![Panthera leo & Crocuta crocuta](https://upload.wikimedia.org/wikipedia/commons/7/7f/Panthera_leo_%26_Crocuta_crocuta.jpg)
Interspecific competition, in
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 overl ...
, is a form of
competition
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, indiv ...
in which individuals of ''different''
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
compete for the same resources in an ecosystem (e.g. food or living space). This can be contrasted with
mutualism, a type of
symbiosis. Competition between members of the same species is called
intraspecific competition
Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to r ...
.
If a tree species in a dense
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
grows taller than surrounding tree species, it is able to absorb more of the incoming sunlight. However, less sunlight is then available for the trees that are shaded by the taller tree, thus interspecific competition.
Leopards and
lions can also be in interspecific competition, since both species feed on the same
prey
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
, and can be negatively impacted by the presence of the other because they will have less food.
Competition is only one of many interacting
biotic and
abiotic
In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
factors that affect community structure. Moreover, competition is not always a straightforward, direct, interaction. Interspecific competition may occur when individuals of two separate species share a
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 ...
in the same area. If the resource cannot support both populations, then lowered
fecundity
Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
, growth, or survival may result in at least one species. Interspecific competition has the potential to alter
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 using a ...
s,
communities and the
evolution
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. Variation ...
of interacting species. On an individual organism level, competition can occur as interference or exploitative competition.
Types
All of the types described here can also apply to
intraspecific competition
Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to r ...
, that is, competition among individuals within a species. Also, any specific example of interspecific competition can be described in terms of both a mechanism (e.g., resource or interference) and an outcome (symmetric or asymmetric).
Based on mechanism
Exploitative competition, also referred to as resource competition, is a form of competition in which one species consumes and either reduces or more efficiently uses a shared limiting resource and therefore depletes the availability of the resource for the other species. Thus, it is an indirect interaction because the competing species interact via a shared resource.
Interference competition is a form of competition in which individuals of one species interacts directly with individuals of another species via antagonistic displays or more aggressive behavior.
In a review and synthesis of experimental evidence regarding interspecific competition, Schoener described six specific types of mechanisms by which competition occurs, including consumptive, preemptive, overgrowth, chemical, territorial, and encounter. Consumption competition is always resource competition, but the others cannot always be regarded as exclusively exploitative or interference.
Separating the effect of resource use from that of interference is not easy. A good example of exploitative competition is found in
aphid
Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
species competing over the sap in plant
phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
. Each aphid species that feeds on host plant sap uses some of the resource, leaving less for competing species. In one study, ''
Fordinae geoica'' was observed to out-compete ''
F. formicaria'' to the extent that the latter species exhibited a reduction in survival by 84%. Another example is the one of competition for calling space in amphibians, where the calling activity of a species prevents the other one from calling in an area as wide as it would in allopatry. A last example is driving of bisexual rock lizards of genus ''
Darevskia'' from their natural habitats by a daughter unisexual form; interference competition can be ruled out in this case, because
parthenogenetic
Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...
forms of the
lizards never demonstrate aggressive behavior.
This type of competition can also be observed in forests where large trees dominate the
canopy
Canopy may refer to:
Plants
* Canopy (biology), aboveground portion of plant community or crop (including forests)
* Canopy (grape), aboveground portion of grapes
Religion and ceremonies
* Baldachin or canopy of state, typically placed over an ...
and thus allow little light to reach smaller competitors living below. These interactions have important implications for the
population dynamics and distribution of both species.
Based on outcome
Scramble and contest competition refer to the relative success of competitors. Scramble competition is said to occur when each competitor is equal suppressed, either through reduction in survival or birth rates. Contest competition is said to occur when one or a few competitors are unaffected by competition, but all others suffer greatly, either through reduction in survival or birth rates. Sometimes these types of competition are referred to as symmetric (scramble) vs. asymmetric (contest) competition. Scramble and contest competition are two ends of a spectrum, of completely equal or completely unequal effects.
Apparent competition
Apparent competition is actually an example of predation that alters the relative abundances of prey on the same
trophic level
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...
. It occurs when two or more species in a
habitat
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
affect shared natural enemies in a higher
trophic level
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...
. If two species share a common
predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
, for example, apparent competition can exist between the two prey items in which the presence of each prey species increases the abundance of the shared enemy, and thereby suppresses one or both prey species. This mechanism gets its name from experiments in which one prey species is removed and the second prey species increases in abundance. Investigators sometimes mistakenly attribute the increase in abundance in the second species as evidence for resource competition between prey species. It is "apparently" competition, but is in fact due to a shared predator, parasitoid, parasite, or pathogen.
