CHAPTER OVERVIEW 15: Competition Learning Objectives Characterize the various types of competition between individuals, both within (intraspecific) and between (interspecific) species Develop an appreciation for the role competition plays in driving evolution Determine competitive outcomes using linked population models of two competitors (Lotka-Volterra Model) 15.1: Introduction and Types of Competition 15.2: Intraspecific (Single Species) Competition 15.3: Interspecific (Two Species) Competition 15.4: Ecological and Evolutionary Consequences of Competition 15.5: Quantifying Competition Using the Lotka-Volterra Model 15.6: Sources and Attributions Summary Competition is any interaction in which two individuals need the same limited supply, resulting on a negative impact on both individuals involved. Competition can occur among two individuals of the same species (intraspecific competition) or among two individuals of different species (interspecific competition) and can involve direct interactions (interference or contest competition) or indirect interactions (exploitation or scramble competition). The logistic growth model is a model of intraspecific competition, as the carrying capacity is a result of competition among individuals for limited resources (food, mates, etc.). To model interspecific competition, ecologists modify the logistic growth model to consider the impact of other species on shared resources. The resulting Lotka-Volterra Model can be used to predict competitive outcomes between two species, which include coexistence or competitive exclusion. 15: Competition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.1: Introduction and Types of Competition Competition 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) (Begon et al. 1996). Competition lowers the fitness of both organisms involved, since the presence of one of the organisms always reduces the amount of the resource available to the other (Lang & Benbow 2013).
Sea anemones compete for the territory in tide pools.
In the study of community ecology, competition within and between members of a species is an important biological interaction. Competition is one of many interacting biotic and abiotic factors that affect community structure, species diversity, and population dynamics (shifts in a population over time) (Lang & Benbow 2013). Competition within and between species for resources is important in natural selection.
There are three major mechanisms of competition: interference, exploitation, and apparent competition (in order from most direct to least direct). Interference and exploitation competition can be classed as "real" forms of competition, while apparent competition is not, as organisms do not share a resource, but instead share a predator (Lang & Benbow 2013). Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. Diffuse competition refers to the summed effects of all competitors.
Studies show that intraspecific competition can regulate population dynamics (changes in population size over time). This occurs because individuals become crowded as a population grows. Since individuals within a population require the same resources, crowding causes resources to become more limited. Some individuals (typically small juveniles) eventually do not acquire enough resources and die or do not reproduce. This reduces population size and slows population growth. The logistic growth model, which includes a term for the species' carrying capacity, is a model of intraspecific competition.
Species also interact with other species that require the same resources. Consequently, interspecific competition can alter the sizes of many species' populations at the same time. Experiments demonstrate that when species compete for a limited resource, one species eventually drives the populations of other species extinct. According to the competitive exclusion principle, species less suited to compete for resources must either adapt or die out, although competitive exclusion is rarely found in natural ecosystems (Lang & Benbow 2013). Ecologists use a modified version of coupled logistic growth models for competing species, known as the Lotka-Volterra Competition model, to model interspecific competition.
: The three different mechanisms of competition. Solid arrows indicate direct relationships, dashed lines represent
indirect relationships. Developed by N. Gownaris.
15.1 Types of Competition 15.1.1 Interference Competition
: Male-male competition in red deer during rut is an example of interference competition within a species.
During interference competition, also called contest competition, organisms of the same species or of two or more different species interact directly by competing for scarce resources. For example, large aphids defend feeding sites on cottonwood leaves by ejecting smaller aphids from better sites. Male-male competition in red deer during rut is an example of interference competition that occurs within a species (intraspecific competition).
Interference competition occurs directly between individuals via aggression when the individuals interfere with foraging, survival, reproduction of others, or by directly preventing their physical establishment in a portion of the habitat. An example of this can be seen between the ant Novomessor cockerelli and red harvester ants, where the former interferes with the ability of the latter to forage by plugging the entrances to their colonies with small rocks (Barton et al. 2002). Male bowerbirds, who create elaborate
structures called bowers to attract potential mates, may reduce the fitness of their neighbors directly by stealing decorations from their structures (Le Bourlot et al. 2014). In animals, interference competition is a strategy mainly adopted by larger and stronger organisms within a habitat. As such, populations with high interference competition have adult-driven generation cycles. At first, the growth of juveniles is stunted by larger adult competitors. However, once the juveniles reach adulthood, they experience a secondary growth cycle (Le Bourlot et al. 2014). Plants, on the other hand, primarily engage in interference competition with their neighbors through allelopathy, or the production of biochemicals (Schenk 2006). Interference competition can be seen as a strategy that has a clear cost (injury or death) and benefit (obtaining resources that would have gone to other organisms) (Case & Gilpin 1975). In order to cope with strong interference competition, other organisms often either do the same or engage in exploitation competition. For example, depending on the season, larger ungulate red deer males are competitively dominant due to interference competition. However, does and fawns have dealt with this through temporal resource partitioning -- foraging for food only when adult males are not present (Stone et al. 2018). 15.1.2 Exploitation Competition Exploitation competition, or scramble competition, occurs indirectly when organisms both use a common limiting resource or shared food item. Instead of fighting or exhibiting aggressive behavior in order to win resources, exploitative competition occurs when resource use by one organism depletes the total amount available for the other organism. These organisms might never interact directly, but compete by responding to changes in resource levels. Very obvious examples of this phenomenon include a diurnal species and a nocturnal species that nevertheless share the same resources, or a plant that competes with neighboring plants for light, nutrients, and space for root growth (Jensen 1987). This form of competition typically rewards those organisms who claim the resource first. As such, exploitation competition is often size-dependent and smaller organisms are favored since smaller organisms typically have higher foraging rates (Le Bourlot et al. 2014). Since smaller organisms have an advantage when exploitative competition is important in an ecosystem, this mechanism of competition might lead to a juvenile-driven generation cycle: individual juveniles succeed and grow fast, but once they mature they are outcompeted by smaller organisms (Le Bourlot et al. 2014). In plants, exploitative competition can occur both above- and below-ground. Aboveground, plants reduce the fitness of their neighbors by vying for sunlight. Plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. Plants that produce many roots typically reduce soil nitrogen to very low levels, eventually killing neighboring plants. Exploitative competition has also been shown to occur both within species (intraspecific) and between different species (interspecific). Furthermore, many competitive interactions between organisms are some combination of exploitative and interference competition, meaning the two mechanisms are far from mutually exclusive. For example, a recent 2019 study found that the native thrips species Frankliniella intonsa was competitively dominant over an invasive thrips species Frankliniella occidentalis because it not only exhibited greater time feeding (exploitative competition) but also greater time guarding its resources (interference competition) (Bhuyain & Lim 2019). Plants may also exhibit both forms of competition, not only scrambling for space for root growth but also directly inhibiting other plants' development through allelopathy. 15.1.3 Apparent Competition Apparent competition occurs when two otherwise unrelated prey species indirectly compete for survival through a shared predator (Holt 1977). This form of competition typically manifests in new equilibrium abundances of each prey species. For example, suppose there are two species (species A and species B), which are preyed upon by food-limited predator species C. Scientists observe an increase in the abundance of species A and a decline in the abundance of species B. In an apparent competition model, this relationship is found to be mediated through predator C; a population explosion of species A increases the abundance of the predator species C due to a greater total food source. Since there are now more predators, species A and B would be hunted at higher rates than before. Thus, the success of species A was to the detriment of species B -- not because they competed for resources, but because their increased numbers had indirect effects on the predator population.
