodied animals tend to be K selected. The relationship between body size and rate of population growth has been demonstrated empirically,[30] and in fact has been shown to scale to M−1/4 across taxonomic groups.[27] The optimal population growth rate for a species is therefore thought to be determined by the allometric constraints outlined by the MTE, rather than strictly as a life history trait that is selected for based on environmental conditions.
Regarding density, MTE predicts carrying capacity of populations to scale as M-3/4, and to exponentially decrease with increasing temperature. The fact that larger organisms reach carrying capacity sooner than smaller one is intuitive, however, temperature can also decrease carrying capacity due to the fact that in warmer environments, higher metabolic rate of organisms demands a higher rate of supply.[31] Empirical evidence in terrestrial plants, also suggests that density scales as -3/4 power of the body size.[32]
Observed patterns of diversity can be similarly explained by MTE. It has long been observed that there are more small species than large species.
MTE's ability to explain patterns of diversity remains controversial. For example, researchers analyzed patterns of diversity of New World
Ecosystem processes
At the ecosystem level, MTE explains the relationship between temperature and production of total biomass.[38] The average production to biomass ratio of organisms is higher in small organisms than large ones.[39] This relationship is further regulated by temperature, and the rate of production increases with temperature.[40] As production consistently scales with body mass, MTE provides a framework to assess the relative importance of organismal size, temperature, functional traits, soil and climate on variation in rates of production within and across ecosystems.[38] Metabolic theory shows that variation in ecosystem production is characterized by a common scaling relationship, suggesting that global change models can incorporate the mechanisms governing this relationship to improve predictions of future ecosystem function.
See also
- Allometry
- Constructal theory
- Dynamic energy budget theory
- Ecology
- Evolutionary physiology
- Occupancy-abundance relationship
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