Life in the biosphere shows an impressive variety of individual shapes and body sizes. From the smallest microorganism (approx. 10-13g) to the largest mammal (>10 g), living things cover more than 21 orders of magnitude of body size. The largest living organisms are actually plants (giant sequoia, Sequoiadendron giganteum (Lindl.) Buchholz), but since most of their bodies are actually dead bark tissues, their living biomass is lower than that of the largest mammals. Given this impressive variability of sizes, consistent body-size patterns, so common at every scale of observation as to be considered universal, can be detected.
The first body-size patterns to be emphasized were that there are many small and few large individuals and species in the biosphere. The range of body sizes from the smallest to the largest individuals may vary substantially, when moving from marine to brackish water to freshwater and terrestrial ecosystems, as well as from tropical to polar ecosystems or from lowlands to highlands, but the pattern of many small and few large individuals still holds. This simple and universal observation was reported by Charles Elton in the first half of the twentieth century in his pivotal book Animal Ecology. This pattern can be explained by means of simple, 'taxon-free', energy-related arguments: since small individuals require less energy per unit of time than large individuals for their maintenance and activity, a fixed productivity will support, at equilibrium, a higher density of small than large individuals. This explanation is actually an oversimplification of the real world; there are at least two other components that need to be taken into account in order to decode the body-size-abundance patterns into a deterministic mechanism of community organization: a phylogenetic and evolutionary component, determining the actual diversity of species and body sizes at continental and global scales; and an interaction component, selecting the body sizes and the species best suited to withstand the locally occurring abiotic conditions and structural habitat architecture (abiotic niche filtering), and determining trophic links and competitive ranking among co-occurring species differing in body size. However, the simple, energy-related, 'taxon-free' explanation emphasizes the ecological relevance of body-size patterns, which are conceptually independent of species-specific resource requirements and species composition.
Body-size patterns include population or species-level patterns, such as the body-size range pattern, and community, landscape, or continental-level patterns. The latter include variations with individual body size in number and biomass of individuals, number of species, population densities, and energy used by populations. The body-size ratio between co-occurring species pairs, known as the Hutchinson ratio, also shows deterministic and consistent patterns of variation within the community-level body-size patterns.
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