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Types of Interactions
Most living things produce more offspring than will survive.
A female frog, for example, might lay hundreds of eggs in a
small pond. In a few months, the population of frogs in that
pond will be about the same as it was the year before. Why
won’t the pond become overrun with frogs? An organism,
such as a frog, interacts with biotic and abiotic factors in its
environment that can control the size of its population
Types of ecological interactions
competition
predation
symbiosis
parasitism
mutualism
commensalism
Limiting Factors
Populations cannot grow without stopping, because the
environment contains a limited amount of food, water,
living space, and other resources. A resource that is so
scarce that it limits the size of a population is called a
limiting factor. For example, food becomes a limiting
factor when a population becomes too large for the amount
of food available. Any single resource can be a limiting
factor to a population’s size.
In Japan, green herons make
interesting use of the biotic
and abiotic parts o ftheir
environment. They will
drop sticks and even bread
crumbs into the water to
attract fish. Sometimes,
they wil lcatch a fish
within 2 or 3 seconds of
dropping the bait
Keystone Species: Although all members of an ecosystem have important
roles, some members are more important than others to the overall integrity of the
ecosystem. Such species are called keystone species. In the same way that
removing an arch’s keystone causes the arch to collapse, removing a keystone
species can cause the whole ecosystem to collapse. This occurred when the sea
otter was hunted to near extinction in the kelp forests of the Pacific Ocean.
With the otters gone, nothing was left to eat the sea urchins, which multiplied
and ate all of the kelp. The kelp was home to dozens of animals, all of which
disappeared, as did most of the sea urchins. When the otter was reintroduced,
populations of all of these organisms returned!
Carrying capacity
The largest population that an environment can
support is known as the carrying capacity. When a
population grows larger than its carrying capacity,
limiting factors in the environment cause
individuals to die off or leave. As individuals die or leave, the
population decreases. For example, after a rainy season, plants may produce a
large crop of leaves and seeds. This large amount of food may cause an
herbivore population to grow. If the next year has less rainfall, there won’t be
enough food to support the large herbivore population. In this way, a
population may become larger than the carrying capacity, but only for a little
while. A limiting factor will cause the population to die back. The population
will return to a size that the environment can
Competition – two species share a requirement for a
limited resource  reduces fitness of one or both species
When two or more individuals or populations try to
use the same resource, such as food, water, shelter,
space, or sunlight, it is called competition. Because
resources are in limited supply in the environment,
their use by one
individual or population decreases the amount available
to other organisms.
Competition happens between individuals within a
population. The elks in Yellowstone National Park are
herbivores that compete with each other for the same
food plants in the park. This competition is a big problem
in winter when many plants die.
Competition also happens between populations. The
different species of trees in at the right are competing
With each other for sunlight and space.
Some of the trees in this
forest grow tall to reach sunlight, which
reduces the amount of sunlight available
to shorter trees nearby.
Predation – one species feeds on another  enhances
fitness of predator but reduces fitness of prey
herbivory is a form of
predation
Predators and Prey
• Predator Adaptations
To survive, predators must be able to catch their prey. Predators
have a wide variety of methods and abilities for doing so.
The cheetah, for example, is able to run very quickly to catch
its prey. The cheetah’s speed gives it an advantage over other
predators competing for the same prey.
Other predators, such as the goldenrod spider, shown at the
right, ambush their prey. The goldenrod spider blends in
so well with the goldenrod flower that all it has to do is wait
for its next insect meal to arrive.
Predators and Prey
• Prey Adaptations
Prey have their own methods and abilities to keep from being
eaten. Prey are able to run away, stay in groups, or camouflage
themselves. Some prey are poisonous. They may advertise their
poison with bright colors to warn predators to stay away. The
fire salamander sprays a poison that burns.
Predators quickly learn to recognize its warning coloration.
Many animals run away from predators. Prairie dogs run
to their underground burrows when a predator approaches.
Many small fishes, such as anchovies, swim in groups called
schools. Antelopes and buffaloes stay in herds. All the eyes,
ears, and noses of the individuals in the group are watching,
listening, and smelling for predators. This behavior increases
1. Chemical Defense
 There are two main ways animals can use
chemicals to defend themselves.
 Animals can synthesize toxin using their
own metabolic processes, or they can
accumulate toxin from the food they eat.
