It is commonly assumed that metabolic rate scales with body mass to the 3/4 power. have adjusted metabolic rate to body size by using this relationship. including some of the most abundant life-forms on the planet, such. The metabolic theory of ecology (MTE) is an extension of Kleiber's law and posits that the level up to the level of the largest animals and plants on the planet. . The relationship between body size and rate of population growth has been. These types of relationships are often expressed as a power law (i.e. In most animals, metabolic rate scales as M (read: body mass to the ¾ x cells , or about % of earth's total inventory of prokaryotes.
Metabolic theory of ecology - Wikipedia
I have chosen to focus on this comparison for three major reasons. Many pelagic animals show little or no decrease in mass-specific metabolic rate as they grow in body mass, contrary to conventional belief. Furthermore, this distinctive ontogenetic metabolic scaling can be seen at many taxonomic levels, from species to phylum. As such, it is a remarkable case of convergent evolution that demands our attention. Consider that pelagic species in five different phyla with very different body designs show isometric or nearly isometric metabolic scaling.
Furthermore, significant differences in metabolic scaling between pelagic and benthic bottom-dwelling animals can be observed within the four of these phyla for which sufficient data are available figures 2345. This distinction can be seen within lower taxonomic groups as well.
For example, within the crustacean order Amphipoda, the mean metabolic scaling exponent of four pelagic species 0. Pelagic versus benthic differences in metabolic scaling occur even within species. Similar biphasic scaling is exhibited by many other aquatic fish and invertebrates with pelagic larvae reviewed in Glazier Although isometric metabolic scaling appears to be common in pelagic animals, there may be some exceptions.
For example, limited data on appendicularians suggest that their metabolic rate scales allometrically rather than isometrically Lombard et al. In addition, analyses that include the very youngest stages of development may reveal shifts from allometric to isometric scaling, as recently reported for the jellyfish Aurelia aurita Kinoshita et al.
Third, pelagic animals clearly show that metabolic scaling does not simply result from a single set of physical constraints, but can apparently evolve in response to ecological circumstances.
- Metabolic theory of ecology
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It is unlikely that structural constraints alone can account for the similar metabolic scaling of pelagic animals that are so different in body design. Furthermore, the models of West and colleagues, Banavar and colleagues, and others that focus on internal transport networks regardless of whether or not they are fractal cannot explain why pelagic versus benthic animals of similar body design with or without overt transport networks exhibit such strong differences in metabolic scaling figures 2345.
It seems to be an inescapable conclusion that the open-water environment of pelagic animals is somehow favoring the evolution of isometric metabolic scaling directly or indirectlyregardless of body design. Possible explanations of isometric metabolic scaling in pelagic animals The isometric or nearly isometric metabolic scaling of pelagic animals during ontogeny is a robust pattern crying out for explanation.
Although several explanations are possible, two seem to show the most promise see also Glazier These explanations are related to two fundamental problems facing all pelagic animals: To stay afloat, many pelagic animals must continuously swim, an energy-expensive activity.
In addition, the energy costs of swimming have been implicated in the switch from isometric to negatively allometric metabolic scaling that occurs in some fish as they develop. It may be no coincidence that this ontogenetic switch in swimming mode occurs at the same body size at which a phase shift in metabolic scaling also occurs Wuenschel et al.
The relatively large energy costs of swimming may contribute to the isometric metabolic scaling of many other pelagic animals and larvae as well.
However, many pelagic animals are neutrally buoyant or nearly so, thus presumably reducing locomotor costs required for maintaining position in the upper levels of open water. Some water-filled, gelatinous forms, such as many ctenophores and cnidarian medusae, are quite sluggish and spend considerable periods of time drifting.
Among pelagic animals generally, there appears to be a negative association between buoyancy and swimming activity see Glazier Therefore, swimming costs probably cannot account for all cases of isometric scaling in pelagic animals.
Nevertheless, buoyancy maintenance itself entails other energy costs that may contribute to isometric metabolic scaling in some pelagic species e. Since pelagic animals live in open water, they are continually exposed to predation, which has been partially offset in many species by the evolution of transparency Johnsen High mortality of both juveniles and adults may select for rapid growth and maturation rates and large reproductive outputs.
Since pelagic animals are generally short-lived, they must rapidly expend energy to grow and reproduce before they die. High sustained production costs at all ages and sizes may, in turn, result in a mass-specific metabolic rate that does not decrease with increasing body mass i. It is probably no coincidence that pelagic salps, which have among the highest known growth rates in animals, and which continue to grow rapidly throughout their lifetime, have the highest metabolic scaling exponents up to 1.
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Pelagic cnidarians, ctenophores, pteropods, squid, cladocerans, and krill also typically have high rates of sustained production during their relatively short lives, though we still have much to learn about the life histories of many of these animals see Glazier The second explanation may be more generally applicable than the first, though these explanations are not mutually exclusive.
For example, high production costs may require high levels of feeding and associated locomotor activity, which together may contribute to isometric metabolic scaling in many pelagic animals. Future testing of these explanations would benefit from the development of quantitative models.
A particularly promising approach would be to use optimization models relating body mass—dependent metabolic rates to body mass—specific mortality and production rates, as proposed by Kozlowski and Weiner Organism level[ edit ] Small animals tend to grow fast, breed early, and die young. This increased growth rate produces trade-offs that accelerate senescence. For example, metabolic processes produce free radicals as a by-product of energy production. Selection favors organisms which best propagate given these constraints.
As a result, smaller, shorter lived organisms tend to reproduce earlier in their life histories. Population and community level[ edit ] MTE has profound implications for the interpretation of population growth and community diversity. 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.
Observed patterns of diversity can be similarly explained by MTE. It has long been observed that there are more small species than large species. For example, researchers analyzed patterns of diversity of New World coral snakes to see whether the geographical distribution of species fit within the predictions of MTE i.