References Barton, K.E., Sanders, N.J., & Gordon, D.M. (2002). The effects of proximity and colony age on interspecific interference competition between the desert ants, Pogonomyrmex barbatus and Aphaenogaster cockerelli. The American Midland Naturalist, 148(2), pp. 376-382. doi:10.1674/0003-0031 Begon, M., Harper, J.L., & Townsend, C.R. (1996). Ecology: Individuals, populations and communities. Blackwell Science. Bhuyain, M.M.H., & Lim, U.T. (2019). Interference and exploitation competition between Frankliniella occidentalis and F. intonsa (Thysanoptera: Thripidae) in laboratory assays. Florida Entomologist, 102(2), pp. 322-328. doi:10.1653/024.102.0206 Case, T.J., & Gilpin, M.E. (1974). Interference competition and niche theory. Proceedings of the National Academy of Sciences of the United States of America, 71(8), pp. 3073-3077. doi:10.1073/pnas.71.8.3073 Holt, R.D. (1977). Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology, 12(2), pp. 197-229. Jensen, A.L. (1987). Simple models for exploitative and interference competition. Ecological Modelling, 35(1), pp. 113-121. doi:10.1016/0304-3800(87)90093-7 Lang, J.M., & Benbow, M.E. (2013). Species interactions and competition. Nature Education Knowledge, 4(4), pp. 8. Le Bourlot, V., Tully, T., & Claessen, D. (2014). Interference versus exploitative competition in the regulation of size-structured populations. The American Naturalist, 184(5), pp. 609-623. doi:10.1086/678083 Schenk, H.J. (2006). Root competition: Beyond resource depletion. Journal of Ecology, 94(4), pp. 725-739. doi:10.1111/j.13652745.2006.01124.x Stone, D.B., Martin, J.A., Cohen, B.S., Prebyl, T.J., Killmaster, C., & Miller, K.V. (2018). Intraspecific temporal resource partitioning at white-tailed deer feeding sites. Current Zoology, 65(2), pp. 139-146. doi:10.1093/cz/zoy05 Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Competition biology by Wikipedia, the free encyclopedia 15.1: Introduction and Types of Competition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.2: Intraspecific (Single Species) Competition
: Male hartebeest locking horns and fiercely defending their territories. An example of direct 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 reproduce (Townsend, 2008). By contrast, interspecific competition occurs when members of different species compete for a shared resource. Members of the same species have rather similar requirements for resources, whereas different species have a smaller contested resource overlap, resulting in intraspecific competition generally being a stronger force than interspecific competition (Connell, 1983). Intraspecific competition does not just involve direct interactions between members of the same species (such as male deer locking horns when competing for mates) but can also include indirect interactions where an individual depletes a shared resource (such as a grizzly bear catching a salmon that can then no longer be eaten by bears at different points along a river). When resources are infinite, intraspecific competition does not occur and populations can grow exponentially. Exponential population growth is exceedingly rare, but has been documented, most notably in humans since 1900. Elephant (Loxodonta africana) populations in Kruger National Park (South Africa) also grew exponentially in the mid-1900s after strict poaching controls were put in place (Young & Ferreira, 2009). However, prolonged exponential growth is rare in nature because resources are finite and so not every individual in a population can survive, leading to intraspecific competition for the scarce resources. Individuals can compete for food, water, space, light, mates, or any other resource which is required for survival or reproduction. The resource must be limited for competition to occur; if every member of the species can obtain a sufficient amount of every resource then individuals do not compete and the population grows exponentially (Townsend, 2008). When resources are limited, an increase in population size reduces the quantity of resources available for each individual, reducing the per capita fitness in the population. As a result, the growth rate of a population slows as intraspecific competition becomes more intense, making it a negatively density dependent process. The falling population growth rate as population increases can be modeled effectively with the logistic growth model. The rate of change of population density eventually falls to zero, the point ecologists have termed the carrying capacity (K). However, a population can only grow to a very limited number within an environment. The carrying capacity, defined by the variable K, of an environment is the maximum number of individuals or species an environment can sustain and support over a longer period of time. The logistic growth equation is an effective tool for modeling intraspecific competition despite its simplicity, and has been used to model many real biological systems. Here N is the size of the population at a given time, r is inherent per-capita growth rate, and K is the carrying capacity. At low population densities, is much smaller than K and so the main determinant for population growth is just the per capita growth rate. However, as approaches the carrying capacity the second term in the logistic equation becomes
smaller, reducing the rate of change of population density (Hanson, 1981). Eventually, the population size equals the carrying capacity, and the population growth rate equals zero.