1. Chemical Defense
Many animals accumulate
toxin from their food
rather than synthesizing it
from scratch.
Photo courtesy of T. W. Davies, Cal. Acad. of Sciences.
For example, the larvae of
Monarch butterflies accumulate
toxins from the plants they
inhabit. Birds that eat the
Monarchs vomit and learn to
avoid them in the future.
Their bright coloration allows
birds to remember and avoid
them.
1. Chemical Defense
•
Animals which synthesize
their own toxin are able to
convert chemical compounds
in their body to a poison.
► There are many amphibians
that produce skin toxins.
The skin toxins are
produced by special poison
glands, usually located on
the animal's back or
throughout the skin.
Photo courtesy of Dr. John Daly
The poison dart frog has
poison glands scattered
all over its body.
2. Camouflage
Animals that camouflage
themselves pretend to be
something they are not.
Either their coloration,
marking patterns, or
entire body resembles
something else in their
environment, here a leaf,
an owl.
2. Camouflage
Here an aptly named
walking stick
pretends to be a
twig, in an attempt
to avoid being seen
by a bird or other
predator. This is
an example of
cryptic coloration.
Photo courtesy of Dr. Lloyd Glenn Ingles, Cal. Acad. of Sciences.
2. Camouflage
In this picture, a foureyed butterfly fish uses
deceptive markings.
The large spot near the
tail resembles an eye.
When predators attack
the wrong end, the
butterfly fish can swim
away in the other
direction!
Symbiosis – two species live together  can include
parasitism, mutualism, and commensalism
Parasitism – one species feeds on another
Benefits one species but hurts the other
Mutualism – two species provide resources or services
to each other  BOTH benefit
Commensalism – one species receives a benefit from
another species  Benefits one species does nothing for the
other.
Exploring Relationships: In India, the chital, a small deer, has trouble finding enough
grass to eat during the dry season. This deer relies on a certain type of monkey to get
enough to eat.
• Can you guess how the monkeys
benefit the deer?
• Which species benefit from
this relationship?
These deer have keen eyesight, hearing, and sense of smell. They therefore warn the
monkeys of predators.
Which species do you think benefit?
The honeyguide, a small African bird, lives up to its name. The purpose of its song
is to lure nearby creatures to a nest of honeybees it has found. Many animals have learned to
listen for this bird! Baboons, mongooses, ratels (or honey badgers),and even people will follow
the bird in order to claim the honey. Out of harm’s way, the bird waits for the leftovers: bee
larvae. The bird’s unique digestive system allows it to eat wax as well.
• Which animals in this story
are in a mutualistic relationship?
• Which animals are prey, and
which are predators?
1. Acacia ants live on the bullhorn
acacia tree, which provides
the ants with food and
shelter. The ants deter grazing
animals from eating the tree.
What type of relationship is this?
2. Plants called epiphytes, such as
certain orchids, live on other
plants, which provide only a
place to grow. Epiphytes
absorb sunlight, water, and
nutrients from their surroundings
and make their own
food.
What type of relationship is this?
3. There is a tiny wasp that lays
its eggs in a variety of other
insects, such as caterpillars,
spiders, aphids, and flies. The
wasp larvae feed on the host
insect and eventually kill it.
What type of relationship is this?
Section Quiz: Match the correct word to the definition
1.a relationship between two organisms in
which one benefits and the other is not affected
2.an organism that eats all or part of another
organism
a.prey
3.a relationship in which two different
b.symbiosis
organisms live in close association with each c.mutualism
other
d.commensalism
4.an organism that is killed and eaten by
e.parasitism
another organism
f.predator
5.a relationship where one organism benefits
and the other is harmed
6. a relationship between two species in which
both species benefit
Write the letter of the correct answer.
7.The largest population an environment can support is its
a. carrying capacity.
c. population.
b. limiting factor.
d. symbiosis.
8. One type of competition involves individuals competing for
resources. The other involves competition between different
a. organisms.
c. environments.
b. populations.
d. relationships.
9. Young wasps are eating the tomato hornworm that is their host.
What is this an example of?
a. commensalism
c. parasitism
b. mutualism d .competition
10. A bird eats a worm. Who is the predator?
a. the worm
b. the bird
c. both the bird and the worm
d. neither the bird nor the worm