Populations and Communities
III. Interactions among Living Things
A. Adapting to the Environment
1. natural selection
A process where individuals that are better adapted to
the environment are more likely to survive and reproduce
than others.
2. adaptations
Behaviors and physical characteristics of a species that
allow them to live successfully in their environment.
3. niche
An organism’s role or how it makes its living.
B. Competition
The struggle between organisms to survive in a habitat with
limited resources.
Example: A grizzly bears and a wolves competing for food
(elk).
C. Predation
An interaction in which one organism kills and eats another
organism.
1. Predator: The organism that does the killing.
Example: wolf, tiger, shark, pelican, snake.
2. Prey: The organism that is being killed.
Example: deer, zebra, seal, fish, mouse.
a. Predator Adaptation: Characteristics that help
predators catch and kill their prey.
Example: Grizzly bear has sharp teeth and claws for
tearing flesh.
b. Prey Adaptation: Characteristics that help prey
avoid being caught and eaten.
Example: Deer have long legs which make them fast
and agile to escape a predator.
c. The Effect of Predation on Population Size
Predation has an effect on population size. If
predators are very effective at hunting their prey,
the result is often a decrease in the size of the prey
population. But this decrease in the prey populations
will then affect the predator population.
D. Symbiosis
Is a close relationship between two species that benefits at
least one species.
1. Mutualism
A symbiotic relationship where both species benefit.
Example: Bacteria that lives in our stomach. The
bacteria help us break down food and they get a place to
live and food.
2. Commensalism
A symbiotic relationship where one species benefits and
the other is neither harmed nor helped.
Example: The red-tailed hawk living in a saguaro plant.
The hawk gets a place to build nest and the plant is
unharmed.
3. Parasitism
A symbiotic relationship where one organism lives on or
inside another organism and harms it.
a. Parasite
The organism that benefits and lives in or on the other
organism.
b. Host
The organism that is harmed and lived on or in.
Example: A tick feeding off a dog. The tick drinks
the blood from the dog and harms it.