16. Community Interactions


16.3. Kinds of Organism Interactions


One of the important components of an organism’s niche is the other living things with which it interacts. Some interactions are harmful to one or both of the organisms, whereas other interactions are beneficial. Ecologists have classified the kinds of interactions between organisms into broad categories.



Competition is an interaction between organisms in which both organisms are harmed to some extent. This is the most common kind of interaction among organisms. Organisms are constantly involved in competition. Competition occurs whenever two organisms need a vital resource that is in short supply (figure 16.5). The vital resource may be such things as soil nutrients, sunlight, or pollinators for plants; or food, shelter, nesting sites, water, mates, or space for animals.



FIGURE 16.5. Competition

Whenever a needed resource is in limited supply, organisms compete for it. Competition between members of the same species is called intraspecific competition. (a) Intraspecific competition for sunlight among these pine trees has resulted in the tall, straight trunks. Those trees that did not grow fast enough died. Competition between different species is called interspecific competition. (b) The lion and vultures are competing for the lion’s zebra kill.


Intraspecific competition takes place between members of the same species. It can involve a snarling tug-of-war between two dogs over a scrap of food or a silent struggle between pine seedlings for access to available light. Interspecific competition occurs between members of different species. The interaction between weeds and tomato plants in a garden is an example of interspecific competition. If the weeds are not removed, they compete with the tomatoes for available sunlight, water, and nutrients, resulting in poor growth of both the tomatoes and weeds. Similarly, there is interspecific competition among species of carnivores (e.g., hawks, owls, coyotes, foxes) for the small mammals and birds they use for food. Competition does not necessarily involve a face-to-face confrontation. For example, if a coyotes kills and eats a rodent, it has had a competitive effect on foxes, hawks, and other carnivores as well as other members of its own species, because there is now one less rodent available to be caught and eaten by others.


Competition and Natural Selection

Competition is a powerful force for natural selection. Although competition results in harm to both organisms, there can still be winners and losers. The two organisms may not be harmed to the same extent, with the result that one gains greater access to the limited resource. Biologists have recognized that, the more similar the requirements of two species of organisms, the more intense the competition between them. This has led to the development of a general rule known as the competitive exclusion principle. According to the competitive exclusion principle, no two species of organisms can occupy the same niche at the same time. If two species of organisms do occupy the same niche, the competition will be so intense that one or more of the following processes will occur: (1) one of the two species will become extinct, (2) one will migrate to a different area where competition is less intense, or (3) the two species will evolve into slightly different niches, so that the intensity of the competition is reduced. For example, a study of the feeding habits of several kinds of warblers shows that, although they live in the same place and feed on similar organisms, their niches are slightly different, because they feed in different places on trees (figure 16.6).



FIGURE 16.6. Niche Specialization

Although all of these warbler species have similar feeding habits, they limit the intensity of competition by feeding on different parts of the tree.


Another example involves the competition of various species of flowering plants for pollinators. Some have bright red tubular flowers that are attractive to hummingbirds. Some have foul odors that attract flies or beetles. Others are open only at night and are pollinated by moths or bats. A few kinds of orchid flowers mimic female wasps and are pollinated when the male wasp tries to mate with the fake female wasp. Many flowers attract several kinds of bees, butterflies, or beetles, but the flowers open only at certain times of the day. All of these differences are niche specializations that reduce competition for pollinators.



Predation is an interaction in which one animal captures, kills, and eats another animal. The organism that is killed is the prey, and the one that does the killing is the predator. Predators benefit from the relationship because they obtain a source of food; obviously, prey organisms are harmed. Most predators are relatively large, compared to their prey, and have specific adaptations to aid them in catching prey. There are many different styles of predation. Many predators, such as leopards, lions, cheetahs, hawks, squid, sharks, and salmon, use speed and strength to capture their prey. Dragonflies, bats, and swallows use a technique that involves flying around in an area where they can capture flying insects. Predators such as frogs, many kinds of lizards, and insects (e.g., praying mantis) blend in with their surroundings and strike quickly when a prey organism happens by. Wolf spiders and jumping spiders have large eyes, which help them find prey, which they pounce on and kill. The webs of other kinds of spiders serve as nets to catch flying insects. The prey are quickly paralyzed by the spider’s bite and wrapped in a tangle of silk threads (figure 16.7). Many kinds of birds, insects, and mammals simply search for slow-moving prey, such as caterpillars, grubs, aphids, slugs, snails, and similar organisms. Many kinds of marine snails and starfish are predators of other slow-moving sea creatures.

