HONORS LIVING ENVIRONMENT
MS. ETRI
TOPIC 23: ECOLOGY
PART 3: FACTORS AND INTERACTIONS IN AN ECOSYSTEM
LIMITING FACTOR: an environmental factor that keeps (limits) a population from growing too large.

Reduces the population density or prevents its growth.

EXAMPLE: food, living space, water, disease, weather.

The mouse population is kept in check by the predator owl population.
o
Without owls preying on
mice, acting as limiting
factors, the mouse
population would
increase greatly and then
decrease slightly when its
food supply was used up.
The food supply would
then become a limiting
factor.
LIMITING NUTRIENT: the element that must be added for production to increase in an area.

A single nutrient that is either scarce or cycles very slowly through an ecosystem

Nitrogen and phosphorus are typically the nutrients that most often limit marine production.
HABITAT VS. NICHE
HABITAT

The place in the ecosystem where an organism lives.

Are determined by both the abiotic and biotic factors an organism needs to survive.

An organism’s “ADDRESS.”

EXAMPLE: An earthworm’s habitat is moist soil.
NICHE

The role an organism plays in the ecosystem.

Includes the organism’s feeding habits, and what it contributes to its surroundings, how it interacts with
other organisms.

An organism’s “PROFESSION.”
**THE NICHE OF AN ORGANISM DETERMINES ITS HABITAT**
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HONORS LIVING ENVIRONMENT
MS. ETRI
**EACH TYPE OF ORGANISM MUST OCCUPY ITS OWN NICHE. THIS ALLOWS MANY SPECIES TO
COEXIST IN ONE PLACE**
EXAMPLE: In a freshwater pond community, a carp (type of fish) eats decaying material from around the
bases of underwater plants.

In the same community, a snail scrapes algae from the leaves and stems of the same plant.

Both organisms live in the same pond habitat, but they occupy different niches because one
eats decaying material from around the bases of underwater plants and the other scrapes algae
from the leaves and stems of the same plant.
COMMUNITY INTERACTIONS
1. COMPETITION: the struggle between organisms for limited resources.
EXAMPLE: Competition between a brown bird population and a blue bird population in the forest.

If both types of birds eat yellow butterflies at the tops of trees, competition would exist
between the two species because they would be fighting for the same food source.

Competition can exist both within and among species.
2. PREDATION

A relationship in which members of one species (the predator) consume members of the other
species (the prey).

True predators help control the size of prey populations.
o
Generally, the predator-prey relationship keeps the population size of both species in
balance.
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HONORS LIVING ENVIRONMENT
MS. ETRI
o Every change in the population size of one species is followed by a corresponding
change in the population size of the other species.
As the prey
population
increases, the predator population starts to rise. With more predators, the prey population starts to decrease,
which, in turn, causes the predator population to decline. This pattern keeps repeating. There is always a slight
lag between changes in one population and changes in the other population.
3. SYMBIOSIS: A close association between two different species in which at least one species
benefits.

For the other species, the outcome of the association may be positive, negative or neutral.
THREE TYPES OF SYMBIOTIC RELATIONSHIPS:
a. MUTUALISM: both species benefit.

EXAMPLE: nearly blind shrimp and goby fish.
o
The shrimp maintains a burrow in the sand in which both the goby and shrimp live.
o
When a predator comes near, the fish
touches the shrimp with its tail as a
warning.
o
Then, both fish and shrimp retreat to the
burrow until the predator is gone.
o
Each gains from this relationship: the
shrimp gets a warning of approaching
danger, and the fish gets a safe home and
a place to lay its eggs.

Bee & Flower
b. COMMENSALISM: one species benefits while the other species is not affected.
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HONORS LIVING ENVIRONMENT
 One animal typically uses another for a purpose other than food.

MS. ETRI
EXAMPLE: barnacles on whales
c. PARASITISM: one species (the parasite) benefits while the other species (the host) is harmed.

Some parasites live on the surface of their host.

Others live inside their host, entering through a break in the skin or in food or water.

EXAMPLE: roundworms are parasites of the human
intestine.
o
The worms produce huge numbers of eggs, which
are passed in the host’s feces to the environment.
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