dN/dt = rate of change of population density N = population size at time t r = per capita growth rate K = carrying capacity
A line graph titled "Figure 1: Logistic Growth of Population Size Over Time" is labeled "Logistic S-shaped Curve" with Time on the x-axis and Population on the y-axis. Text to the side reads: "the S-shaped logistic curve is formed when growth rate decreases as carrying capacity is approached by the population". The graphed curve is labeled "Slow Growth" close to the origin with a gradually increasing positive slope. Adjacent text reads: "Slow growth occurs when natality is slightly above mortality, for fast growth natality is greater than mortality". The middle portion of the curve labeled "fast growth" is nearly linear, and shows the transition from an increasing positive slope to a decreasing positive slope. An additional horizontal red dotted line labeled "Carrying Capacity" intersects with the "Stable Equilibrium" labeled portion of the curve, with text that reads "Carrying capacity is the amount of organisms within a region that the environment can support sustainably". The stable equilibrium portion of the curve is horizontal with adjacent text that reads: "Stable equilibrium is met when the population aligns with the carrying capacity line".
: The growth of a population following a logistic curve, resulting in the S-shaped graph. This model reaches a stable
equilibrium, sustaining the population at the carrying capacity as time continues. Developed by Nchisick under CC-BY-SA.
The logistic growth curve is initially very similar to the exponential growth curve. When population density is low, individuals are free from competition and can grow rapidly. However, as the population reaches its maximum (the carrying capacity), intraspecific competition becomes fiercer and the per capita growth rate slows until the population reaches a stable size. At the carrying capacity, the rate of change of population density is zero because the population is as large as possible based on the resources available. Experiments on Daphnia growth rates showed a striking adherence to the logistic growth curve (Schoener, 1973). The inflexion point in the Daphnia population density graph occurred at half the carrying capacity, as predicted by the logistic growth model. In summary, the resources within an environment are limited. Therefore, the environment can only support a certain number of individuals before its resources completely diminish. Numbers larger than this will suffer a negative population growth until eventually reaching the carrying capacity, whereas populations smaller than the carrying capacity will grow until they reach it.
References Connell, J. (1983). On the prevalence and relative importance of interspecific competition: Evidence from field experiments. American Naturalist, 122(5), pp. 661-696. doi:10.1086/284165 Hanson, F. (1981). Logistic growth with random density independent disasters. Theoretical Population Biology, 19(1), pp. 1-18. doi:10.1016/0040-5809(81)90032-0 Schoener, T. (1973). Population growth regulated by intraspecific competition for energy or time: Some simple representations. Theoretical Population Biology, 4(1), pp. 56-84. doi:10.1016/0040-5809(73)90006-3 Townsend. (2008). Essentials of Ecology. pp. 103-105. ISBN 978-1-4051-5658-5 Young, K., Ferreira, S., & van Aarde, R. (2009). The influence of increasing population size and vegetation productivity on elephant distribution in the Kruger National Park. Austral Ecology, 34(3), pp. 329-342. doi:10.1111/j.1442-9993.2009.01934.x
Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Intraspecific competition by Wikipedia, the free encyclopedia
15.2: Intraspecific (Single Species) Competition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.3: Interspecific (Two Species) Competition
: Subadult male lion and spotted hyena in the Masai Mara. The two species share the same ecological niche, and are thus in competition with each other.
Interspecific competition may occur when individuals of two separate species share a limiting resource in the same area. If the resource cannot support both populations, then lowered fecundity, growth, or survival may result in at least one species. Interspecific competition has the potential to alter populations, communities, and the evolution of interacting species. An example among animals could be the case of cheetahs and lions; since both species feed on similar prey, they are negatively impacted by the presence of the other because they will have less food, however, they still persist together, despite the prediction that under competition one will displace the other. In fact, lions sometimes steal prey items killed by cheetahs. Potential competitors can also kill each other, in so-called 'intraguild predation'. For example, in southern California coyotes often kill and eat gray foxes and bobcats, all three carnivores sharing the same stable prey (small mammals) (Fedriani et al., 2000).
15.3.1 Interspecific Competition: The Lotka-Volterra Model Given two populations, N1 and N2, with logistic dynamics, the Lotka-Volterra formulation adds an additional term to account for the species' interactions. Thus the competitive Lotka-Volterra equations are: Population 1:
Here, 12 represents the effect species 2 has on the population of species 1 and 21 represents the effect species 1 has on the population of species 2. These values do not have to be equal. Because this is the competitive version of the model, all interactions must be harmful (competition) and therefore all -values are positive. Note that each species can have its own growth rate and
carrying capacity. The Lotka-Volterra model for competition and how it is used to predict competitive outcomes is described in more detail in the Quantifying Competition section of this chapter. 15.3.2 Resource-Ratio Hypothesis (R* rule) The R* rule (also called the resource-ratio hypothesis) is a hypothesis that attempts to predict which species will become dominant as the result of competition for resources (Tilman, 1982). It predicts that if multiple species are competing for a single limiting resource, then whichever species can survive at the lowest equilibrium resource level (i.e., the R*) can outcompete all other species (Tilman, 1982). If two species are competing for two resources, then coexistence is only possible if each species has a lower R* on one of the resources (Tilman, 1982). For example, two phytoplankton species may be able to coexist if one is more limited by nitrogen, and the other is more limited by phosphorus. Consider a community with multiple species. We will assume that each species competes for a single resource, and ignore the effects of interference or apparent competition. Each population increases by consuming resources, and declines when resources are too scarce. For example, we could model their population dynamics as
where is the density of species j, R is the density of the resource, a is the rate at which species j eats the resource, d is species j's
death rate, and r is the rate at which resources grow when not consumed. It is easy to show that when species j is at equilibrium by
= 0), that the equilibrium resource density, , is
When R > , species j's population will increase; when R is less than , species j's population will decline. Because of this, the
species with the lowest R* will eventually dominate. Consider the two species case, where
equilibrium, R = , and species 1's population will be increasing. When species 1 is at equilibrium, R = , and species 2's
population will be decreasing (Tilman, 1982).