Often predators are useful to humans because they control populations of organisms that do us harm. For example, snakes eat rats and mice that eat stored grain and other agricultural products. Birds and bats eat insects that are agricultural pests.



FIGURE 16.7. The Predator-Prey Relationship

(a) Many predators, such as lions and cheetahs, use speed and strength to capture prey. (b) Other predators, such as frogs and chameleons, blend in with their surroundings, lie in wait, and ambush their prey. (c) Some spiders use nets to capture prey. Obviously, predators benefit from the food they obtain, to the detriment of the prey.


It is even possible to think of a predator as having a beneficial effect on the prey species. Certainly, the individual organism that is killed is harmed, but the population can benefit. Predators can control the size of a prey population and thus, prevent large populations of prey organisms from destroying their habitat or they can reduce the likelihood of epidemic disease by eating sick or diseased individuals. Furthermore, predators act as selecting agents. The individuals that fall prey to them are likely to be less well-adapted than the ones that escape predation. Predators usually kill slow, unwary, poorly hidden, sick, or injured individuals. Thus, the genes that may have contributed to slowness, inattention, poor camouflage, illness, or the likelihood of being injured are removed from the gene pool.


Symbiotic Relationships

Symbiosis means “living together.” Unfortunately, this word is used in several ways, none of which is very precise. However, the term symbiosis is usually used for interactions that involve a close physical relationship between two kinds of organisms. The three kinds of relationships discussed in the following sections—parasitism, commensalism, and mutualism—are often referred to as symbiotic relationships because they usually involve organisms that are physically connected to one another.



In parasitism, one organism lives in or on another living organism, from which it derives nourishment. The parasite derives benefit and harms the host, the organism it lives in or on. Parasites are smaller than their hosts. In general, they do not kill their host quickly but, rather, use it as a source of food for a long time. However, the parasite’s activities may weaken the host so that it eventually dies. Parasitism is a very common kind of interrelationship. Nearly every category of living thing has species that are parasites. There are parasitic bacteria, fungi, protozoa, plants, fish, insects, worms, mites, and ticks. In fact, there are more species of parasites in the world than there are nonparasites.

There are many styles of parasitism. Parasites that live on the outside of their hosts are called external parasites. For example, ticks live on the outside of the bodies of animals, such as rats, turtles, and humans, where they suck blood and do harm to their hosts. Internal parasites live on the inside of their hosts. For example, tapeworms live in their hosts’ intestines. Several kinds of plants are parasitic; mistletoe invades the tissues of the tree it is living on to derive its nourishment. Some flowering plants, such as beech drops and Indian pipe, lack chlorophyll and are not able to do photosynthesis. They derive their nourishment by obtaining nutrients from the roots of trees or soil fungi and grow aboveground for a short period when they flower. Indian pipe is interesting in that it is parasitic on the fungi that assist tree roots in absorbing water. The root fungi receive nourishment from the tree and the Indian pipe obtains nourishment from the fungi. So, the Indian pipe is an indirect parasite on trees (figure 16.8).



FIGURE 16.8. The Parasite-Host Relationship

Parasites benefit from the relationship because they obtain nourishment from the host. (a) Tapeworms are internal parasites in the guts of their host, where they absorb food from the host’s gut. (b) The tick is an external parasite that sucks body fluids from its host. (c) Indian pipe (Monotropa uniflora) is a flowering plant that lacks chlorophyll and is parasitic on fungi that have a mutualistic relationship with tree roots. The host in any of these three situations may not be killed directly by the relationship, but it is often weakened, becoming more vulnerable to predators and diseases.


Many kinds of fungi are parasites of plants, including commercially valuable plants. Farmers spend millions of dollars each year to control fungus parasites. Many kinds of insects, worms, protozoa, bacteria, and viruses are important human parasites.

