15.1: Community Ecology- Species Interactions
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Populations rarely, if ever, live in isolation from populations of other species. In most cases, numerous species share a habitat. The interactions between these populations play a major role in regulating population growth and abundance. All populations occupying the same habitat form a community: populations inhabiting a specific area at the same time. The number of species occupying the same habitat and their relative abundance is known as species diversity. Areas with low diversity, such as the glaciers of Antarctica, still contain a wide variety of living things, whereas the diversity of tropical rainforests is so great that it cannot be counted. Ecology is studied at the community level to understand how species interact with each other and compete for the same resources.
Competition
Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the a community, which describes how they acquire the resources they need, in what habitat they live, and how they interact with other species in the community (Figure \(\PageIndex{1}\)). In othere words, the niche includes all the ways that the species interacts with the biotic and abiotic factors of the environment.
Figure \(\PageIndex{1}\): Bird Niches. Each of these species of birds has a beak that suits it for its niche. For example, the long slender beak of the nectarivore allows it to sip liquid nectar from flowers. The short sturdy beak of the granivore allows it to crush hard, tough grains.
Competition – occurs when the biological demand for an ecological resource exceeds the supply, causing organisms to interfere with each other. Plants, for example, often compete for access to limited supplies of sunlight, water, nutrients, and space. Animals may compete for food, nesting sites, mates, and other resources. Intraspecific competition occurs when individuals of the same species vie for access to resources, while interspecific competition occurs between species. Because use of resources by one individual takes those resources away from another individual we think of competition as an interaction that has a negative effect on both individuals or species in the interaction. For example, if flying squirrels and a species of bird are both using tree cavities for nests, having more squirrels is bad for the birds and having more birds is bad for the squirrels.
If a species is particularly effective at co-opting resources to its own benefit, it may displace other species, a phenomenon known as competitive displacement (or in extreme cases, competitive exclusion). This affects the presence and relative abundance of species in the community. For example, sugar maple (Acer saccharum) is a highly competitive tree in hardwood forests of eastern Canada. Where environmental conditions are well suited for this species, it can dominate mature stands. If large sugar maple trees are removed from a stand, perhaps by a selective timber harvest, other tree species (as well as small sugar maples) will benefit from the reduced competition and will grow more vigorously.
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat indefinitely. In other words, different species cannot coexist forever in a community if they are competing for all the same resources. An example of this principle is shown in Figure \(\PageIndex{2}\), with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different microniches. These organisms coexist by minimizing direct competition.
Facilitation or Symbiosis
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the broader definition will be used.
A commensal relationship (commensalism) occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Many potential commensal relationships are difficult to identify because it is difficult to demonstrate that one partner is unaffected by the presence of the other. Birds nesting in trees provide an example of a commensal relationship (Figure \(\PageIndex{3}\)). The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators.

Another example or a commensal relationship involves the Little Blue Heron and the White Ibis, which are both wading birds. The Little Blue Heron catches more fish in the presence of the White Ibis, but the White Ibis is unaffected. Interestingly, Little Blue Herons attempt to catch fish more often in the presence of the species, but the success rate of their attempts does not change. Nevertheless, more frequent attempts still increases the total number of fish caught. The White Ibis may make fish more visible to Little Blue Herons, causing changes in their behavior (figure \(\PageIndex{4}\)).
Figure \(\PageIndex{4}\): The Little Blue Heron (left) and the White Ibis (right) have a commensal relationship. Image by Russ (CC-BY).
A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut (Figure \(\PageIndex{5}\)a). The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite.


Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria (Figure \(\PageIndex{5}\)b). As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae. Another set of mutualisms are pollinators, such as bees, butterflies, and hummingbirds. Polinators benefit because they eat the collect pollen and/or nectar that they collect from flowers. The plants also benefit because their pollen is dispersed to other plants, allowing them to reproduce. Both the pollinators and the plants benefit, demonstrating a mutualism (figure \(\PageIndex{6}\)).
Figure \(\PageIndex{6}\): A male Broad-Tailed Hummingbird visits a scarlet gilia flower at the Rocky Mountain Biological Laboratory. The hummingbird gains food (nectar) while aiding the gilia flower with reproduction. Image by David W. Inouye (CC-BY).
Predation and Herbivory
Perhaps the classical example of species interaction is the predator-prey relationship. Predation occurs when one species (the predator) kills and eats multiple prey over its lifetime. Nature shows on television highlight the drama of one living organism killing another. Populations of predators and prey in a community are not constant over time: in most cases, they vary in cycles that appear to be related. Population sizes of predators and prey in a community are not constant over time, and they may vary in cycles that appear to be related.
Figure \(\PageIndex{7}\): The Canada lynx is an example of a predator. Image by Michael Zahra (CC-BY-SA).
Figure \(\PageIndex{8}\): The snowshoe hare is the prey of the Canada lynx. Image by the National Park Service (public domain).
The most often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator, Figure \(\PageIndex{7}\)) and the snowshoe hare (prey, Figure \(\PageIndex{8}\)), using nearly 200 year-old trapping data from North American forests (Figure \(\PageIndex{8}\)). This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, they kill so many hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.

