Prey And Predator


One habitat might contain hundreds or even thousands of species. Look at the rotting log habitat shown in Figure 16. A rotting log in a forest can be home to many species of insects, including termites that eat decaying wood and ants that feed on the termites. Other species that live on or under the rotting log include millipedes, centipedes, spiders, and worms. You might think that competition for resources would make it impossible for so many species to live in the same habitat. However, each species has different requirements for its survival. As a result, each species has its own niche (NICH). An organism's niche is its role in its environment—how it obtains food and shelter, finds a mate, cares for its young, and avoids danger.

l^lEmtHTmEEji Why does each species have its own niche?

Special adaptations that improve survival are often part of an organism's niche. Milkweed plants contain a poison that prevents many insects from feeding on them. Monarch butterfly caterpillars have an adaptation that allows them to eat milkweed. Monarchs can take advantage of a food resource that other species cannot use. Milkweed poison also helps protect monarchs from predators. When the caterpillars eat milkweed, they become slightly poisonous. Birds avoid eating monarchs because they learn that the caterpillars and adult butterflies have an awful taste and can make them sick.

Plant Poisons The poison in milkweed is similar to the drug digitalis. Small amounts of digitalis are used to treat heart ailments in humans, but it is poisonous in large doses. Research the history of digitalis as a medicine. In your Science Journal, list diseases for which it was used but is no longer used.

Figure 16 Different adaptations enable each species living in this rotting log to have its own niche. Termites eat wood. They make tunnels inside the log. Millipedes feed on plant matter and find shelter beneath the log. Wolf spiders capture insects living in and around the log.

Wolf Spider Interactions


Wolf spider

SECTION 3 Interactions Within Communities E ♦ 23

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Wolf spider

SECTION 3 Interactions Within Communities E ♦ 23

.1 nd ^^^pdez/Animals Animals, (bl)Donald Specker/Animals Animals, (br)Joe McDonald/Animals Animals

Prey Running Away From Predator
Figure 17 The alligator is a predator. The turtle is its prey.

Predator and Prey When you think of survival in the wild, you might imagine an antelope running away from a lion. An organism's niche includes how it avoids being eaten and how it finds or captures its food. Predators, like the one shown in Figure 17, are consumers that capture and eat other consumers. The prey is the organism that is captured by the predator. The presence of predators usually increases the number of different species that can live in an ecosystem. Predators limit the size of prey populations. As a result, food and other resources are less likely to become scarce, and competition between species is reduced.

Cooperation Individual organisms often cooperate in ways that improve survival. For example, a white-tailed deer that detects the presence of wolves or coyotes will alert the other deer in the herd. Many insects, such as ants and honeybees, live in social groups. Different individuals perform different tasks required for the survival of the entire nest. Soldier ants protect workers that go out of the nest to gather food. Worker ants feed and care for ant larvae that hatch from eggs laid by the queen. These cooperative actions improve survival and are a part of the specie's niche.


Obtaining Energy

• All life requires a constant supply of energy.

• Most producers make food by photosynthesis using light energy.

• Consumers cannot make food. They obtain energy by eating producers or other consumers.

• A food chain models the feeding relationships between species.

Symbiotic Relationships

• Symbiosis is any close relationship between species.

• Mutualism, commensalism, and parasitism are types of symbiosis.

• An organism's niche describes the ways in which the organism obtains food, avoids danger, and finds shelter.

Self Check

1. Explain why all consumers depend on producers for food.

2. Describe a mutualistic relationship between two imaginary organisms. Name the organisms and explain how each benefits.

3. Compare and contrast the terms habitat and niche.

4. Think Critically A parasite can obtain food only from a host organism. Explain why most parasites weaken, but do not kill, their hosts.

Applying Skills

5. Design an experiment to classify the symbiotic relationship that exists between two hypothetical organisms. Animal A definitely benefits from its relationship with Plant B, but it is not clear whether Plant B benefits, is harmed, or is unaffected.

24 ♦ E CHAPTER 1 Interactions of Life

Ted Levin/Animals Animals

FBBdíng Habits of Planaria

You probably have watched minnows darting about in a stream. It is not as easy to observe organisms that live at the bottom of a stream, beneath rocks, logs, and dead leaves. Countless stream organisms, including insect larvae, worms, and microscopic organisms, live out of your view. One such organism is a type of flat-worm called a planarian. In this lab, you will find out about the eating habits of planarians.