Many parasites have extremely complicated life cycles (chapter 23 discusses the life cycle of several kinds of worm parasites). In many of these life cycles, some species carry the parasite from one host to the next. Such a carrier organism is known as a vector. For example, the protozoan that causes malaria is carried from one human to another by certain species of mosquitos, and the bacterium Borrelia burgdorferi, which causes lyme disease, is carried by certain species of ticks (figure 16.9).



FIGURE 16.9. Lyme Disease—Hosts, Parasites, and Vectors

Lyme disease is a bacterial disease originally identified in a small number of people in the Old Lyme, Connecticut, area. Today, it is found throughout the United States and Canada. The parasite Borrelia burgdorferi, is a bacterium that can live in a variety of mammalian hosts (e.g., humans, mice, horses, cattle, domestic cats, and dogs). Certain ticks are vectors that suck blood from an infected animal and carry the disease to another animal when the tick feeds on it.


Special Kinds of Predation and Parasitism

Both predation and parasitism are relationships in which one member of the pair is helped and the other is harmed. But there are many kinds of common interactions in which one is harmed and the other aided that don’t fit neatly into the categories of interactions dreamed up by scientists. For example, when a deer eats the leaves off a tree or a goose eats grass, they are doing harm to the plant they are eating while deriving a benefit. In essence, these herbivores are plant predators or parasites (figure 16.10). In aquatic habitats there are many kinds of organisms (sponges, clams, barnacles, shrimp, etc.) that live as filter-feeders. They are essentially grazers on tiny organisms in the water around them. Most consume a mixture of algae and tiny animals, but they consume the entire organism and can be considered a kind of predator. In addition, there are many animals, such as mosquitoes, biting flies, vampire bats, and ticks, that take blood meals but don’t usually live permanently on the host or kill it. Are they temporary parasites or specialized predators?



FIGURE 16.10. Special Kinds of Predation and Parasitism

Herbivores have a relationship with plants that is very similar to that of carnivores with their prey and parasites with their hosts. The herbivores are aided and the plants they feed on are harmed. Mosquitoes and other kinds of blood-sucking animals can be considered temporary parasites or predators. They do harm to the animal they feed on and benefit from the relationship.


Finally, birds such as cowbirds and some species of European cuckoos do not build nests but, rather, lay their eggs in the nests of other species of bird, which raise these foster young rather than their own. The adult cowbird and cuckoo often remove eggs from the host nest. In addition, cowbird and cuckoo offspring typically push the hosts’ eggs or young out of the nest. For these reasons, typically only the cowbird or cuckoo chick is raised by the foster parents. This kind of relationship has been called nest parasitism. The surrogate parents (hosts) are harmed, and the cowbird or cuckoo is aided by having others expend the energy needed to raise its young (figure 16.11).




FIGURE 16.11. Nest Parasitism

This cowbird chick in the nest is being fed by its host parent, a yellow warbler. The cowbird chick and its cowbird parents both benefit but the host is harmed because it is not raising any of its own young.



Commensalism is a relationship in which one organism benefits and the other is not affected. For example, sharks often have another fish, the remora, attached to them. The remora has a sucker on the top side of its head, which allows it to attach to the shark for a free ride (figure 16.12a). Although the remora benefits from the free ride and by eating leftovers from the shark’s meals, the shark does not appear to be troubled by this uninvited guest, nor does it benefit from its presence.

Another example of commensalism is the relationship between trees and epiphytic plants. Epiphytes are plants that live on the surface of other plants but that do not derive nourishment from them (figure 16.12b ) . Many kinds of plants (e.g., orchids, ferns, and mosses) use the surfaces of trees as places to live. These kinds of organisms are particularly common in tropical rainforests. Many epiphytes derive a benefit from the relationship because they are able to be located in the tops of the trees, where they receive more sunlight and moisture. The trees derive no benefit from the relationship, nor are they harmed; they simply serve as support surfaces for the epiphytes.



FIGURE 16.12. Commensalism

In commensalism, one organism benefits and the other is not affected. (a) The remora shown here hitchhikes a ride on the shark. It eats scraps of food left over from the shark’s messy eating habits. The shark does not seem to be hindered in any way. (b) The grey Spanish moss hanging on this oak tree is a good example of an epiphyte. The spanish moss does not harm the tree but benefits from using the tree surface as a place to grow.