References Fedriani, J.M., Fuller, T.K., Sauvajot, R.M., & York, E.C. (2000). Competition and intraguild predation among three sympatric carnivores. Oecologia, 125, pp. 258-270. doi:10.1016/0040-5809(77)90042-9 Tilman, D. (1982). Resource competition and community structure. Princeton: Princeton University Press. ISBN 9780691083025.
Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Interspecific competition by Wikipedia, the free encyclopedia R*_rule_(ecology) by Wikipedia, the free encyclopedia
15.3: Interspecific (Two Species) Competition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.4: Ecological and Evolutionary Consequences of Competition 15.4.1 Competitive Exclusion
: 1) A smaller (yellow) species of bird forages across the whole tree. 2) A larger (red) species competes for resources.
3) Red dominates in the middle for the more abundant resources. Yellow shifts to a new niche, avoiding competition.
The competitive exclusion principle postulates that two species which compete for the same limited resource cannot coexist at constant population values. When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche. The principle has been paraphrased in the maxim "complete competitors cannot coexist" (Hardin, 1960). Georgy Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, P. aurelia and P. caudatum. The conditions were to add fresh water every day and input a constant flow of food. Although P. caudatum initially dominated, P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition. However, Gause was able to let the P. caudatum survive by differing the environmental parameters (food, water). Thus, Gause's law is valid only if the ecological factors are constant.
: Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, P. aurelia outcompetes P. caudatum.
Competitive exclusion is predicted by mathematical and theoretical models such as the Lotka-Volterra models of competition. However, competitive exclusion is rarely observed in natural ecosystems and many biological communities appear to violate Gause's law. The best-known example is the so-called "paradox of the plankton" (Hutchinson, 1960). All plankton species live on a very limited number of resources, primarily solar energy and minerals dissolved in the water. According to the competitive exclusion principle, only a small number of plankton species should be able to coexist on these resources. Nevertheless, large numbers of plankton species coexist within small regions of open sea.
15.4.2 Niche Differentiation Niche differentiation (also known as 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. When two species differentiate their niches, they tend to compete less strongly, and are thus more likely to coexist. Species can differentiate their niches in many ways, such as by consuming different foods, or using different areas of the environment. As an example of niche partitioning, several anole lizards in the Caribbean islands share common diets--mainly insects. They avoid competition by occupying different physical locations. Although these lizards might occupy different locations, some species can be found inhabiting the same range, with up to 15 in certain areas. For example, some live on the ground while others are arboreal. Species who live in different areas compete less for food and other resources, which minimizes competition between species. However, species who live in similar areas typically compete with each other (Pacala, 1985).
: Niche differentiation by size: greater duckweed, lesser duckweed and rootless dwarf duckweed.
Competing species can partition their niche in different ways. This list is not exhaustive, but illustrates several classic examples. Resource Partitioning is the phenomenon where two or more species divide out resources like food, space, resting sites etc. to coexist. For example, some lizard species appear to coexist because they consume insects of differing sizes (Caldwell & Vitt, 1999). Alternatively, species can coexist on the same resources if each species is limited by different resources, or differently able to capture resources. Different types of phytoplankton can coexist when different species are differently limited by nitrogen, phosphorus, silicon, and light (Grover, 1997). In the Galapagos Islands, finches with small beaks are more able to consume small seeds, and finches with large beaks are more able to consume large seeds. If a species' density declines, then the food it most depends on will become more abundant (since there are so few individuals to consume it). As a result, the remaining individuals will experience less competition for food. Although "resource" generally refers to food, species can partition other non-consumable objects, such as parts of the habitat. For example, warblers are thought to coexist because they nest in different parts of trees (MacArthur, 1958). Species can also partition habitat in a way that gives them access to different types of resources. As previously stated, anole lizards appear to coexist because each uses different parts of the forests as perch locations (Grover, 1997). This likely gives them access to different species of insects.
Predator Partitioning occurs when species are attacked differently by different predators (or natural enemies more generally). For example, trees could differentiate their niche if they are consumed by different species of specialist herbivores, such as herbivorous insects. If a species density declines, so too will the density of its natural enemies, giving it an advantage. Thus, if each species is constrained by different natural enemies, they will be able to coexist (Grover, 1994). Early work focused on specialist predators; however, more recent studies have shown that predators do not need to be pure specialists, they simply need to affect each prey species differently (Chesson & Kuang, 2008; Sedio et al., 2013). Conditional Differentiation (sometimes called temporal niche partitioning) occurs when species differ in their competitive abilities based on varying environmental conditions. For example, in the Sonoran Desert, some annual plants are more successful during wet years, while others are more successful during dry years (Angert et al., 2013). As a result, each species will have an advantage in some years, but not others. When environmental conditions are most favorable, individuals will tend to compete most strongly with members of the same species. For example, in a dry year, dry-adapted plants will tend to be most limited by other dry-adapted plants. Competition-Predation Trade-Off: Species can differentiate their niche via a competition-predation trade-off if one species is a better competitor when predators are absent, and the other is better when predators are present. Defenses against predators, such as toxic compounds or hard shells, are often metabolically costly. As a result, species that produce such defenses are often poor competitors when predators are absent. Species can coexist through a competition-predation trade-off if predators are more abundant when the less defended species is common, and less abundant if the well-defended species is common (Holt et al. 1994). This effect has been criticized as being weak, because theoretical models suggest that only two species within a community can coexist because of this mechanism (Chase et al. 2002). 15.4.3 Coexistence
: Coexistence theory attempts to explain the paradox of the plankton - how can ecologically similar species coexist without competitively excluding each other?
