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Size-asymmetric competition refers to situations in which larger individuals exploit disproportionately greater amounts of
resources Resource refers to all the materials available in our environment which are technologically accessible, economically feasible and culturally sustainable and help us to satisfy our needs and wants. Resources can broadly be classified upon their av ...
when
competing Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
with smaller individuals.Schwinning, S. & Weiner, J. Mechanisms determining the degree of size asymmetry in competition among plants. Oecologia 113 doi:10.1007/s004420050397 (1998). This type of competition is common among
plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
Weiner, J. Asymmetric competition in plant-populations. Trends in Ecology & Evolution 5, 360-364, doi:10.1016/0169-5347(90)90095-u (1990) but also exists among
animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
. Size-asymmetric competition usually results from large individuals monopolizing the resource by "pre-emption". i.e. exploiting the resource before smaller individuals are able to obtain it. Size-asymmetric competition has major effects on
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction usi ...
structure and diversity within
ecological communities In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community ...
.Rajaniemi, T. K. Explaining productivity-diversity relationships in plants. Oikos 101, 449-457, doi:10.1034/j.1600-0706.2003.12128.x (2003)Lamb, E. G., Kembel, S. W. & Cahill, J. F., Jr. Shoot, but not root, competition reduces community diversity in experimental mesocosms. Journal of Ecology 97, 155-163, doi:10.1111/j.1365-2745.2008.01454.x (2009)DeMalach, N., Zaady, E., Weiner, J. & Kadmon, R. Size asymmetry of resource competition and the structure of plant communities. Journal of Ecology 104, 899-910, doi:10.1111/1365-2745.12557 (2016)May, F., Grimm, V. & Jeltsch, F. Reversed effects of grazing on plant diversity: the role of below-ground competition and size symmetry. Oikos 118, 1830-1843, doi:10.1111/j.1600-0706.2009.17724.x (2009)


Definition of size asymmetry

Resource competition can vary from complete symmetric (all individuals receive the same amount of resources, irrespective of their size, known also as
scramble competition In ecology, scramble competition (or complete symmetric competition or exploitation competition) refers to a situation in which a resource is accessible to all competitors (that is, it is not monopolizable by an individual or group). However, sin ...
) to perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass) to absolutely size asymmetric (the largest individuals exploit all the available resource). The degree of size asymmetry can be described by the parameter θ in the following equation focusing on the partition of the resource r among n individuals of sizes Bj.Schwinning, S. & Fox, G. A. Population-dynamic consequences of competitive symmetry in annual plants. Oikos 72, 422-432, doi:10.2307/3546128 (1995) ri refers to the amount of resource consumed by individual i in the neighbourhood of j. When θ =1, competition is perfectly size symmetric, e.g. if a large individual is twice the size of its smaller competitor, the large individual will acquire twice the amount of that resource (i.e. both individuals will exploit the same amount of resource per biomass unit). When θ >1 competition is size-asymmetric, e.g. if large individual is twice the size of its smaller competitor and θ =2, the large individual will acquire four times the amount of that resource (i.e. the large individual will exploit twice the amount of resource per biomass unit). As θ increases, competition becomes more size-asymmetric and larger plants get larger amounts of resource per unit biomass compared with smaller plants.


Differences in size-asymmetry among resources in plant communities

Competition among plants for
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
is size-asymmetric because of the directionality of its supply. Higher
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
shade lower leaves but not vice versa. Competition for
nutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...
appears to be relatively size-symmetric, although it has been hypothesized that a patchy distribution of nutrients in the soil may lead to size-asymmetry in competition among roots. Nothing is known about the size-asymmetry of competition for
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
.


Implication for plant communities

Various ecological processes and patterns have been shown to be affected by the degree of size-asymmetry e.g.
succession Succession is the act or process of following in order or sequence. Governance and politics *Order of succession, in politics, the ascension to power by one ruler, official, or monarch after the death, resignation, or removal from office of ...
,
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 bio ...
distribution,
grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and ot ...
response,
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 ...
,
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
functioning, coexistence and
species richness Species richness is the number of different species represented in an ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the abundances of the species or their relative ab ...
. A large body of evidence shows that species loss following nutrient enrichment (
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
) is related to light competition (5, 15, 16). However, there is still a debate whether this phenomenon is related to the size-asymmetry of light competition or to other factors. Contrasting assumptions about size-asymmetry characterise the two leading and competing theories in plant ecology, the R* theory and the CSR theory. The R* theory assumes that competition is size-symmetric and therefore predicts that competitive ability in nature results from the ability to withstand low level of resources (known as the
R* rule The R* rule (also called the resource-ratio hypothesis) is a hypothesis in community ecology that attempts to predict which species will become dominant as the result of competition for resources. The hypothesis was formulated by American ecologis ...
). In contrast the CSR theory assumes that competition is size-asymmetric and therefore predicts that competitive ability in nature results from the ability to grow fast and attain a large size. Size-asymmetric competition affects also several
evolutionary Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
processes in relation to trait selection. Evolution of plant height is highly affected by asymmetric light competition.Tilman, D. Plant strategies and the dynamics and structure of plant communities. Princeton University Press, Princeton, New Jersey, USA. 360. p. (1988)Falster, D. S. & Westoby, M. Plant height and evolutionary games. Trends in Ecology & Evolution 18, 337-343, doi:10.1016/s0169-5347(03)00061-2 (2003) Theory predicts that only under asymmetric light competition, plants will grow upward and invest in
wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
production at the expense of investment in leaves, or in
reproductive The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are a ...
organs (
flowers A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanism ...
and
fruits In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
). Consistent with this, there is evidence that plant height increases as water availability increases,Klein, T., Randin, C. & Korner, C. Water availability predicts forest canopy height at the global scale. Ecology Letters 18, 1311-1320, doi:10.1111/ele.12525 (2015) presumably due to increase in the relative importance of size-asymmetric competition for light. Similarly, investment in the size of seeds at the expense of their number may be more effective undersize-asymmetric resource competition, since larger seeds tend to produce larger seedlings that are better competitors.Falster, D. S., Moles, A. T. & Westoby, M. A general model for the scaling of offspring size and adult size. American Naturalist 172, 299-317, doi:10.1086/589889 (2008) Size-asymmetric competition can be exploited in managing plant communities, such as the suppression of
weed A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place", or a plant growing where it is not wanted.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. ...
in
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...
fields. Weeds are a greater problem for farmer in dry than in moist environments, in large part because crops can suppress weeds much more effectively undersize-asymmetric competition for light than under more size-symmetric competition below ground.


See also

*
competition (biology) Competition is an interaction between organisms or species in which both require a resource that is in limited supply (such as food, water, or territory). Competition lowers the fitness of both organisms involved since the presence of one of th ...
*
Asymmetric competition Asymmetric competition refers to forms of business competition where firms are considered competitors in some markets or contexts but not in others. In such cases a firm may choose to allocate competitive resources and marketing actions among its co ...
* resource (biology) *
Resource partitioning In ecology, niche differentiation (also known as niche segregation, niche separation and niche partitioning) refers to the process by which competing species use the environment differently in a way that helps them to coexist. The competitive exclu ...
*
plant ecology Plant ecology is a subdiscipline of ecology which studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among and between plants and other organisms. Examples ...
* Jacob Weiner


References

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