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Species Interactions and Competition
By: Jennifer M. Lang (University of Dayton) & M. Eric Benbow (University of Dayton) © 2013 Nature
Education
Citation: Lang, J. M. & Benbow, M. E. (2013) Species Interactions and
Competition. Nature Education Knowledge 4(4):8
Species Interactions and Competition
Introduction
Organisms live within an ecological community, which is defined as an assemblage of populations
of at least two different species that interact directly and indirectly within a defined geographic area
(Agrawal et al. 2007; Ricklefs 2008; Brooker et al. 2009). Species interactions form the basis for
many ecosystem properties and processes such as nutrient cycling and food webs. The nature of
these interactions can vary depending on the evolutionary context and environmental conditions in
which they occur. As a result, ecological interactions between individual organisms and entire
species are often difficult to define and measure and are frequently dependent on the scale and
context of the interactions (Harrison & Cornell 2008; Ricklefs 2008; Brooker et al. 2009).
Nonetheless, there are several classes of interactions among organisms that are found throughout
many habitats and ecosystems. Using these classes of interactions as a framework when studying an
ecological community allows scientists to describe naturally occurring processes and aids in
predicting how human alterations to the natural world may affect ecosystem properties and
processes.
At the coarsest level, ecological interactions can be defined as either intra-specific or inter-specific.
Intra-specific interactions are those that occur between individuals of the same species, while
interactions that occur between two or more species are called inter-specific interactions.
However, since most species occur within ecological communities, these interactions can be affected
by, and indirectly influence, other species and their interactions. The ones that will be discussed in
this article are competition, predation, herbivory and symbiosis. These are not the only types of
species interactions, just the most studied — and they are all parts of a larger network of
interactions that make up the complex relationships occurring in nature.
Competition
Competition is most typically considered the interaction of individuals that vie for a common
resource that is in limited supply, but more generally can be defined as the direct or indirect
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interaction of organisms that leads to a change in fitness when the organisms share the same
resource. The outcome usually has negative effects on the weaker competitors. There are three
major forms of competition. Two of them, interference competition and exploitation competition,
are categorized as real competition. A third form, apparent competition, is not. Interference
competition occurs directly between individuals, while exploitation competition and apparent
competition occur indirectly between individuals (Holomuzki et. al 2010) (Figure 1).
Figure 1: The three major types of competitive interactions.
Diagrams illustrating the three major types of competitive interactions where the dashed lines indicate indirect
interactions and the solid lines direct interactions that are part of ecological communities. C1 = Competitor #1, C2
= Competitor #2, P = Predator, R = Resource.
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Nature Education All rights reserved.
When an individual directly alters the resource-attaining behavior of other individuals, the
interaction is considered interference competition. For example, when a male gorilla prohibits
other males from accessing a mate by using physical aggression or displays of aggression, the
dominant male is directly altering the mating behavior of other males. This is also an example of an
intra-specific interaction. Exploitation competition occurs when individuals interact indirectly as
they compete for common resources, like territory, prey or food. Simply put, the use of the resource
by one individual will decrease the amount available for other individuals. Whether by interference or
exploitation, over time a superior competitor can eliminate an inferior one from the area, resulting in
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competitive exclusion (Hardin 1960). The outcomes of competition between two species can be
predicted using equations, and one of the most well known is the Lotka-Volterra model (Volterra
1926, Lotka 1932). This model relates the population density and carrying capacity of two species to
each other and includes their overall effect on each other. The four outcomes of this model are: 1)
species A competitively excludes species B; 2) species B competitively excludes species A; 3) either
species wins based on population densities; or 4) coexistence occurs. Species can survive together if
intra-specific is stronger than inter-specific competition. This means that each species will inhibit
their own population growth before they inhibit that of the competitor, leading to coexistence.
Another mechanism for avoiding competitive exclusion is to adopt alternative life history and
dispersal strategies, which are usually reinforced through natural selection. This mechanism reduces
competitive interactions and increases opportunities for new colonization and nutrient acquisition.
