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Symbiosis
Year 8 Science
Review of previous lesson
• In our last lesson we looked at Living things
grouped in the ecosystems as either:
• Producers
• Consumers
• Decomposers
• We also developed the idea of a Food Chain
showing how when food is eaten energy is passed
from one living thing to another.
Grass → grasshopper → kookaburra
• Finally, we realised that ecosystems often have
many different food chains and paths and we
developed the idea of a Food Web which shows
the interactions of possible Food Chains
Living together
• Organisms in ecosystems do not always simply interact
with each other based on producers and consumers.
• Sometimes organisms co-exist with other organisms to
obtain protection and shelter.
• We use the term Symbiosis to describe the interaction
between two different organisms where at least one of
them benefits. The other organism may also benefit, be
unaffected or be harmed or even killed.
• Consider the case of the clown fish which shelters in a sea
anemone……
Types of Symbiosis
There are three main types of Symbiosis.
They are:
• Mutualism
• Parasitism
• Commensalism
Let us look at each of these in detail…
Mutualism
• An interaction between two different
organisms that benefits both is called
Mutualism.
• In many cases neither species could survive
under natural conditions without the help of
the other.
• The key point is that both depend on each
other and both benefit.
Parasitism
• An interaction between two different organisms
where one species (the parasite) lives in or on
another species (the host) from which it obtains
food, shelter and other requirements is called
Parasitism.
• Some parasites harm their hosts but do not usually
kill them. Why do think the parasite does not kill
the host?
• The key point is that one species depends on the
other and only one species benefits while the
other species is affected in some way.
Commensalism
• An interaction between two different organisms
where one species (the commensal) benefits from
another species (sometimes called the host)
without affecting the host.
• Unlike parasitism, in commensalism the host is
not affected, harmed or killed.
• The key point is that one species benefits from
another species but does not affect the other
species in any way.
Epiphytes and Saprophytes
• Some symbiotic interactions do not fall into the
three major categories.
• Epiphytes are plants that grow on the outside of
plants without taking nourishment (Staghorn
ferns, orchids etc.).
• Saprophytes are organism that live on dead and
decaying plants (e.g fungi).