Coexistence theory is a framework to understand how competitor traits can maintain species diversity and stave-off competitive exclusion even among similar species living in ecologically similar environments. Coexistence theory explains the stable coexistence of species as an interaction between two opposing forces: fitness differences between species, which should drive the best-adapted species to exclude others within a particular ecological niche, and stabilizing mechanisms, which maintains diversity via niche differentiation. For many species to be stabilized in a community, population growth must be negative density-dependent, i.e. all participating species have a tendency to increase in density as their populations decline. In such communities, any species that becomes rare will experience positive growth, pushing its population to recover and making local extinction unlikely. As the population of one species declines, individuals of that species tend to compete predominantly with individuals of other species. Thus, the tendency of a population to recover as it declines in density reflects
reduced intraspecific competition (within-species) relative to interspecific competition (between-species), the signature of niche differentiation.
: Groundhog and a raccoon eating together.
Two qualitatively different processes can help species to coexist: a reduction in average fitness differences between species or an increase in niche differentiation between species. These two factors have been termed equalizing and stabilizing mechanisms, respectively (Chesson, 2000). For species to coexist, any fitness differences that are not reduced by equalizing mechanisms must be overcome by stabilizing mechanisms. Equalizing mechanisms reduce fitness differences between species. As its name implies, these processes act in a way that merge the competitive abilities of multiple species closer together. Equalizing mechanisms affect interspecific competition (the competition between individuals of different species). For example, when multiple species compete for the same resource, competitive ability is determined by the minimum level of resources a species needs to maintain itself (known as an R*, or equilibrium resource density) (Tilman, 1980). Thus, the species with the lowest R* is the best competitor and excludes all other species in the absence of any niche differentiation. Any factor that reduces the differences in R* level between species (like increased harvest of the dominant competitor) is classified as an equalizing mechanism. Environmental variation (which is the focus of the Intermediate Disturbance Hypothesis) can be considered an equalizing mechanism. Since the fitness of a given species is intrinsically tied to a specific environment, when that environment is disturbed (e.g. through storms, fires, volcanic eruptions, etc.) some species may lose components of their competitive advantage which were useful in the previous version of the environment. Stabilizing mechanisms promote coexistence by concentrating intraspecific competition relative to interspecific competition. In other words, these mechanisms "encourage" an individual to compete more with other individuals of its own species, rather than with individuals of other species. Stabilizing mechanisms increase the low-density growth rate of all species. Resource partitioning (a type of niche differentiation) is a stabilizing mechanism because interspecific competition is reduced when different species primarily compete for different resources. Similarly, if species are differently affected by environmental variation (e.g., soil type, rainfall timing, etc.), this can create a stabilizing mechanism called the storage effect. The theory proposes one way for multiple species to coexist: in a changing environment, no species can be the best under all conditions (Chesson & Warner, 1981). Instead, each species must have a unique response to varying environmental conditions, and a way of buffering against the effects of bad years. The storage effect gets its name because each population "stores" the gains in good years or microhabitats (patches) to help it survive population losses in bad years or patches.
: Several species of Galapagos finches display character displacement. Each closely related species differs in beak
size and beak depth, allowing them to coexist in the same region since each species eats a different type of seed: the seed best fit
for its unique beak. The finches with the deeper, stronger beaks consume large, tough seeds, while the finches with smaller beaks
Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition. For example, Darwin's finches can be found alone or together on the Galapagos Islands. Both species' populations actually have more individuals with intermediate-sized beaks when they live on islands without the other species present. However, when both species are present on the same island, competition is intense between individuals that have intermediate-sized beaks of both species because they all require intermediate sized seeds. Consequently, individuals with small and large beaks have greater survival and reproduction on these islands than individuals with intermediate-sized beaks. Different finch species can coexist if they have traits--for instance, beak size--that allow them to specialize on particular resources. When Geospiza fortis and Geospiza fuliginosa are present on the same island, G. fuliginosa tends to evolve a small beak and G. fortis a large beak. The observation that competing species' traits are more different when they live in the same area than when competing species live in different areas is called character displacement. For the two finch species, beak size was displaced: beaks became smaller in one species and larger in the other species.