The success of this is often dependent upon events (such as tide, flood, or fire disturbances) that
create opportunities for dispersal and nutrient acquisition. Consider that Plant Species A is more
efficient than Plant Species B at nutrient uptake, but Plant B is a better disperser. In this example, the
resource under competition is nutrients, but nutrient acquisition is related to availability. If a
disturbance opens up new space for colonization, Plant B is expected to arrive first and maintain its
presence in the community until Plant A arrives and begins competing with Plant B. Eventually Plant
A will outcompete Plant B, perhaps by growing faster because Plant A is more efficient at nutrient
acquisition. With an increasing Plant A population, the Plant B population will decline, and given
enough time, can be excluded from that area. The exclusion of Plant B can be avoided if a local
disturbance (for example, prairie fires) consistently opens new opportunities (space) for
colonization. This often happens in nature, and thus disturbance can balance competitive
interactions and prevent competitive exclusion by creating patches that will be readily colonized by
species with better dispersal strategies (Roxburgh et al. 2004) (Figure 2). The success of the dispersal
versus nutrient acquisition trade-off depends, however, on the frequency and spatial proximity (or
how close they are) of disturbance events relative to the dispersal rates of individuals of the
competing species. Coexistence can be achieved when disturbances occur at a frequency or distance
that allows the weaker, but often better dispersing, competitor to be maintained in a habitat. If the
disturbance is too frequent the inferior competitor (better disperser) wins, but if the disturbance is
rare then the superior competitor slowly outcompetes the inferior competitor, resulting in
competitive exclusion. This is known as the intermediate disturbance hypothesis (Horn 1975,
Connell 1978).
Figure 2: The results of simulation models on the role disturbances play in maintaining species coexistence
between patches over time.
Schematics showing the results of simulation models on the role disturbances play in maintaining species
coexistence between patches over time. The black pixels represent a superior competitor with low dispersal ability
and grey pixels indicate an inferior competitor species with greater dispersal ability. The white indicates the extent
of each disturbance. Consistent disturbances may facilitate coexistence and prevent competitive exclusion.
© 2013
Nature Education Modified and reproduced with permission from Roxburgh et al. 2004. All
rights reserved.
Apparent competition occurs when two individuals that do not directly compete for resources affect
each other indirectly by being prey for the same predator (Hatcher et al. 2006). Consider a hawk
(predator, see below) that preys both on squirrels and mice. In this relationship, if the squirrel
population increases, then the mouse population may be positively affected since more squirrels will
be available as prey for the hawks. However, an increased squirrel population may eventually lead to
a higher population of hawks requiring more prey, thus, negatively affecting the mice through
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increased predation pressure as the squirrel population declines. The opposite effect could also
occur through a decrease in food resources for the predator. If the squirrel population decreases, it
can indirectly lead to a reduction in the mouse population since they will be the more abundant food
source for the hawks. Apparent competition can be difficult to identify in nature, often because of
the complexity of indirect interactions that involve multiple species and changing environmental
conditions.
Predation and Herbivory
Predation requires one individual, the predator, to kill and eat another individual, the prey (Figure
3). In most examples of this relationship, the predator and prey are both animals; however,
protozoans are known to prey on bacteria and other protozoans and some plants are known to trap
and digest insects (for example, pitcher plant) (Figure 4). Typically, this interaction occurs between
species (inter-specific); but when it occurs within a species (intra-specific) it is cannibalism.
