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9.1: Species Interactions

Difficulty Level: At Grade Created by: CK-12

How do different species affect one another?

As you can see from what you've read so far, and from what you've observed in your own environment, no organism lives by itself. Plants are crowded out by other plants, eaten by animals, used for shelter, stepped on, and uprooted. Animals are working hard looking for food, watching out for other animals that want to eat them, and trying to find something to use for shelter. In other words, every plant and every animal interacts with other plants and animals in some way. This section will introduce some different ways animals and plants interact in the environment.

“We need the tonic of wilderness to wade sometimes in marshes where the bittern and the meadow-hen lurk, and hear the booming of the snipe; to smell the whispering sedge where only some wilder and more solitary fowl builds her nest, and the mink crawls with its belly close to the ground.”

Henry David Thoreau

quoted in The Earth speaks

Ecologists spend a lot of time trying to understand interactions between organisms. They try to take the point of view of each organism in the interaction to see if it is good or bad for that organism. For example, if they see a spider catch a fly in its web, the ecologist observes that the interaction is good for the spider, because the spider gets to have dinner. On the other hand, they observe that the interaction is not very good for the fly, because the fly is becoming the spider's dinner! Sometimes one organism wins while the other loses. Sometimes both organisms lose. And sometimes interactions benefit both organisms.

Did You Know?

Two species of barnacles compete for space on rocks in tide pools in Scotland. One barnacle actually crowds the other off the rocks, undercutting it and replacing it even where it had begun to grow.

Organisms Compete

Ecologists define competition as what happens when one organism uses a resource in such a way that another organism can't use it. For example, if two species of birds eat the same kind of seeds, they may be competing with each other to get the most seeds. One species of tree may compete for water with another by growing roots that spread out over a larger area or go down farther. Competition usually occurs only when a resource is in short supply. For example, trees may compete for water but not for carbon dioxide and oxygen. There's usually plenty of carbon dioxide and oxygen to go around.

How Do I Interact With Other Species? List ten ways you interact directly with other species in your environment. Are these generally harmful or helpful for the other species? For example, do you provide a habitat for animals in your backyard? List ten ways you affect and are affected by other species on a global level. Are these harmful or helpful interactions for the other species? How could you make these more positive interactions?

Figure 8.1 Competition occurs when two species or individuals try to gain control of the same limited resource. Who do you think is competing in this picture? Who do you think will win?

Sometimes organisms compete without ever directly interacting with one another. They do this by using the same resource that happens to be in short supply. For example, squirrels and jays both like to eat acorns. If the squirrels manage to gather all of the acorns in an area and store them before the jays can find them, the squirrels and the jays are competing (and the squirrels are winning).

Did You Know?

A scientist in Chicago put two kinds of flour beetles in a container of flour to study competition between species. One species always out competed the other. Which one survived depended on the conditions. One species survived if it was hot and damp. The other species survived if it was cool and dry.

Another example of this type of competition occurs between two types of trout that live in the Ausable River in Michigan. Both species of trout like to rest on the bottom of streambeds, but they rest in the same location. The brook trout like to rest in shady areas of the stream where the water isn't flowing very fast. However, when brown trout are present, they take the spots and the brook trout give them up. In this case, the two types of trout compete for space in the river. And often they don't even come in contact with each other.

Humans compete for space, too. The central plains of the United States were once a vast prairie that supported many diverse species of plants and animals. However, when large numbers of humans arrived in the area, they cleared the native grasses to plant crops. They killed or drove away native animals and fenced off grazing areas for cattle and sheep. The humans, their crop plants, and domestic animals took over the resources of the plains area to meet their own needs. As a result, these resources became unavailable to other species.

What Do You Think?

Humans are usually pretty good at competing with other species for resources. Does this mean that humans have a right to use any resources they can get in any way that they think fit? Explain your answer.

Did You Know?

How does grass keep growing back when grazing animals keep eating it? Unlike most plants that grow from the tips, new grass tissue grows from the base of the plant. So when the top of the plant is cut off, the grass continues to grow.

Figure 8.2 Hummingbirds compete for sources of nectar by guarding distinct territories.

Sometimes competition occurs when two organisms actually fight to gain control of the resources within a certain area. For example, hummingbirds eat nectar from flowers. They may guard several flowers and chase away any other hummingbirds that try to feed from their flowers. Sometimes the hummingbirds carry this behavior so far that they even chase away moths, which try to sip nectar at “their” flowers.

Did You Know?

Owls are very successful predators partly because of their silent flight. The leading flight feathers of their wings form a cushion that reduces the noise they make as they slice through the air.

