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TPO17
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Sym
biotic Relationships
TPO17-3
：
Symbiotic Relationships
A symbiotic relationship is
an interaction between two or more species in
which
one species lives in
or on another species. There are three
main
types
of symbiotic
relationships: parasitism,
commensalism, and mutualism. The first and the
third can
be key factors in the
structure of a biological community; that is, all
the populations of
organisms living
together and potentially interacting in a
particular area.
Parasitism
is
a
kind
of
predator-
prey
relationship
in
which
one
organism,
the
parasite, derives its food at the
expense of its symbiotic associate, the host.
Parasites
are usually smaller than
their hosts. An example of a parasite is a
tapeworm that lives
inside
the
intestines
of
a
larger
animal
and
absorbs
nutrients
from
its
host.
Natural
selection favors the parasites that are
best able to find and feed on hosts. At the same
time,
defensive
abilities
of
hosts
are
also
selected
for.
As
an
example,
plants
make
chemicals toxic to fungal and bacterial
parasites, along with ones toxic to predatory
animals (sometimes they are the same
chemicals). In vertebrates, the immune system
provides a multiple defense against
internal parasites.
At
times,
it
is
actually
possible
to
watch
the
effects
of
natural
selection
in
host-parasite
relationships. For example, Australia during the
1940 s was overrun by
hundreds
of
millions
of
European
rabbits.
The
rabbits
destroyed
huge
expanses
of
Australia
and
threatened
the
sheep
and
cattle
industries.
In
1950,
myxoma
virus,
a
parasite
that affects rabbits, was deliberately introduced
into Australia to control the
rabbit
population.
Spread
rapidly
by
mosquitoes,
the
virus
devastated
the
rabbit
population. The virus was less deadly
to the offspring of surviving rabbits, however,
and it caused less and less
harm over the
years.
Apparently,
genotypes
(the
genetic
make-up of an organism) in the
rabbit population were selected that were better
able
to resist the parasite. Meanwhile,
the deadliest strains of the virus perished with
their
hosts as natural selection
favored strains that could infect hosts but not
kill them. Thus,
natural selection
stabilized this host-parasite relationship.
In
contrast
to
parasitism,
in
commensalism,
one
partner
benefits
without
significantly
affecting the other. Few cases of absolute
commensalism probably exist,
because
it
is
unlikely
that
one
of
the
partners
will
be
completely
unaffected.
Commensal
associations
sometimes
involve
one
species'
obtaining
food
that
is
inadvertently exposed by another. For
instance, several kinds of birds feed on insects
flushed
out
of
the
grass
by
grazing
cattle.
It
is
difficult
to
imagine
how
this
could
affect
the
cattle, but
the
relationship
may help
or hinder them
in
some
way not
yet
recognized.
The third type of
symbiosis, mutualism, benefits both partners in
the relationship
Legume
plants
and
their
nitrogen-fixing
bacteria,
and
the
interactions
between
flowering plants and their pollinators,
are examples of mutualistic association. In the
first
case,
the
plants
provide
the
bacteria
with
carbohydrates
and
other
organic
compounds, and the bacteria have
enzymes that
act
as catalysts that eventually add
nitrogen to the soil, enriching it. In
the second case, pollinators (insects, birds)
obtain
food
from
the
flowering
plant,
and
the
plant
has
its
pollen
distributed
and
seeds
dispersed much more efficiently than
they would be if they were carried by the wind
only.
Another
example
of
mutualism
would
be
the
bull's
horn
acacia
tree,
which
grows in Central and
South America. The tree provides a place to live
for ants of the
genus Pseudomyrmex. The
ants live in large, hollow thorns and eat sugar
secreted by
the tree. The ants also eat
yellow structures at the tip of leaflets: these
are protein rich
and seem to have no
function for the tree except to attract ants. The
ants benefit the
host tree by attacking
virtually anything that touches it. They sting
other insects and
large herbivores
(animals that eat only plants) and even clip
surrounding vegetation
that grows near
the tree. When the ants are removed, the trees
usually die, probably
because
herbivores
damage
them
so
much
that
they
are
unable
to
compete
with
surrounding vegetation
for light and growing space.
The
complex
interplay
of
species
in
symbiotic
relationships
highlights
an
important
point
about
communities:
Their
structure
depends
on
a
web
of
diverse
connections among organisms.