& Real-World Question-

What food items do planarians prefer to eat? Goals

■ Observe the food preference of planarians.

■ Infer what planarians eat in the wild.

Materials small bowl planarians (several) lettuce leaf raw liver or meat

Safety Precautions guppies (several) pond or stream water magnifying lens

Magnification: Unknown


1. Fill the bowl with stream water.

2. Place a lettuce leaf, piece of raw liver, and several guppies in the bowl. Add the planarians. Wash your hands.

3. Observe what happens inside the bowl for at least 20 minutes. Do not disturb the bowl or its contents. Use a magnifying lens to look at the planarians.

4. Record all of your observations in your Science Journal.

Conclude and Apply-

1. Name the food the planarians preferred.

2. Infer what planarians might eat when in their natural environment.

3. Describe, based on your observations during this lab, a planarian's niche in a stream ecosystem.

4. Predict where in a stream you might find planarians. Use references to find out whether your prediction is correct.


Magnification: Unknown


Your Ont h

Your Ont h

Share your results with other students in your class. Plan an adult-supervised trip with several classmates to a local stream to search for planarians in their native habitat. For more help, refer to the Science Skill Handbook.

Richard L. Carlton/Photo Researchers


■ Identify the environmental factors needed by a population of fruit flies.

■ Design an experiment to investigate how a change in one environmental factor affects in any way the size of a fruit fly population.

■ Observe and measure changes in population size.

Possible Materials fruit flies standard fruit fly culture kit food items (banana, orange peel, or other fruit) water heating or cooling source culture containers cloth, plastic, or other tops for culture containers magnifying lens

Safety Precautions


roPULArion Growth in Fruit Flies

& Real-World Question-

Populations can grow at an exponential rate only if the environment provides the right amount of food, shelter, air, moisture, heat, living space, and other factors. You probably have seen fruit flies hovering near ripe bananas or other fruit. Fruit flies are fast-growing organisms often raised in science laboratories. The flies are kept in culture tubes and fed a diet of specially prepared food flakes. Can you improve on this standard growing method to achieve faster population growth? Will a change in one environmental factor affect the growth of a fruit fly population?

Form a Hypothesis-

Based on your reading about fruit flies, state a hypothesis about how changing one environmental factor will affect the rate of growth of a fruit fly population.

& Test Your Hypothesis-

Make a Plan

1. As a group, decide on one environmental factor to investigate. Agree on a hypothesis about how a change in this factor will affect population growth. Decide how you will test your hypothesis, and identify the experimental results that would support your hypothesis.

2. List the steps you will need to take to test your hypothesis. Describe exactly what you will do. List your materials.

3. Determine the method you will use to measure changes in the size of your fruit fly populations.

26 ♦ E CHAPTER 1 Interactions of Life

(t)Jean Claude Revy/PhotoTake, NYC, (b)OSF/Animals Animals

4. Prepare a data table in your Science Journal to record weekly measurements of your fruit fly populations.

5. Read the entire experiment and make sure all of the steps are in a logical order.

6. Research the standard method used to raise fruit flies in the laboratory. Use this method as the control in your experiment.

7. Identify all constants, variables, and controls in your experiment.

Follow Your Plan

1. Make sure your teacher approves your plan before you start.

2. Carry out your experiment.

3. Measure the growth of your fruit fly populations weekly and record the data in your data table.

© Analyze Your Data

1. Identify the constants and the variables in your experiment.

2. Compare changes in the size of your control population with changes in your experimental population. Which population grew faster?

3. Make and Use Graphs Using the information in your data table, make a line graph that shows how the sizes of your two fruit fly populations changed over time. Use a different colored pencil for each population's line on the graph.

ft Conclude and Apply-

1. Explain whether or not the results support your hypothesis.

2. Compare the growth of your control and experimental populations. Did either population reach exponential growth?

How do you know?

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Compare the results of your experiment with those of other students in your class. For more help, refer to the Science Skill Handbook.

Runk/Schoenberger from Grant Heilman

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    What planarians might eat when in their natural environment?
    6 years ago

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