To know the following interspecies relationships and
examples of each:
Detailed learning outcomes
Symbiotic relationships are co-evolved and specific intimate
relationships between members of two different species.
A parasite benefits in terms of energy or nutrients, whereas a
host is harmed by their loss. Parasites often have a limited
metabolism so cannot survive out of contact with a host.
Both mutualistic partner species benefit from the relationship.
One example is that of cellulose-digesting protozoa/bacteria
living in the guts of many herbivores.
Symbiosis is the close relationship between
two different species, eg the relationship
between nitrogen-fixing soil bacteria (Rhizobia)
and leguminous plant roots (peas, beans and
There are two types of symbiosis:
A parasite lives on another organism
(the host) from which it obtains food. A B
This: Harmed Benefits
• benefits the parasite
• harms (or kills) the host.
Some parasites (ectoparasites) live on the outer skin
of the host:
ticks leech fleas
Some parasites (endoparasites) live inside the
host (usually in vertebrates):
Living with parasites
Only poor parasites kill their host as this is not
to their advantage, especially for obligate
There is a balance between parasitic damage
and host defence, so the relationship is
Diseases often occur in cycles as parasite and
Secondary host species or vectors
Vectors are organisms that transmit parasites,
usually insects and ticks. These vectors are
essential for the development of the infectious
stage of the lifecycle of some parasites before
they are transmitted to the next host (eg
mosquitoes transmit the malarial parasite).
Some of these vectors therefore have a double
aspect of parasitism.
As a host evolves defence mechanisms against
the parasite, so the parasite evolves
mechanisms to overcome the defence.
This has led to many host–parasite interactions
being species specific, in other words most
parasites only infect a particular host.
Types of parasite
Parasites may be animals or plants.
Viruses are parasites.
Unicellular organisms, such as some
bacteria, are parasites.
Multicellular organisms, such as worms
and insects, may be parasites.
Mutualism is a specialist
relationship between two A B
organisms that is beneficial to both. Benefits Benefits
Herbivores have special
Lichens are composed of fungus cellulose-digesting micro-
and algae in a beneficial organisms in their stomachs.
Leguminous plants and
These bacteria live freely in
They are able to infect the
roots of leguminous plants
and form nodules.
Inside the nodules bacteria
obtain energy from their
host and supply nitrogen
compounds in return.
Fungi and algae (lichens)
Lichens are able to colonise
areas where other plants are
unable to grow.
The fungal part of the lichen
produces tough cellulose walls
and secretes mineral-digesting
chemicals while receiving
The algal part of the lichen
photosynthesises and receives
protection and essential
Herbivores and gut bacteria
Bacteria which can do
this are found in the
alimentary tract of
These bacteria receive
food and shelter, and
aid plant digestion.
Ants and aphids
Some species of ants ‘farm’
aphids. They move them from
plant to plant and protect them
In return the aphids secrete a
sugary liquid that the ants
Terrestrial plants and pollinating
Many plants have evolved
special flower shapes which
allow only one kind of animal
The animal depends on the
nectar it receives as a reward
for its visit.
The plant depends on the
animal to transport its pollen
Species A + +
Species B - +
Many thanks to R. Purdie from Sanquhar
Academy for the use of some materials.