Another example of this sort of competition happens between the common loon and people who fish in the loons' lake. Loons eat large amounts of fish. Sometimes humans who fish feel that the loons are eating the fish that they would like to catch. They get angry about the lost catch and shoot the loons. This is illegal, and more than one person has been arrested for trying to stop loons from eating fish. Generally, in this and other kinds of competitive interactions, one species wins and the other loses.

Figure 8.3 A lynx preys on hares. The graph in Figure 8.4 shows the effect lynx and hares have on each other.


Another sort of species interaction in which one species wins and the other loses is when a predator eats its prey. This interaction, called predation (preh-DAY-shun), occurs everywhere. When a cat catches a bird, that is predation. When an owl catches a mouse or a jay eats the eggs of another bird, that is also predation. Some ecologists even claim that a deer eating grass is a kind of predation.

One of the most famous examples of the interaction between predators and their prey is the story of lynx and hares in northern Canada. Lynx are cats that eat snowshoe hares (rabbit-like animals). Both of these species have been hunted for their fur. For more than a hundred years hunters have been trapping lynx and hares and selling their pelts to the Hudson Bay Company. Throughout that time the company has kept careful records of the number of pelts they bought each year.

Figure 8.4 The lines on this graph show the changing abundance of lynx and hares in northern Canada during the years 1845-1935.

Ecologists realized that the Hudson Bay Company records were great sources of information on lynx and hare populations. The ecologists assumed that the number of pelts reflected how well the lynx and hare populations were doing. So they graphed the numbers of each type of pelt bought each year. What they discovered is shown in Figure 8.4.

Figure 8.5 American bitterns are masters of camouflage.

Look at the graph in Figure 8.4. The dotted line represents the number of hares caught each year, and the solid line represents the number of lynx caught. You can see that the numbers move up and down in cycles. For example, in 1869 almost no hares were caught. But over 100,000 were caught only six years later in 1875. This data shows that the number of hares can go up and down dramatically.

The interesting thing is that the number of lynx closely follows the number of hares. When there are a lot of hares, there are also a lot of lynx. This pattern makes sense because lynx eat the hares. When there are a lot of hares around, the lynx can easily feed themselves and their kittens. So the number of lynx increases. On the other hand, in bad years when there are few hares to be caught, not many lynx kittens survive and the number of lynx goes down.

What helps a prey animal avoid being eaten by a predator? One answer is camouflage. Camouflage is the color, markings, or body shape that helps to hide an animal or plant in its surroundings. With camouflage, an animal (or plant) can blend in with its surroundings. Blending in keeps the predator from easily seeing its prey. You can probably think of many examples. Insects seem to be the masters of camouflage. Moths often look like the bark of the trees upon which they rest. Katydids look like leaves when they fold up their wings. Walking sticks look like little sticks, and you would never know that they were insects unless you saw them move.

Larger animals use camouflage, too. One example is the American bittern. The bittern is a big brown and white wading bird that likes to hunt in the plants growing at the edge of freshwater lakes. Bitterns have long brown streaks that run up their white necks. They stand with their beaks in the air so that the brown streaks line up with the plants around them. To top it off, the bitterns sway with the breeze like the plants around them. Some animals such as the stoat, a mink-like animal, change color in different seasons. In winter, where it snows, the stoat changes from its normal brown and black to white except the black end of its tail.

Did You Know?

Monarchs are poisonous to birds because monarch caterpillars feed on milkweed. Milkweed has a toxic compound in it that remains in the monarchs after they emerge as adults.

Figure 8.6 Viceroy butterflies mimic monarchs. What differences can you find between the monarch and viceroy butterflies?

Mimicry is another way that prey fool their predators. In mimicry, one species of organism looks like another species. One classic example is the viceroy butterfly. Viceroy butterflies look very much like monarch butterflies, which are the big orange and black butterflies that you may see flying south every fall. But why would it help a viceroy butterfly to look like a monarch butterfly? Birds like to prey on butterflies. But they don't prey on monarch butterflies because monarchs are toxic to birds. In fact, if a bird does eat a monarch it vomits almost immediately. This situation works to the advantage of the viceroy. Birds, which have learned not to try to eat a monarch, will avoid viceroys, too. And this relationship may be beneficial to the monarch butterfly. Viceroys, too, are distasteful to birds. Consequently, if a bird eats either a monarch or a viceroy it will learn to leave any big orange and black butterfly alone!

Figure 8.7 Flower flies mimic another flying insect. Which other insect do you think the flower fly is mimicking? Why do you think this might be a good insect to mimic?

Another example of mimicry is the flower fly, which is shown in Figure 8.7. It is actually a fly and a lot like the ones that buzz around garbage on a hot summer day. But it is black with bright yellow stripes. What do you think the flower fly is mimicking and why? What might be the advantage of this mimicry for the flower fly?

