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).