References Angert, A.L., Huxman, T.E., Chesson, P., & Venable, D.L. (2009). Functional tradeoffs determine species coexistence via the storage effect. Proceedings of the National Academy of Sciences, 106(28), pp. 11641-11645. doi:10.1073/pnas.0904512106 Caldwell, J.P., & Vitt, L.J. (1999). Dietary asymmetry in leaf litter frogs and lizards in a transitional northern Amazonian rain forest. Oikos, 84(3), pp. 383-397. doi:10.2307/3546419 Chase, J.M., Abrams, P.A., Grover, J.P., Diehl, S., Chesson, P., Holt, R.D., Richards, S.A., Nisbet, R.M., & Case, T.J. (2002). The interaction between predation and competition: A review and synthesis. Ecology Letters, 5(2), pp. 302-315. doi:10.1046/j.14610248.2002.00315.x Chesson, P., & Warner, R. (1981). Environmental variability promotes coexistence in lottery competitive systems. The American Naturalist, 117(6), pp. 923-943. doi:10.1086/283778
Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31, pp. 343-366. doi:10.1146/annurev.ecolsys.31.1.343 Chesson, P., & Kuang, J.J. (2008). The interaction between predation and competition. Nature, 456(7219), pp. 235-238. doi:10.1038/nature07248 Grover, J.P. (1997). Resource competition (1st ed.). London: Chapman & Hall. ISBN 978-0412749308 Hardin, G. (1960). The competitive exclusion principle. Science, 131(3409), pp. 1292-1297. doi:10.1126/science.131.3409.1292 Holt, R.D., Grover, J., & Tilman, D. (1994). Simple rules for interspecific dominance in systems with exploitative and apparent competition. The American Naturalist, 144(5), pp. 741-771. doi:10.1086/285705 Hutchinson, G.E. (1961). The paradox of the plankton. The American Naturalist, 95(882), pp. 137-145. doi:10.1086/282171 MacArthur, R.H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39(4), pp. 599-619. Pacala, S.W., & Roughgarden, J. (1985). Population experiments with the Anolis lizards of St. Maarten and St. Eustatius. Ecology, 66(1), pp. 129-141. doi:10.2307/1941313 Sedio, B.E., Ostling, A.M. (2013). How specialised must natural enemies be to facilitate coexistence among plants? Ecology Letters, 16(8), pp. 995-1003. doi:10.1111/ele.12130 Tilman, D. (1980). Resources: A graphical-mechanistic approach to competition and predation. The American Naturalist, 116(3), pp. 362-393. doi:10.1086/283633 Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Competitive exclusion principle by Wikipedia, the free encyclopedia Ecological niche by Wikipedia, the free encyclopedia Coexistence theory by Wikipedia, the free encyclopedia Character displacement by Wikipedia, the free encyclopedia 15.4: Ecological and Evolutionary Consequences of Competition is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.5: Quantifying Competition Using the Lotka-Volterra Model Real populations do not exist in isolation, but share habitats with populations of other species. In many cases, coexisting species will interact by interspecific competition, predation, parasitism, mutualism, or other ecological interactions. More realistic models must take such interactions into account. In the 1920s, Vito Volterra and Alfred Lotka (1932) independently developed models of interspecific competition (competition between two species), and investigated the conditions that would permit competing species to coexist indefinitely. An important ecological generalization, the competitive exclusion principle, has grown out of the Lotka-Volterra model and from other sources. This principle states that two species cannot coexist unless their niches are sufficiently different that each limits its own population growth more than it limits that of the other. In other words, if there is too much niche overlap, one species will competitively exclude the other. In reality, whether two species coexist depends not only on their competitive interactions with each other, but also on their interactions with the abiotic environment and with other species not included in this simple model. Nevertheless, the competitive exclusion principle has proven fruitful in stimulating research and understanding ecological interactions in the natural world. In broad terms, the questions Lotka and Volterra asked were: What will happen to the population dynamics of these two populations, given various values of the model parameters? Are there parameter values that will produce a winner and a loser--one population that persists while the other goes extinct? This would be competitive exclusion. Will other values result in coexistence, in which both competing populations persist indefinitely? Model Development - Starting with One Species The logistic model of population growth focuses on intraspecific competition (competition between individuals of the same species). To keep things (relatively) simple, we will develop our model of interspecific competition beginning with this form of the logistic model:
where K is the carrying capacity, or largest sustainable population. The value of K is set by available resources and by each
individual's resource demand. This version of the logistic model has intraspecific competition built into it in the term
This term reduces the population growth rate in response to the addition of each new member of the population, representing the
reduction in per capita birth rate, and increase in per capita death rate, caused by competition for limited resources.
The Lotka-Volterra model of interspecific competition builds on the logistic model of a single population. It begins with a separate logistic model of the population of each of the two, competing species.
Note the use of subscripts 1 and 2 to denote which species' population is being modeled. Each population has its own rate of increase r and carrying capacity K, and these may differ between the two species. Model Development - Coupling Two Competitors Next, we build interspecific competition into each of these equations. We assume that each new member of Population 1 reduces resources available to each member of Population 2, and thus reduces population growth rate. Additionally, new members of Population 2 will also reduce resources available to members of Population 1--this is, after all, the meaning of interspecific competition.
The simplest way to model this would be to modify the
each additional member of Population 2 will affect Population 1 exactly as much as an additional member of Population 1. That is
not necessarily the case, so we multiply in this term by a competition coefficient, 12 to express how much effect each
additional member of Population 2 has on Population 1, relative to the effect of a new member of Population 1. We modify the
model for Population 2 in a parallel way. The resulting Lotka-Volterra model of two-species competition is:
Note the subscripts on the competition coefficients: 12 expresses the effect of one member of Population 2 on the growth rate of Population 1; 21 expresses the effect of one member of Population 1 on the growth rate of Population 2. The value of the competition coefficient tells us something about the relative importance of interspecific and intraspecific competition on the population dynamics of a species.
When the competition coefficient is less than 1, intraspecific competition has a stronger per capita impact on resource availability for that species. When the competition coefficient is greater than 1, interspecific competition has a stronger per capita impact on resource availability for that species. When the competition coefficient is equal to 1, inter and intraspecific competition have a similar per capita impact on resource availability for that species.
Equilibrium Solutions for Coupled Competitors
To understand the outcome of competition between these two species, we must first find the equilibrium solutions to the coupled
equations above. The equilibrium is the population size at which the population stops growing. We can solve for this equilibrium by
for each species to zero and solving for N.
The resulting equation is the equation of a line, y = a + bx. We call this line a zero net growth isocline, or ZNGI, because anywhere along it, Population 1 has zero net growth. In other words, this is an equilibrium solution for Population 1. Just as x and y in the general linear equation y = a + bx can be used as coordinates for graphing, so we can use N1 and N2 as coordinates to graph each species ZNGI. We can graph this isocline by finding any two points along it and connecting them with a straight line. Two convenient points are where = 0 and where = 0. If we solve for these intercepts, we wind up with the following two coordinates for Population 1: [0, K1/a12] (setting x, or the size of Population 1, to 0) and [K1, 0] (setting y, or the population size of Population 2, to 0). In words, if there are no members of Population 2 in the habitat, Population 1 will stabilize at its own carrying capacity, K1. This seems a reasonable solution. If there are no members of Population 1, Population 2 will stabilize at the carrying capacity of Population 1 accounting for the relative per capita resource requirements of Population 2 relative to Population 1. These points can be plotted and connected to visualize the ZNGI. The same is done for Population 2:
This results in the following ZNGI coordinates: [0, K2] and [K2/a21,0].