Did You Know?

Many parasites actually live inside their hosts. One example is the tapeworm, which lives inside the intestines of animals.


Another type of interaction between two species in which one species wins and the other loses is the interaction between a parasite and its host. Parasites are organisms that get their food from a host organism without killing it. The parasite you may know best is the flea. Fleas live on a host such as a dog and feed on the host's blood. A host organism is an organism that is used as food by a parasitic organism without being killed. The difference between parasites and predators is that parasites usually don't kill their hosts.

Figure 8.8 Parasites such as the lampreys attached to this fish usually don't kill their host immediately.

One parasite, the lamprey, is damaging fish populations in the Great Lakes. Lampreys are jawless fish that attach themselves to the sides of other fish. There they scrape out a shallow wound and feed on the body fluids that leak out. Generally, lampreys don't kill the fish to which they attach. But they do leave it in a weakened state with a big hole in its side. Lampreys moved into the Great Lakes from the Atlantic Ocean along the St. Lawrence Seaway. They are causing economic problems because they are contributing to the decline of the Great Lakes fishery.

Brown-headed cowbirds are birds that act as parasites by laying their eggs in other birds' nests. The host bird doesn't know that the cowbird has laid an egg in its nest, and will incubate the cowbird egg and feed the chick when it hatches. The host bird is being parasitized because it is spending time and energy feeding the brown-headed cowbird chick instead of its own chicks. In fact, the brown-headed cowbird chick usually hatches before the host's chicks. Some cowbirds may even push the other nestlings out of the nest to gain the full attention of its host parent!

Figure 8.9 The indigo bunting nestling is being pushed from the nest by a cowbird nestling. A video recording of this type of nest interaction was taken by D. C. Dearborne.

Plants can be parasites, too. Mistletoe is a parasitic plant that grows in trees. When a mistletoe seed lands in the crotch of a tree, it germinates and sends its roots into the tree instead of to the ground. As the mistletoe grows, it parasitizes its host tree by using some of the tree's water and nutrients.


Every interaction between two species isn't necessarily a win-lose situation. Sometimes two species actually help each other out. Ecologists call this kind of interaction mutualism.

Did You Know?

The rhinoceros and the tickbird are two unlikely animals to help each other. The tickbird eats ticks living on the rhinoceros and receives protection by riding on the back of the rhinoceros. The rhinoceros is healthier because it has the ticks removed.

One example of mutualism occurs between one ant species and a type of violet. Violets are flowering plants and reproduce with seeds. One of the problems a plant has is that it can't move around and drop its seeds where they are most likely to grow. To overcome this problem, this violet produces seeds that have nutritious tidbits on them that the ants eat. The ants look for these seeds, carry them back to their nests, and eat the tidbits. When they're done, the ants abandon the rest of the seed that may sprout and grow. Both the violet and the ant benefit in this interaction, shown in Figure 8.10. The violet's seeds are carried away to new places to grow and the ants have a meal!

Figure 8.10 Mutualism occurs when both species benefit in the interaction. In this interaction, the violet has its seeds carried to new places and the ants receive something to eat.

Activity 8-1: Once upon an Oak Tree


You have been studying how organisms live together and how they can help or harm each other. In this activity you have an opportunity to think more about species interactions by considering what happens in a community of organisms that live on and around an oak tree.


  • Writing materials
  • Drawing materials (optional)
  • Resource


Step 1 Listen carefully as the story is read to you. Close your eyes and imagine that you are observing the species interactions in the story.

Step 2 After hearing the story, read the story on the Resource. Identify each interaction described in the story and discuss the questions below as they apply to each organism in the interaction.

  • Is the interaction primarily for food?
  • Is the interaction helpful or harmful?
  • Is the interaction intentional or unintentional?
  • What else do you notice about these interactions?

Step 3 After you have identified the types of interactions that have already been described in the story, think about other interactions that might exist. Select an organism in the story, or a new organism that could survive on the oak tree. Complete the story by writing about the interactions the organism has with other species. You may get some helpful ideas by looking at the following questions:

  • What other animals and plants can be included in the oak tree environment? In the forest? In the meadow nearby?
  • How do these species interact with each other?
  • Are these interactions harmful or helpful to each organism?
  • How do these interactions relate to the overall number of other organisms in the area?
  • Do you think these organisms interact with organisms from other areas? How? Give some examples.

Review Questions

  1. What do ecologists mean by the word competition?
  2. What is the difference between a predator and a parasite?
  3. Describe an example of mutualism with which you are familiar, and that isn't explained in this book.

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