S7L4d
Categorize relationships between organisms that
are competitive or mutually beneficial.
Each organism in an ecosystem
interrelates with the other members.
These relationships fall into one of
three categories:
1. Symbiosis
2. Competition
3. Predation
A long-term association between two
members of a community in which
one or both parties benefit
 Three types of symbiotic relationships:
◦ Commensalism
◦ Mutualism
◦ Parasitism

Beneficial to both organisms
 Example:
◦ Protozoa living in termite intestine
 Protozoa break down wood the
termite eats, while the termite
provides food and shelter for the
protozoa


One member benefits and the other is
unaffected
◦ Barnacles that live on a whale
 Barnacles don’t harm or feed on
whale. They hitch a ride on the slow
moving whale in order to catch
plankton and other food in the water
 Whale is neither benefited nor harmed

Benefits one organism (parasite), but
harms the other (host)
◦ Tapeworms in a human are parasites
◦ Tapeworm benefits by getting its
nutrition from the intestines of its
human host
◦ Host is harmed because there are not
as many nutrients to absorb into its
body.
Competitive Relationship
When two or more organisms seek the
same limited resource, they compete with
each other.
 A resource could be food, water, light,
ground space or nesting space
 Competion can be intraspecific or
interspecific.
◦ Intraspecies competetion occurs between
members of the same species
◦ Interspecies competition occurs between
members of different species

Predator is an organism that feeds off of
another organism
 Prey is the organism it feeds on
 Sea lions hunt down and kill herring
◦ They harry the edges of the school of fish
and eat until they have killed all the weak
and sick animals
◦ Sounds cruel, but without predation,
competition and disease, there would be
no natural limitations on population
growth

• May seem brutal that organisms feed off
one another and awful that organisms
perish from losing resources or getting
disease, but the alternative is even more
disastrous:
 AN EVER-MORE-CROWDED WORLD IN
WHICH EVERY POPULATION
INCREASED UNTIL THERE WAS NO
SPACE OR FOOD LEFT FOR ANYTHING
OR ANYONE!
• Ecological balances are important!