The idea that the population cycling of the two species is entirely controlled by predation models has come under question. More recent studies have pointed to undefined density-dependent factors as being important in the cycling, in addition to predation. One possibility is that the cycling is inherent in the hare population due to density-dependent effects such as lower fecundity (maternal stress) caused by crowding when the hare population gets too dense. The hare cycling would then induce the cycling of the lynx because it is the lynxes’ major food source. The more we study communities, the more complexities we find, allowing ecologists to derive more accurate and sophisticated models of population dynamics.
Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.
Adaptations of Predators
Predators have a variety of adaptations to catch and consume prey, and the specific adaptations depend on the predator. For example, raptors, such as owls and hawks, have hooked beaks for tearing flesh and talons for grabbing prey. Mammal predators often have sharp teeth and claws. Some predators can run quickly to chase prey, and others "sit and wait", lunging forward when prey pass them. Some predators, like rattlesnakes and tarantulas, subdue their prey by injecting them with venom (figure \(\PageIndex{9}\)). Predators often have large eyes located forward (like a wolf) rather than eyes spaced far apart on the sides of the head (like a sheep). Forward-facing eyes allow for depth perception, which is key to tracking prey. In contrast, peripheral vision is expanded when eyes are located to the sides, and this helps prey identify threats.
Figure \(\PageIndex{9}\): This Asian birdeater tarantula uses its fangs to inject its prey with poisonous venom. Image by Matt Reinbold (CC-BY-SA).
Defense Mechanisms against Predation and Herbivory
The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.
Prey evolve mechanical, chemical, physical, or behavioral defenses against predators. Some prey have armor (such as a turtle shell or the bony plates protecting armadillos), a mechanical defense that reduces predation by discouraging physical contact. Many animals produce or obtain chemical defenses from plants and store them to prevent predation. Other species use their body shape and coloration as camouflage to avoid being detected by predators, an example of a physical defense. The tropical walking stick is an insect with the coloration and body shape of a twig, which makes it very hard to see when it is stationary against a background of real twigs (figure \(\PageIndex{10}\)-a). In another example, the chameleon can change its color to match its surroundings (figure \(\PageIndex{10}\)-b).
Figure \(\PageIndex{10}\): (a) The tropical walking stick and (b) the chameleon use their body shape and/or coloration to prevent detection by predators. (credit a: modification of work by Linda Tanner; credit b: modification of work by Frank Vassen)
Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey (Figure \(\PageIndex{11}\). Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten. shows some organisms’ defenses against predation and herbivory.

Many species use their body shape and coloration to avoid being detected by predators, an example of a physical defense. The tropical walking stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary against a background of real twigs (Figure \(\PageIndex{12}\)a). In another example, the chameleon can change its color to match its surroundings (Figure \(\PageIndex{12}\)b). Both of these are examples of camouflage, or avoiding detection by blending in with the background.


Some species use coloration as a way of warning predators that they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemical, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration (Figure \(\PageIndex{13}\)). Warning coloration only works if a predator uses eyesight to locate prey and can learn—a naïve predator must experience the negative consequences of eating one before it will avoid other similarly colored individuals. For example, the monarch butterfly caterpillar sequesters poisons from its food (plants and milkweeds) to make itself poisonous or distasteful to potential predators. The caterpillar is bright yellow and black to advertise its toxicity. The caterpillar is also able to pass the sequestered toxins on to the adult monarch, which is also dramatically colored black and red as a warning to potential predators.

While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation (Figure \(\PageIndex{14}\)).


In Müllerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. Figure \(\PageIndex{15}\) shows a variety of foul-tasting butterflies with similar coloration.

Figure \(\PageIndex{15}\): Several unpleasant-tasting Heliconius butterfly species share a similar color pattern with better-tasting varieties, an example of Müllerian mimicry. (credit: Joron M, Papa R, Beltrán M, Chamberlain N, Mavárez J, et al.)
Running from predators, hiding, and playing dead are examples of behavioral defenses. Some prey also exhibit behaviors that threaten predators. For example, snapping turtles stretch their legs to appear larger and snap aggressively at predators.
Parasitism
A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits, but the organism being fed upon, the host, is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle by spreading to another host.
The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed (Figure \(\PageIndex{16}\)). The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is bringing into its gut by eating, and may grow to be over 50 ft long by adding segments. The parasite moves from species to species in a cycle, making two hosts necessary to complete its life cycle. Another common parasite is Plasmodium falciparum, the protozoan cause of malaria, a significant disease in many parts of the world. Living in human liver and red blood cells, the organism reproduces asexually in the gut of blood-feeding mosquitoes to complete its life cycle. Thus malaria is spread from human to human by mosquitoes, one of many arthropod-borne infectious diseases.

Disease – is a pathological relationship in which the health of plants or animals suffers from an infestation of another species, usually a microbe. Virulent diseases can cause enormous changes in the composition of ecological communities. In the early 1900s, the American chestnut (Castanea dentata) was afflicted by chestnut blight (Endothia parasitica), an introduced fungal pathogen. Because chestnuts have little immunity to this disease, the species was virtually eliminated from the forests of eastern North America by the 1950s. This change released other tree species from competition with the previously dominant chestnut, and they quickly filled gaps in the canopy created by its demise.
Contributors and Attributions
Modified by Kyle Whittinghill and Melissa Ha from
- 45.6: Community Ecology by OpenStax, is licensed CC BY by
Connie Rye (East Mississippi Community College), Robert Wise (University of Wisconsin, Oshkosh), Vladimir Jurukovski (Suffolk County Community College), Jean DeSaix (University of North Carolina at Chapel Hill), Jung Choi (Georgia Institute of Technology), Yael Avissar (Rhode Island College) among other contributing authors. Original content by OpenStax (CC BY 4.0; Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87).
- Community Ecology from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)
- 6.2: Ecosystems by CK-12: Biology Concepts, is licensed CC BY-NC
- Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0.
- Ecology: From Individuals to the Biosphere from Environmental Science: A Canadian Perspective by Bill Freedman