12-4 How do living things interact?
Competition Resources in an ecosystem are limited. Animals sharing a habitat struggle for
resources, such as food, water, shelter, territory, and mates. Competition is the struggle
among organisms for resources in an ecosystem. Animals can compete with members of
the same species or member of a different species. In competition, animals that are better
adapted to conditions in the habitat are more likely to survive and reproduce.
Plants also compete for resources. Competition among plants is less active than
competition among animals. Plants compete for sunlight, water, growing space. Some
plants release poisonous chemicals into the ground around them to prevent grass and
weeds from crowding their growing space and using their resources.
Figure 12-10 These rams are in competition for a mate
Figure 12-11 This spider is about to kill its prey.
Predation All animals get energy from the foods they eat. Some animals eat plants,
whereas other animals eat animals. A relationship in which an organism kills and eats
other organisms is called predation. An organism that kills and eats another organism is
called a predator. An organism that is killed an eaten by another organism is called prey.
Look at figure 12-11. The spider is the predator. It is about to eat an insect, its prey.
Predators obviously benefit from predation because they eat the prey. However, the prey
species can also benefit from predation.
Predators are more likely to catch, kill, and eat weak or unhealthy animals than strong,
healthy ones. This means that the strongest members of the prey population are left to
reproduce and pass on their traits to their offspring.
Symbiosis In an ecosystem, there are many relationships between species. Symbiosis is a
close relationship between two organisms from different species that may help or harm
the organism.
12-4 How do living things interact?
Lesson Review
Skills: researching, using resources
Circle the term that best completes each statement.
1. Resources in an ecosystem are (abundant / limited).
2. (Predation / Competition) is the struggle among organisms for resources in an ecosystem.
3. In competition, animals that are better adapted to conditions in the habitat are (more / less) likely to
survive and reproduce.
4. Competition among plants is (less / more) active than competition among animals.
5. A relationship in which an organism kills and eats another organism is called
(competition / predation).
6. An organism that kills and eats another organism is called (a predator / prey).
7. (Symbiosis / Competition) is a close relationship between two organisms from different species that
may help or harm one of the organisms.
8. Prey species (can / cannot) benefit from predation.
9. Predators are (less / more) likely to catch, kill, and eat weak or unhealthy animals than strong,
healthy ones.
10. An organism that is killed and eaten by another organism is called (a predator / prey).
Skill Challenge
Skills: applying concepts, inferring
Explain how each of the following interactions can help a species to maintain a
strong, healthy population. You may use reference materials if necessary.
Interaction
Competition
Predation
Symbiosis
How the Interaction Helps Species
CHECKING CONCEPTS
1. What is competition?
2. For what do animals compete?
3. For what do plants compete?
4. What is predation?
5. What is symbiosis?