Metabolic theory of ecology
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The metabolic theory of ecology (MTE) is the ecological component of the more general Metabolic Scaling Theory and
Kleiber's law Kleiber's law, named after Max Kleiber for his biology work in the early 1930s, is the observation that, for the vast majority of animals, an animal's metabolic rate scales to the power of the animal's mass. Symbolically: if is the animal's metab ...
. It posits that the
metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
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. MTE is based on an interpretation of the relationships between body size, body temperature, and
metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
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 An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
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.


Fundamental concepts in MTE


Metabolism

Metabolic pathways consist of complex networks, which are responsible for processing of both energy and material. The metabolic rate of a
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
is defined as the rate of respiration in which energy is obtained by oxidation of carbon compound. The rate of
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
on the other hand, indicates the metabolic rate of an
autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...
. According to MTE, both body size and temperature affect the metabolic rate of an organism. Metabolic rate scale as 3/4 power of body size, and its relationship with temperature is described by Van’t Hoff-Arrhenius equation over the range of 0 to 40 °C.


Stoichiometry

From the ecological perspective, stoichiometry is concerned with the proportion of elements in both living organisms and their environment. 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. 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, 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.


Theoretical background

Metabolic rate scales with the mass of an organism of a given species according to
Kleiber's law Kleiber's law, named after Max Kleiber for his biology work in the early 1930s, is the observation that, for the vast majority of animals, an animal's metabolic rate scales to the power of the animal's mass. Symbolically: if is the animal's metab ...
where ''B'' is whole organism metabolic rate (in watts or other unit of power), ''M'' is organism mass (in kg), and ''B''o is a mass-independent normalization constant (given in a unit of power divided by a unit of mass. In this case, watts per kilogram): :B = B_oM ^ \, At increased temperatures, chemical reactions proceed faster. This relationship is described by the
Boltzmann factor Factor, a Latin word meaning "who/which acts", may refer to: Commerce * Factor (agent), a person who acts for, notably a mercantile and colonial agent * Factor (Scotland), a person or firm managing a Scottish estate * Factors of production, su ...
, where ''E'' is
activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
in
electronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defi ...
s or
joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...
s, ''T'' is absolute temperature in kelvins, and ''k'' is the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
in eV/K or J/K: :e^ While ''B''o 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 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 ''b''o is a normalization constant that is independent of body size or temperature: :B = b_oM^e^ 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 SMR = (B/M) = b_oM^e^ 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 or w, or whether either of these can even be considered a universal exponent. In addition to debates concerning the exponent, some researchers also disagree about the underlying mechanisms generating the scaling exponent. Various authors have proposed at least eight different types of mechanisms that predict an
allometric scaling Allometry is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in ''On Growth and Form'' and by Julian Huxley in 1932. Overview Allo ...
exponent of either or . The majority view is that while the exponent is indeed the mean observed exponent within and across taxa, there is intra- and interspecific variability in the exponent that can include shallower exponents such as. Past debates on the exact value of the exponent are settled in part because the observed variability in the metabolic scaling exponent is consistent with a 'relaxed' version of metabolic scaling theory where additional selective pressures lead to a constrained set of variation around the predicted optimal exponent. Much of past debate have focused on two particular types of mechanisms. One of these assumes energy or resource transport across the ''external'' surface area of three-dimensional organisms is the key factor driving the relationship between metabolic rate and body size. The surface area in question may be skin, lungs, intestines, or, in the case of unicellular organisms, cell membranes. In general, the surface area (SA) of a three dimensional object scales with its volume (V) as ''SA = cV'', where c is a proportionality constant. The
Dynamic Energy Budget The dynamic energy budget (DEB) theory is a formal metabolic theory which provides a single quantitative framework to dynamically describe the aspects of metabolism (energy and mass budgets) of all living organisms at the individual level, based o ...
model predicts exponents that vary between – 1, depending on the organism's developmental stage, basic body plan and resource density. DEB is an alternative to metabolic scaling theory, developed before the MTE. DEB also provides a basis for population, community and ecosystem level processes to be studied based on energetics of the constituent organisms. In this theory, the biomass of the organism is separated into structure (what is built during growth) and reserve (a pool of polymers generated by assimilation). DEB is based on the first principles dictated by the kinetics and thermodynamics of energy and material fluxes, has a similar number of parameters per process as MTE, and the parameters have been estimated for over 3000 animal species While some of these alternative models make several testable predictions, others are less comprehensive and of these proposed models only DEB can make as many predictions with a minimal set of assumptions as metabolic scaling theory. In contrast, the arguments for a
scaling factor In affine geometry, uniform scaling (or isotropic scaling) is a linear transformation that enlarges (increases) or shrinks (diminishes) objects by a ''scale factor'' that is the same in all directions. The result of uniform scaling is similarit ...
are based on resource transport network models, where the
limiting resource A limiting factor is a variable of a system that causes a noticeable change in output or another measure of a type of system. The limiting factor is in a pyramid shape of organisms going up from the producers to consumers and so on. A factor not l ...
s are distributed via some optimized network to all resource consuming cells or
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s. These models are based on the assumption that metabolism is proportional to the rate at which an organism's distribution networks (such as circulatory systems in animals or xylem and phloem in plants) deliver nutrients and energy to body tissues. Larger organisms are necessarily less efficient because more resource is in transport at any one time than in smaller organisms: size of the organism and length of the network imposes an inefficiency due to size. It therefore takes somewhat longer for large organisms to distribute nutrients throughout the body and thus they have a slower mass-specific metabolic rate. An organism that is twice as large cannot metabolize twice the energy—it simply has to run more slowly because more energy and resources are wasted being in transport, rather than being processed. Nonetheless, natural selection appears to have minimized this inefficiency by favoring resource transport networks that maximize rate of delivery of resources to the end points such as cells and organelles. This selection to maximize metabolic rate and energy dissipation results in the allometric exponent that tends to ''D''/(''D+1''), where ''D'' is the primary dimension of the system. A three dimensional system, such as an individual, tends to scale to the 3/4 power, whereas a two dimensional network, such as a river network in a landscape, tends to scale to the 2/3 power. 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
metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
, and the influence of body size and
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
on metabolic rate, provide the fundamental constraints by which ecological processes are governed. If this holds true from the level of the individual up to ecosystem level processes, then life history attributes,
population dynamics Population dynamics is the type of mathematics used to model and study the size and age composition of populations as dynamical systems. History Population dynamics has traditionally been the dominant branch of mathematical biology, which has ...
, and ecosystem processes could be explained by the relationship between metabolic rate, body size, and body temperature. While different underlying mechanisms make somewhat different predictions, the following provides an example of some of the implications of the metabolism of individuals.


