If two organisms are in competition for the same resources, they are said to be competitors. One organism may be unsuccessful, and if it can’t find an alternative source for that resource, it may grow more slowly, or it may die.

Intraspecific competition is competition between individuals of the same species. A lion may compete with other lions for the same herd of antelope.

For example, lions and hyenas often compete for the same food source.

Interactions between predators and prey are important in many communities. If a predator eats one or more organisms, the size of the prey population decreases. If the community is to remain stable, the numbers of predators and prey need to be balanced.

Several different adaptations are beneficial to prey. Mimicry is an evolved resemblance to another organism. You may be familiar with Batesian mimicry, named after an English nineteenth century naturalist. In Batesian mimicry, an animal resembles another animal, called the model, that is unpalatable to the predator.

Several insect species mimic the yellow banded pattern of wasps. Once the predator has been stung by a “real” wasp, it may be wary of trying to eat anything that looks like it.

A second type of mimicry is named for the German naturalist Fritz Müller. Müllerian mimicry is the convergence, over evolutionary time, of the appearance of two unpalatable species. A number of tropical butterfly species exhibit Müllerian mimicry. For example, these butterflies belonging to two different families have very similar markings. Both kinds are poisonous to predators. By converging in this way, predators learn more quickly to avoid butterflies with this color pattern, thereby reducing the mortality rate of both kinds of butterflies.

Animals may also mimic their environment. A classic example is the English pepper moth Biston betularia. These moths fly at night and rest by day on tree trunks. There are both light-colored and dark-colored varieties of pepper moths.

In areas far from industrial activity, the trunks of trees are encrusted with lichens. As you can see, the light-colored moth it is almost invisible against its background. In these areas, light-colored moths are much more prevalent than dark-colored moths.

During the early Industrial Revolution, environmental pollution killed much of the lichens on trees. The dark-colored moths now had the advantage, being nearly invisible to predators against the black sooty deposits that covered much of the countryside.

See if you can identify which organisms are Batesian mimics and which are Müllerian mimics on this screen.

Drag the labels over the pair to which you think they refer.

Correct Answer: That’s right!

Incorrect answer: That’s not quite right.

All: The hawkmoth larva and king snake are both Batesian mimics. Their resemblance to a dangerous animal helps them avoid being eaten. The sea slug species are Müllerian mimics: All are unpleasant to eat. By sharing a similar color pattern, there is a greater chance that a predator will already have learned not to eat an animal with that pattern.

In addition to predator-prey interactions, some organisms live together in contact. This kind of interaction is called symbiosis, and the smaller of the two organisms that take part in this kind of relationship is called a symbiont.

Parasites, like the fish tapeworm living in this bear’s intestines, are symbionts that absorb nutrients from the body tissues or fluids of living prey, called hosts.

When the bear deposits feces, the tapeworm’s eggs get into the water, where they develop into a free-swimming form that infects tiny crustaceans. When the crustaceans are eaten by fish, the tapeworm’s larva makes its way to the fish’s muscles. When a bear or a human eats the fish raw, the larva embeds into the wall of the host’s small intestine and feeds on the nutrients that pass through.

In host-parasite interactions like this example, one organism is usually attached to a second organism that is harmed in the process. Internal parasites like tapeworms and roundworms are called endoparasites.

In contrast, ticks and aphids are external parasites, or ectoparasites.

Deer tick’s eggs develop into larvae that look like tiny adult ticks. Each larva climbs the stem of a plant and waits for a small host animal, such as a rabbit or mouse, to brush against the plant. The larva falls onto the animal and sucks its blood for 2 to 3 days, then drops off and molts, forming a larger nymph. The nymph repeats the process, and when it falls off the host, it develops into an adult tick. The adult often feeds on a larger host like a deer, where adult male and female ticks may meet and mate. When the female falls from the host, she lays her eggs, and the cycle begins again.

Host-parasite interactions are partnerships that favor only one of the partners. But associations called mutualisms are beneficial to both partners. These two lichens are good examples.

A lichen is a symbiotic organism made up of a fungus and an alga. The fungus provides structure and absorbs nutrients and water, while the alga carries out photosynthesis and provides the chemical products to the fungus.

Lichens can take the form of encrustations on rocks, like the species on the left, or they can be free growing and plantlike in form, like the species on the right.

You learned that mitochondria are thought to have been derived from prokaryotes that infected other cells in the evolution of eukaryotic organisms—another example of mutualism.

Some plants, such as legumes like peas and beans, have bacteria living in their root systems. These bacteria invade the root hairs, forming tumorlike lumps or nodules. The bacteria receive protection and nutrients from the plant and supply the plant with the nitrogen it needs for growth.

Commensalism is a form of symbiosis that benefits only one partner, but doesn’t help or harm the host. One example is the cattle egret of the tropics, which lives around large grazing mammals like this cape buffalo. The egret catches insects that are attracted to the buffalo and its droppings, but doesn’t seem to give the buffalo anything in return.

Amensalism is the harming of one organism by another, without the organism benefiting from the encounter. For example, large animals may trample plants, destroying them for no gain. Some crab species dig into the mud on the seashore, disturbing the burrows of worms and mollusks.

Two species of barnacle grow along the Scottish coastline. One species is typically found higher up the rocks, in an area that's covered only at high tide. The other species is found lower down, toward the low-tide mark.

The swimming larvae of the barnacles can settle and mature on any part of the rocks, so why are the species separated?

Species A is less resistant to drying out, so it prefers the area near the low-tide mark. Species B can withstand several hours of dryness, so it is better able to colonize the area nearer the high-tide mark.

When scientists removed species A from its preferred area, species B was able to colonize the low-tide area.

What was the relationship between the two species of barnacle—mutualism, competition, or commensalism? Click on the relationship that you think is correct.

Correct answer: That’s right!

Incorrect answer: That’s not quite right!

All: In mutualism and commensalism, one or both partners benefit. In our example, neither species of barnacle benefits from the other. Competition can restrict the distribution of organisms, and that is what happens. Species B is normally unable to colonize the low tide area because of the presence of species A. If species A is removed, there is no competition, and species B can proliferate.

We’ve looked at some of the ways organisms can interact, sometimes for mutual benefit but often to the advantage of just one of the species.

Now we’ll explore how communities can interact as a whole.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education