To determine the outcome of competition, we must be able to find the ZNGI lines and the initial sizes of each population and graph them. We can graph the populations of the two species at any time by a point on a graph. Population 1 should always be plotted on the x-axis and Population 2 is on the y-axis. If the point falls below (Population 1) and/or to the left (Population 2) of a species' isocline, that population will continue to increase. If the point falls above (Population 1) and/or to the right (Population 2) of a species' isocline, that population will decrease. This will continue to occur as the coupled populations change sizes, and the point describing the two populations will trace some trajectory across the graph, eventually reaching an equilibrium of coexistence or competition exclusion. Notice that time does not appear on either axis of this graph. The plots above are called phase diagrams, and the space bounded by its axes is called phase space. You can plot the trajectory of two changing populations through the phase space and from that determine whether one species excludes the other, or if they coexist.
A Quick Summary of Coupled Interspecific Competition Models Based on the Logistic Model: Alone, Species 1 and 2 increase to their K Species 1 and 2 reduce each others' K Outcomes Depend On: Competition coefficient Species 1 and Species 2 carrying capacity Species 1 and Species 2 starting population size Steps to Determining Outcomes: Find and plot Species 1 isocline (K1 on x; K1/12 on y) Find and plot Species 2 isocline (K2/21 on x; K2 on y) Plot initial population size Determine vectors and outcome Models Review - Exponential, Logistic, Competition 15.5.5
Exponential (Review) - No Carrying Capacity
= intrinsic growth rate of species 1 = population size of species 1 Logistic (Review) - Single Species, Carry Capacity
= carrying capacity of species 1 (when K = N,
Competition - Two Species Logistic Model, Equations Linked
12 or = a measure of the per capita effect of species 2 on the growth of species 1 21 or = a measure of the per capita effect of species 1 on the growth of species 2
Species 1 and Species 2 Isoclines Species 1 and Species 2 Outcomes
Predator/Prey Relations Exercise Blue crabs have a death rate of 0.005 and an initial population size of 50. Oysters have an intrinsic growth rate of 0.1 and an initial population size of 200. For each 50 oysters a crab consumes, they can produce one additional baby blue crab. For every 400 additional oysters added to the population, each crab can consume one more oyster per unit time. 1) Where are the crab and oyster isoclines? 2) Draw a phase diagram with the isoclines. Note where the starting populations of both species are, as well as the directions of the vectors of the population changes in each quadrant. 3) Draw the boom-and-bust oscillations for these species. 4) If oysters evolved to have thicker and harder to open shells, which term of the predator-prey model would be impacted, and in what direction? 5) Although the oyster population has crashed due to overfishing, the blue crab population continues to increase. What might have caused this dynamic? 6) The oyster's predators and competitors have been eradicated, but their population stops growing after a long period of increase. Why has their population stopped growing? Answer
1) Isocline for predators (0 growth rate of predators): Isocline for prey (0 growth rate for prey): 2)
= 0.005/(0.02*0.0025) = 100 = 0.1/0.0025 = 40
4) Handing time would increase, so p would decrease. 5) Prey-switching is a possible explanation. 6) The population has reached their carrying capacity. Phase Diagram Exercise Two species of monkey in West Africa, putty-nosed monkeys (Species 1) and Diana monkeys (Species 2), have nearly identical niches and breed at the same time annually. Diana monkeys eat insects and fruit, but the putty-nosed monkey eats only insects. A team of ecologists conducting a study on competition between these two species find that: For the putty-nosed monkey population, Diana monkeys have a per capita impact equivalent to 0.8 putty-nosed monkeys. For the Diana monkey population, putty-nosed monkeys have a per capita impact equivalent to 0.6 Diana monkeys. Putty-nosed monkeys have a carrying capacity of 300 and starting population of 250. Diana monkeys have a carrying capacity of 500 and starting population of 400. 1) Why does it make sense for both populations to have a competition coefficient of less than one? 2) What is a change that one of these populations could make to reduce competition? 3) What are the isoclines for these competitors? Graph the isoclines. Which species wins, if any, and what is the name for this type outcome? Answer 1) Since the two monkey species have differences in their diets, they are more likely to impact themselves than they are to impact each other--putty-nosed monkeys will not take fruit from Diana monkeys. For both species, intraspecific
competition is stronger than interspecific competition. 2) One of the populations could change the time of year that they breed; the Diana monkeys can focus their diets on fruit; the populations could focus their diets on different types of insects. 3) References Lotka, A.J. (1932). The growth of mixed populations: Two species competing for a common food supply. Journal of the Washington Academy of Sciences, 22, pp. 461 469. Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Donovan, T. M. and C. Welden. 2002. Spreadsheet exercises in ecology and evolution. Sinauer Associates, Inc. Sunderland, MA, USA. 15.5: Quantifying Competition Using the Lotka-Volterra Model is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