Cannibalism is actually quite common in both aquatic and terrestrial food webs (Huss et al. 2010;
Greenwood et al. 2010). It often occurs when food resources are scarce, forcing organisms of the
same species to feed on each other. Surprisingly, this can actually benefit the species (though not
the prey) as a whole by sustaining the population through times of limited resources while
simultaneously allowing the scarce resources to rebound through reduced feeding pressure (Huss et
al. 2010). The predator-prey relationship can be complex through sophisticated adaptations by both
predators and prey, in what has been called an "evolutionary arms race." Typical predatory
adaptations are sharp teeth and claws, stingers or poison, quick and agile bodies, camouflage
coloration and excellent olfactory, visual or aural acuity. Prey species have evolved a variety of
defenses including behavioral, morphological, physiological, mechanical, life-history synchrony and
chemical defenses to avoid being preyed upon (Aaron, Farnsworth et al. 1996, 2008).
Figure 3: Crocodiles are some of the evolutionarily oldest and dangerous predators.
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Figure 4: A carnivorous pitcher plant.
A carnivorous pitcher plant that preys upon insects by luring
them into the elongated tube where the insects get trapped,
die and are then digested.
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Nature Education Courtesy of M. E. Benbow.
All rights reserved.
Another interaction that is much like predation is herbivory, which is when an individual feeds on all
or part of a photosynthetic organism (plant or algae), possibly killing it (Gurevitch et al. 2006). An
important difference between herbivory and predation is that herbivory does not always lead to the
death of the individual. Herbivory is often the foundation of food webs since it involves the
consumption of primary producers (organisms that convert light energy to chemical energy through
photosynthesis). Herbivores are classified based on the part of the plant consumed. Granivores eat
seeds; grazers eat grasses and low shrubs; browsers eat leaves from trees or shrubs; and frugivores
eat fruits. Plants, like prey, also have evolved adaptations to herbivory. Tolerance is the ability to
minimize negative effects resulting from herbivory, while resistance means that plants use defenses
to avoid being consumed. Physical (for example, thorns, tough material, sticky substances) and
chemical adaptations (for example, irritating toxins on piercing structures, and bad-tasting
chemicals in leaves) are two common types of plant defenses (Gurevitch et al. 2006) (Figure 5).
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Figure 5: Sharp thorns on the branch of a tree, used as
anti-herbivory defense.
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Nature Education Courtesy of M. E. Benbow.
All rights reserved.
Symbiosis: Mutualism, Commensalism and Parasitism
Symbiosis is an interaction characterized by two or more species living purposefully in direct
contact with each other. The term "symbiosis" includes a broad range of species interactions but
typically refers to three major types: mutualism, commensalism and parasitism. Mutualism is a
symbiotic interaction where both or all individuals benefit from the relationship. Mutualism can be
considered obligate or facultative. (Be aware that sometimes the term "symbiosis" is used
specifically to mean mutualism.) Species involved in obligate mutualism cannot survive without the
relationship, while facultative mutualistic species can survive individually when separated but often
not as well (Aaron et al. 1996). For example, leafcutter ants and certain fungi have an obligate
mutualistic relationship. The ant larvae eat only one kind of fungi, and the fungi cannot survive
without the constant care of the ants. As a result, the colonies activities revolve around cultivating
the fungi. They provide it with digested leaf material, can sense if a leaf species is harmful to the
fungi, and keep it free from pests (Figure 6). A good example of a facultative mutualistic relationship
is found between mycorrhizal fungi and plant roots. It has been suggested that 80% of vascular
plants form relationships with mycorrhizal fungi (Deacon 2006). Yet the relationship can turn
parasitic when the environment of the fungi is nutrient rich, because the plant no longer provides a
benefit (Johnson et al. 1997). Thus, the nature of the interactions between two species is often
relative to the abiotic conditions and not always easily identified in nature.
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Figure 6: Leaf cutter ants.
Leaf cutter ants carrying pieces of leaves back to the colony where the leaves will be used to grow a fungus that is
then used as food. The ants will make "trails" to an acceptable leaf source to harvest it rapidly.
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Nature Education Courtesy of M. E. Benbow. All rights reserved.