Consequences
Many studies, including those cited previously, have shown major impacts on both individuals and populations from interspecific competition. Documentation of these impacts has been found in species from every major branch of organism. The effects of interspecific competition can also reach communities and can even influence the evolution of species as they adapt to avoid competition. This evolution may result in the
exclusion
Exclusion may refer to:
Legal or regulatory
* Exclusion zone, a geographic area in which some sanctioning authority prohibits specific activities
* Exclusion Crisis and Exclusion Bill, a 17th-century attempt to ensure a Protestant succession in En ...
of a species in the habitat,
niche separation
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 exc ...
, and
local extinction. The changes of these species over time can also change communities as other species must adapt.
Competitive exclusion
The competitive exclusion principle, also called "
Gause's law" which arose from mathematical analysis and simple competition models states that two species that use the same limiting resource in the same way in the same space and time cannot coexist and must diverge from each other over time in order for the two species to coexist. One species will often exhibit an advantage in resource use. This superior competitor will out-compete the other with more efficient use of the limiting resource. As a result, the inferior competitor will suffer a decline in population over time. It will be excluded from the area and replaced by the superior competitor.
A well-documented example of competitive exclusion was observed to occur between Dolly Varden charr (Trout)(''
Salvelinus malma'') and white spotted char (Trout)(''
S. leucomaenis'') in Japan. Both of these species were morphologically similar but the former species was found primarily at higher elevations than the latter. Although there was a zone of overlap, each species excluded the other from its dominant region by becoming better adapted to its habitat over time. In some such cases, each species gets displaced into an exclusive segment of the original habitat. Because each species suffers from competition, natural selection favors the avoidance of competition in such a way.
Niche differentiation
Niche differentiation is a process by which competitive exclusion leads to differences in resource use. In the previous example, niche differentiation resulted in spatial displacement. In other cases it may result in other changes that also avoid competition. If competition avoidance is achievable, each species will occupy an edge of the niche and will become more specialized to that area thus minimizing competition. This phenomenon often results in the separation of species over time as they become more specialized to their edge of the niche, called niche differentiation. The species do not have to be in separate habitats however to avoid niche overlap. Some species adapt regionally to utilizing different resources than they ordinarily would in order to avoid competition.
There have been several well-documented cases in birds where species that are very similar change their habitat use where they overlap. For example, they may consume different food resources or use different nesting habitat or materials. On the
Galapagos Islands,
finch
The true finches are small to medium-sized passerine birds in the family Fringillidae. Finches have stout conical bills adapted for eating seeds and nuts and often have colourful plumage. They occupy a great range of habitats where they are usua ...
species have been observed to change dietary specializations in just a few generations in order to utilize limited resources and minimize competition.
In some cases, third party species interfere to the detriment or benefit of the competing species. In a laboratory study, coexistence between two competing bacterial species was mediated by
phage
A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacter ...
parasites. This type of interaction actually helped to maintain diversity in bacterial communities and has far reaching implications in medical research as well as ecology. Similar effects have been documented for many communities as a result of the action of a keystone predator that preys on a competitively superior species.
Local extinction
Although local extinction of one or more competitors has been less documented than niche separation or competitive exclusion, it does occur. In an experiment involving
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
in artificial rock pools, local extinction rates were significantly higher in areas of interspecific competition. In these cases, therefore, the negative effects are not only at the population level but also species richness of communities.
Impacts on communities
![Cooper Marsh - Purple-loosestrife](https://upload.wikimedia.org/wikipedia/commons/b/b9/Cooper_Marsh_-_Purple-loosestrife.jpg)
As mentioned previously, interspecific competition has great impact on community composition and structure. Niche separation of species, local extinction and competitive exclusion are only some of the possible effects. In addition to these, interspecific competition can be the source of a cascade of effects that build on each other. An example of such an effect is the introduction of an
invasive species to the United States,
purple-loosestrife
''Lythrum salicaria'' or purple loosestrifeFlora of NW Europe''Lythrum salicaria'' is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the family P ...
. This plant when introduced to wetland communities often outcompetes much of the native flora and decreases species richness, food and shelter to many other species at higher trophic levels. In this way, one species can influence the populations of many other species as well as through a myriad of other interactions. Because of the complicated web of interactions that make up every ecosystem and habitat, the results of interspecific competition are complex and site-specific.