15.6: Sources and Attributions Full Chapter Sources Angert, A.L., Huxman, T.E., Chesson, P., & Venable, D.L. (2009). Functional tradeoffs determine species coexistence via the storage effect. Proceedings of the National Academy of Sciences, 106(28), pp. 11641-11645. doi:10.1073/pnas.0904512106 Barton, K.E., Sanders, N.J., & Gordon, D.M. (2002). The effects of proximity and colony age on interspecific interference competition between the desert ants Pogonomyrmex barbatus and Aphaenogaster cockerelli. The American Midland Naturalist, 148(2), pp. 376-382. doi:10.1674/0003-0031 Begon, M., Harper, J.L., & Townsend, C.R. (1996). Ecology: Individuals, populations and communities. Blackwell Science. Bhuyain, M.M.H., & Lim, U.T. (2019). Interference and exploitation competition between Frankliniella occidentalis and F. intonsa (Thysanoptera: Thripidae) in laboratory assays. Florida Entomologist, 102(2), pp. 322-328. doi:10.1653/024.102.0206 Brown, W.L., & Wilson, E.O. (1956). Character displacement. Systematic Zoology, 5, pp.49-65. Case, T.J., & Gilpin, M.E. (1974). Interference competition and niche theory. Proceedings of the National Academy of Sciences of the United States of America, 71(8), pp. 3073-3077. doi:10.1073/pnas.71.8.3073 Caldwell, J.P., & Vitt, L.J. (1999). Dietary asymmetry in leaf litter frogs and lizards in a transitional northern Amazonian rain forest. Oikos, 84(3), pp. 383-397. doi:10.2307/3546419 Chase, J.M., Abrams, P.A., Grover, J.P., Diehl, S., Chesson, P., Holt, R.D., Richards, S.A., Nisbet, R.M., & Case, T.J. (2002). The interaction between predation and competition: A review and synthesis. Ecology Letters, 5(2), pp. 302-315. doi:10.1046/j.14610248.2002.00315.x Chesson, P., & Warner, R. (1981). Environmental variability promotes coexistence in lottery competitive systems. The American Naturalist, 117(6), pp. 923-943. doi:10.1086/283778 Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31, pp. 343-366. doi:10.1146/annurev.ecolsys.31.1.343 Chesson, P., & Kuang, J.J. (2008). The interaction between predation and competition. Nature, 456(7219), pp. 235-238. doi:10.1038/nature07248 Connell, J. (1983). On the prevalence and relative importance of interspecific competition: Evidence from field experiments. The American Naturalist, 122(5), pp. 661-696. doi:10.1086/284165 Fedriani, J.M., Fuller, T.K., Sauvajot, R.M., & York, E.C. (2000). Competition and intraguild predation among three sympatric carnivores. Oecologia, 125, pp. 258-270. doi:10.1016/0040-5809(77)90042-9 Grover, J.P. (1994). Assembly rules for communities of nutrient-limited plants and specialist herbivores. The American Naturalist, 143(2), pp. 258-82. doi:10.1086/285603 Grover, J.P. (1997). Resource competition (1st ed.). London: Chapman & Hall. ISBN 978-0412749308 Hanson, F. (1981). Logistic growth with random density independent disasters. Theoretical Population Biology, 19(1), pp. 1-18. doi:10.1016/0040-5809(81)90032-0 Hardin, G. (1960). The competitive exclusion principle. Science, 131(3409), pp. 1292-1297. doi:10.1126/science.131.3409.1292 Holt, R.D. (1977). Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology, 12(2), pp. 197-229. Holt, R.D., Grover, J., & Tilman, D. (1994). Simple rules for interspecific dominance in systems with exploitative and apparent competition. The American Naturalist, 144(5), pp. 741-771. doi:10.1086/285705 Hutchinson, G.E. (1961). The paradox of the plankton. The American Naturalist, 95(882), pp. 137-145. doi:10.1086/282171 Jensen, A.L. (1987). Simple models for exploitative and interference competition. Ecological Modelling, 35(1), pp. 113-121. doi:10.1016/0304-3800(87)90093-7
Lang, J.M. & Benbow, M.E. (2013). Species interactions and competition. Nature Education Knowledge, 4(4), pp. 8. Le Bourlot, V., Tully, T., & Claessen, D. (2014). Interference versus exploitative competition in the regulation of size-structured populations. The American Naturalist, 184(5), pp. 609-623. doi:10.1086/678083 Lotka, A.J. (1932). The growth of mixed populations: Two species competing for a common food supply. Journal of the Washington Academy of Sciences, 22, pp. 461 469. MacArthur, R.H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39(4), pp. 599-619. Pacala, S.W., & Roughgarden, J. (1985). Population experiments with the Anolis lizards of St. Maarten and St. Eustatius. Ecology, 66(1), pp. 129-141. doi:10.2307/1941313 Sedio, B.E., Ostling, A.M. (2013). How specialised must natural enemies be to facilitate coexistence among plants? Ecology Letters, 16(8), pp. 995-1003. doi:10.1111/ele.12130 Schenk, H.J. (2006). Root competition: Beyond resource depletion. Journal of Ecology, 94(4), pp. 725-739. doi:10.1111/j.13652745.2006.01124.x Schoener, T. (1973). Population growth regulated by intraspecific competition for energy or time: Some simple representations. Theoretical Population Biology, 4(1), pp. 56-84. doi:10.1016/0040-5809(73)90006-3 Stone, D.B., Martin, J.A., Cohen, B.S., Prebyl, T.J., Killmaster, C., & Miller, K.V. (2018). Intraspecific temporal resource partitioning at white-tailed deer feeding sites. Current Zoology, 65(2), pp. 139-146. doi:10.1093/cz/zoy05 Tilman, D. (1980). Resources: A graphical-mechanistic approach to competition and predation. The American Naturalist, 116(3), pp. 362-393. doi:10.1086/283633 Tilman, D. (1982). Resource competition and community structure. Princeton: Princeton University Press. ISBN 9780691083025. Townsend. (2008). Essentials of Ecology. pp. 103-105. ISBN 978-1-4051-5658-5 Young, K.D., Ferreira, S.M., & Van Aarde, R.J. (2009). The influence of increasing population size and vegetation productivity on elephant distribution in the Kruger National Park. Austral Ecology, 34(3), pp. 329-342. doi:10.1111/j.1442-9993.2009.01934.x Contributors and Attributions This chapter was written by N. Gownaris and T. Zallek, with text taken from the following CC-BY resources: Competition_biology by Wikipedia, the free encyclopedia Interspecific competition by Wikipedia, the free encyclopedia Intraspecific competition by Wikipedia, the free encyclopedia R*_rule_(ecology) by Wikipedia, the free encyclopedia Coexistence theory by Wikipedia, the free encyclopedia Donovan, T. M. and C. Welden. 2002. Spreadsheet exercises in ecology and evolution. Sinauer Associates, Inc. Sunderland, MA, USA. 15.6: Sources and Attributions is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.