Mutualism is an interrelationship in which two species live in close association with one another, and both benefit. Many kinds of animals that eat plants have a mutualistic relationship with the bacteria and protozoa that live in their guts. One of the major components of plant material is the cellulose material that makes up the cell wall. Most animals are unable to digest cellulose but rely on a collection of microorganisms in their guts to perform that function. For example, mammals such as cows, goats, camels, giraffes, and sheep have specialized portions of their guts, called a rumen, in which microorganisms live. These microbes produce enzymes, known as cellulases, that break down the cellulose in the food the animal eats. The microorganisms benefit because the gut provides them with a moist, warm, nourishing environment in which to live. The animals benefit because the breakdown of cellulose provides nutrients the animal could not get otherwise. Termites, plant-eating lizards, and many other kinds of herbivores have similar relationships with the bacteria and protozoa living in their digestive tracts which help them digest cellulose.

Lichens and corals exhibit a more intimate kind of mutualism. The bodies of lichens and corals are composed of the intermingled cells of two different kinds of organisms. Lichens consist of fungal cells and algal cells in a partnership; corals consist of the cells of the coral organism intermingled with algal cells. In both, the algae carry on photosynthesis and provide nutrients, and the fungus or coral provides a moist, fixed structure for the algae to live in.

Another kind of mutualistic relationship exists between flowering plants and insects. Bees and other insects visit flowers to obtain nectar from the blossoms (figure 16.13). Usually, the flowers are constructed so that the bees pick up pollen (sperm-containing packages) from the plant on their hairy bodies, which they transfer to the female part of the next flower they visit. Because bees normally visit many individual flowers of the same species for several minutes and ignore other species of flowers, they can serve as pollen carriers between two flowers of the same species. Plants pollinated in this manner produce less pollen than do plants that rely on the wind to transfer pollen. This saves the plant energy, because it doesn’t need to produce huge quantities of pollen. It does, however, need to transfer some of its energy savings into the production of showy flowers and nectar to attract the bees. The bees benefit from both the nectar and the pollen; they use both for food.



FIGURE 16.13. Mutualism

Mutualism is an interaction between two organisms in which both benefit. (a) The British soldier lichen in this photograph consists of a mutualistic association between a fungus and an alga. (b) Ruminant animals have a mutualistic relationship with the microorganisms in their gut that helps them obtain nutrients from the plants they eat. (c) Insects obtain nectar from plants; the plants benefit by being pollinated. (Note the yellow pollen on the bee.)


Some plants use birds, bats, mice, beetles, flies, and other kinds of organisms to get their pollen distributed. Each kind of flower is specialized to the kind of pollinating animal. Flowers that are pollinated by bats flower at night; and many of those that are pollinated by hummingbirds have long, tubular flowers. Table 16.1 summarizes features of these various kinds of organism interactions.

Another way in which plants and animals participate in a mutually beneficial relationship is in the production and consumption of fruit. The fruit that plants produce contains its seeds. The fruit is attractive to animals that eat it. When the seeds pass through the gut of the animal, they are typically deposited some distance from the plant that produced the fruit. Similarly animals that bury fruits typically carry the fruit away from the plant that produced it.


TABLE 16.1. Summary of Kinds of Organism Interactions


Kinds of Interaction Organism 1

Organism 2





Usually one is harmed more than the other.


Predator benefited

Prey harmed

Predators have special adaptations for capturing prey. Prey organisms have adaptations to avoid predators.


Parasite benefited

Host harmed

Usually the host and parasite are in physical contact.


Commensal benefited

Host unaffected

Usually the host and commensal are in physical contact.




Usually the two organisms are in physical contact.


Although they may eat some of the seeds, animals like squirrels do not find all of the fruits they buried and thus are involved in planting new plants. Therefore, animals serve as dispersal agents for the seeds of the plant. The plant encourages this activity by providing a nutritious fruit for the animal.



5. What do parasites, commensal organisms, and mutualistic organisms have in common? How are they different?

6. Describe two situations in which competition can involve combat and two that do not involve combat.

7. How is the competitive exclusion principle related to the theory of natural selection?

8. In what way are predators and parasites similar in the way they interact with other species? How do they differ in how they interact with other species?