Organism level

Small animals tend to grow fast, breed early, and die young. According to MTE, these patterns in life history traits are constrained by metabolism. An organism's metabolic rate determines its rate of food consumption, which in turn determines its rate of growth. This increased growth rate produces trade-offs that accelerate
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
. For example, metabolic processes produce
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...
as a by-product of energy production. These in turn cause damage at the cellular level, which promotes
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
and ultimately death. Selection favors organisms which best propagate given these constraints. As a result, smaller, shorter lived organisms tend to
reproduce Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...
earlier in their life histories.


Population and community level

MTE has profound implications for the interpretation of
population growth Population growth is the increase in the number of people in a population or dispersed group. Actual global human population growth amounts to around 83 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to ...
and
community diversity A community is a social unit (a group of living things) with commonality such as place, norms, religion, values, customs, or identity. Communities may share a sense of place situated in a given geographical area (e.g. a country, village, town ...
. 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 The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
). 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. 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, and in fact has been shown to scale to ''M''−1/4 across taxonomic groups. 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. Empirical evidence in terrestrial plants, also suggests that density scales as -3/4 power of the body size. Observed patterns of diversity can be similarly explained by MTE. It has long been observed that there are more small species than large species. In addition, there are more species in the tropics than at higher latitudes. Classically, the latitudinal gradient in species diversity has been explained by factors such as higher productivity or reduced seasonality. In contrast, MTE explains this pattern as being driven by the kinetic constraints imposed by temperature on metabolism. The rate of
molecular evolution Molecular evolution is the process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics ...
scales with metabolic rate, such that organisms with higher metabolic rates show a higher rate of change at the molecular level. If a higher rate of molecular evolution causes increased
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
rates, then adaptation and ultimately speciation may occur more quickly in warm environments and in small bodied species, ultimately explaining observed patterns of diversity across body size and latitude. MTE's ability to explain patterns of diversity remains controversial. For example, researchers analyzed patterns of diversity of New World
coral snake Coral snakes are a large group of elapid snakes that can be divided into two distinct groups, the Old World coral snakes and New World coral snakes. There are 16 species of Old World coral snakes, in three genera (''Calliophis'', '' Hemibungar ...
s to see whether the geographical distribution of species fit within the predictions of MTE (i.e. more species in warmer areas). They found that the observed pattern of diversity could not be explained by temperature alone, and that other spatial factors such as primary productivity, topographic heterogeneity, and habitat factors better predicted the observed pattern. Extensions of metabolic theory to diversity that include eco-evolutionary theory show that an elaborated metabolic theory can account for differences in diversity gradients by including feedbacks between ecological interactions (size-dependent competition and predation) and evolutionary rates (speciation and extinction)


Ecosystem processes

At the ecosystem level, MTE explains the relationship between temperature and production of total
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
. The average production to biomass ratio of organisms is higher in small organisms than large ones. This relationship is further regulated by temperature, and the rate of production increases with temperature. 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. 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 Allometry is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in ''On Growth and Form'' and by Julian Huxley in 1932. Overview Allom ...
*
Constructal theory Adrian Bejan is an American professor who has made contributions to modern thermodynamics and developed his constructal law. He is J. A. Jones Distinguished Professor of Mechanical Engineering at Duke University and author of the books Design ...
*
Dynamic energy budget theory The dynamic energy budget (DEB) theory is a formal metabolic theory which provides a single quantitative framework to dynamically describe the aspects of metabolism (energy and mass budgets) of all living organisms at the individual level, based o ...
*
Ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
*
Evolutionary physiology Evolutionary physiology is the study of the biological evolution of physiological structures and processes; that is, the manner in which the functional characteristics of individuals in a population of organisms have responded to natural selectio ...
* Occupancy-abundance relationship


References

{{DEFAULTSORT:Metabolic Theory Of Ecology Ecological theories Theoretical ecology