Competitive Lotka–Volterra model
The impacts of interspecific competition on populations have been formalized in a mathematical model called the
Competitive Lotka–Volterra equations
The competitive Lotka–Volterra equations are a simple model of the population dynamics of species competing for some common resource. They can be further generalised to the Generalized Lotka–Volterra equation to include trophic interactions ...
, which creates a theoretical prediction of interactions. It combines the effects of each species on the other. These effects are calculated separately for the first and second population respectively:
:
:
In these formulae, N is the population size, t is time, K is the carrying capacity, r is the intrinsic rate of increase and α and β are the relative competition coefficients. The results show the effect that the other species has on the species being calculated. The results can be graphed to show a trend and possible prediction for the future of the species. One problem with this model is that certain assumptions must be made for the calculation to work. These include the lack of
migration
Migration, migratory, or migrate may refer to: Human migration
* Human migration, physical movement by humans from one region to another
** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
and constancy of the carrying capacities and competition coefficients of both species. The complex nature of ecology determines that these assumptions are rarely true in the field but the model provides a basis for improved understanding of these important concepts.
An equivalent formulation of these models is:
:
:
In these formulae,
is the effect that an individual of species 1 has on its own population growth rate. Similarly,
is the effect that an individual of species 2 has on the population growth rate of species 1. One can also read this as the effect on species 1 of species 2. In comparing this formulation to the one above, we note that
, and
.
Coexistence between competitors occurs when
and
. We can translate this as coexistence occurs when the effect of each species on itself is greater the effect of the competitor.
There are other mathematical representations that model species competition, such as using non-polynomial functions.
Interspecific competition in macroevolution
Interspecific competition is a major factor in
macroevolution
Macroevolution usually means the evolution of large-scale structures and traits that go significantly beyond the intraspecific variation found in microevolution (including speciation). In other words, macroevolution is the evolution of taxa abov ...
.
Darwin assumed that interspecific competition limits the number of species on Earth, as formulated in his wedge metaphor: "Nature may be compared to a surface covered with ten-thousand sharp wedges ... representing different species, all packed closely together and driven in by incessant blows, . . . sometimes a wedge of one form and sometimes another being struck; the one driven deeply in forcing out others; with the jar and shock often transmitted very far to other wedges in many lines of direction." (From ''Natural Selection'' - the "big book" from which Darwin abstracted the ''Origin''). The question whether interspecific competition limits global
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
is disputed today, but analytical studies of the global Phanerozoic fossil record are in accordance with the existence of global (although not constant) carrying capacities for marine biodiversity. Interspecific competition is also the basis for Van Valen's
Red Queen hypothesis
The Red Queen hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended ...
, and it may underlie the positive correlation between origination and extinction rates that is seen in almost all major taxa.
In the previous examples, the macroevolutionary role of interspecific competition is that of a limiting factor of biodiversity, but interspecific competition also promotes niche differentiation and thus
speciation and diversification. The impact of interspecific competition may therefore change during phases of diversity build-up, from an initial phase where positive feedback mechanisms dominate to a later phase when niche-peremption limits further increase in the number of species; a possible example for this situation is the re-diversification of marine faunas after the end-Permian mass extinction event.
See also
*
Minimum viable population
Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. This term is commonly used in the fields of biology, ecology, and conservation biology. MVP refers to the smallest possible siz ...
*
Symbiosis
*
Mutualism (biology)
*
Macroevolution
Macroevolution usually means the evolution of large-scale structures and traits that go significantly beyond the intraspecific variation found in microevolution (including speciation). In other words, macroevolution is the evolution of taxa abov ...
*
Red Queen hypothesis
The Red Queen hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended ...
References
Further reading
* Begon, M., C.R. Townsend and J.L. Harper. 2006. ''
Ecology: From Individuals to Ecosystems''. Blackwell Publishing, Malden, MA.
*
* Giller, P. S. 1984. Community Structure and the Niche. Chapman & Hall, London.
* Holekamp, K.E. 2006. Interspecific competition and anti-predator behavior. National Science Foundation. https://www.nsf.gov/
*
*
* Solomon, E. P., Berg, L. R., & Martin, D. W. (2002). ''Biology, sixth edition''. (N. Rose, Ed.). Stamford, CT: Thomson Learning
*
* Weiner, J. 1994. The Beak of the Finch. Cambridge University Press, New York.
External links
Competition for Territory: The Levins Model for Two Species''Wolfram Demonstrations Project'' — require
{{modelling ecosystems, expanded=other
Biological interactions
Competition