Commensalism is an interaction in which one individual benefits while the other is neither helped
nor harmed. For example, orchids (examples of epiphytes) found in tropical rainforests grow on the
branches of trees in order to access light, but the presence of the orchids does not affect the trees
(Figure 7). Commensalism can be difficult to identify because the individual that benefits may have
indirect effects on the other individual that are not readily noticeable or detectable. If the orchid
from the previous example grew too large and broke off the branch or shaded the tree, then the
relationship would become parasitic.
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Figure 7: Epiphytic bromeliads that grow on the limbs of large tropical rainforest trees.
The bromeliads benefit by occupying space on the limb receiving rain and sunlight, but do not harm the tree.
© 2013
Nature Education Courtesy of M. E. Benbow. All rights reserved.
Parasitism occurs when one individual, the parasite, benefits from another individual, the host,
while harming the host in the process. Parasites feed on host tissue or fluids and can be found
within (endoparasites) or outside (ectoparasites) of the host body (Holomuzki et al. 2010). For
example, different species of ticks are common ectoparasites on animals and humans. Parasitism is
a good example of how species interactions are integrated. Parasites typically do not kill their hosts,
but can significantly weaken them; indirectly causing the host to die via illness, effects on
metabolism, lower overall health and increased predation potential (Holomuzki et al. 2010). For
instance, there is a trematode that parasitizes certain aquatic snails. Infected snails lose some of
their characteristic behavior and will remain on the tops of rocks in streams where food is
inadequate and even during peaks of waterfowl activity, making them easy prey for the birds (Levri
1999). Further, parasitism of prey species can indirectly alter the interactions of associated
predators, other prey of the predators, and their own prey. When a parasite influences the
competitive interaction between two species, it is termed parasite-mediated competition (Figure
8). The parasite can infect one or both of the involved species (Hatcher et al. 2006). For example, the
malarial parasite Plasmodium azurophilum differentially infects two lizard species found in the Caribbean,
Anolis gingivinius and Anolis wattsi. A. gingivinius is a better competitor than A. wattsi but is susceptible to P.
azurophilum, while A. wattsi rarely contracts the parasite. These lizards are found coexisting only when
the parasite is present, indicating that the parasite lowers the competitive ability of A. gingivinius'
(Schall 1992). In this case, the parasite prevents competitive exclusion, therefore maintaining
species diversity in this ecosystem.
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Figure 8: Multiple conceptual models of species interactions that involve parasites.
The + and – indicate positive and negative influence, respectively, between resources, hosts, predators and
parasites.
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Nature Education Reproduced with permission from Hatcher et al. 2006. All rights reserved.
Summary
The species interactions discussed above are only some of the known interactions that occur in
nature and can be difficult to identify because they can directly or indirectly influence other intraspecific and inter-specific interactions. Additionally, the role of abiotic factors adds complexity to
species interactions and how we understand them. That is to say, species interactions are part of the
framework that forms the complexity of ecological communities. Species interactions are extremely
important in shaping community dynamics. It was originally thought that competition was the
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driving force of community structure, but it is now understood that all of the interactions discussed
in this article, along with their indirect effects and the variation of responses within and between
species, define communities and ecosystems (Agrawal 2007).
References and Recommended Reading
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Agrawal, A. A. et al. Filling Key Gaps in Population and Community Ecology. Frontiers in Ecology and the Environment 5, 145-152
(2007).
Brooker, R. W. et al. Don't Diss Integration: A Comment on Ricklefs's Disintegrating Communities. American Naturalist 174, 919-927
(2009).
Connell, J.H. Diversity in Tropical Rain Forests and Coral Reefs. Science 199 (4335): 1302-1310 (1978).
Deacon, J. in Fungal Biology Vol. 4, 256 (Blackwell Publishing, 2006).
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Gurevitch, J., Scheiner, S. M. & Fox, G. A. The Ecology of Plants Vol. 2, 257 (Sineaur Associates, Inc., 2006).
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Horn, H.S. Markovian Properties of Forest Succession. In Cody, M.L., and J.M. Diamond. Ecology and Evolution of Communities. Belknap
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