Download 15.6 Close relationships - species and symbiotes

15.6 CLOSE RELATIONSHIPS – SPECIES AND SYMBIOTES
This module is about close relationships. Are horses and donkeys the same species? What about
lions and tigers? And why do some different species always live together – sometimes actually on
or inside each other? But first we need to ask what, exactly, is a species?
Species. In Module 15.3 we learnt that species is the lowest and most exclusive group in the
biologists’ taxonomy. Together with Genus, species provides the scientific name for a particular
kind of organism. A species can be defined as a group of organisms that interbreed to produce
fertile offspring. For example, two dogs may look quite different, but if
they mate they will produce puppies, and when
their puppies grow up, they can mate and have
puppies too. Therefore the two dogs are the
same species. In fact, all dogs belong to the
same species, Canis lupus. However, if a horse
and a donkey mate, they produce an offspring
called a mule. Mules are infertile; they cannot
produce young. Therefore horses and donkeys
are different species. A lion and a tiger can mate to produce ligers (if
the father is a lion) or tigons (if the father is a tiger). But unfortunately
ligers and tigons are infertile. They cannot breed therefore lions and tigers are separate species.
Their scientific names are Pantheria leo for the lion and Pantheria tigris for the tiger.
Symbiosis means living together. Biologists recognise three different ways in which two species
live very closely together: mutualism benefits both species, commensalism benefits one species
and is harmless to the other, and parasitism benefits one species and is harmful to the other.
Mutualism. A good example of mutualism is the clown fish (Amphiprion
ocellaris) and the sea anemone (Heteractis magnificus). These two species
always live together. The clown fish is immune to the sting of the anemone
so it can hide from predators amongst the tentacles of its host. The
anemone benefits because the clown fish eats other small fish and
invertebrates that might otherwise harm
it. Also, the faeces of the clown fish
provide the anemone with nutrients.
Cattle and antelopes often have close
mutual relationships with various
species of birds. The birds feed on ticks and other parasites
which infest the skin of the mammal. The bird benefits from a
good source of food and the mammal benefits in health and
comfort. Some very important mutual relationships are those
between insects and flowers. Many insects depend on flowers
for food in the form of nectar, and the flowers depend on the insects for pollination.
Commensalism. On open grasslands, some birds always follow browsing mammals. The
mammals disturb insects in the grass and the birds feed on these. The birds benefit from this
relationship and the mammals are not harmed. Another example is the bacteria that live all over
our skin. The bacteria get a place to live and dead skin cells to feed on, but they do us no harm.
Parasitism. The fleas, ticks and lice that live on the skins of
mammals and suck their blood are good examples of
external parasites. They often cause disease as well
as discomfort for their hosts. The picture (left) is a tick
feeding on a human. Skin diseases like ringworm and
athletes foot are caused by parasitic fungi (Tinea sp.
– sp. means there are various species). Parasitic fungi also cause
diseases in many plants. The picture (right) shows a parasitic plant
called dodder (Cuscuta sp.) that is invading and killing a tree.
Dodder has special roots that burrow into the xylem and phloem of
the tree and steal its sap. Internal parasites are parasites that live
inside their hosts. They include various species of worm that live in
the intestines of many animals including humans.

1. Look up mutualism and parasitism. List all the new examples you can find and
give brief descriptions of two of them. Add pictures if possible.
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