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SYMBIOSIS
Symbiosis is any
intimate relationship or association between members of two or more
different species. The partners of a symbiotic relationship, called
symbionts, may benefit from, be unaffected by, or be harmed by the
relationship. The thousands,
or even millions, of symbiotic associations in nature are all products
of Coevolution, and they fall into several categories.
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Mutualism: Sharing Benefits
Mutualism
is a symbiotic relationship in which both partners benefit. One
example of mutualism is the association between reef-building coral
animals and microscopic algae. These symbiotic algae, which are
called zooxanthellae, live inside cells of the coral, where they
photosynthesize and provide the animal with carbon and nitrogen
compounds as well as oxygen. Zooxanthellae have a stimulatory effect
on the growth of corals, which deposit calcium carbonate skeletons
around their bodies much faster when the algae are present. The
coral, in turn, supplies its zooxanthellae with waste products such
as ammonia, which the algae use to make nitrogen compounds for both
partners. |
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Mycorrhizae are
mutualistic associations that take place between fungi and the roots of
almost alt plants. The fungus absorbs essential minerals from the soil
and provides them to the plant, and the plant provides the fungus with
food produced by photosynthesis. Plants grow more vigorously in the
presence of mycorrhizae, and they are better able to tolerate
environmental stresses such as drought and high soil temperatures.
Frequently,
mutualistic partners are completely dependent on one another. For
example, an obligatory relationship exists between the yucca, a plant
with stiff leaves found in the southwestern United States, and the yucca
moth. The moth transfers, pollen between plants, and the planes provide
food and a mi ft habitat
for the moth larvae, which hatch from eggs laid inside the flower.
Neither species could exist without the other. Without the yucca moth,
pollination—and therefore successful reproduction—would not occur in
the yucca, and it would die out.
Commensalisms: Taking Without Harming
Commensalisms
are a type of symbiosis in which one organism benefits and the other
one is neither harmed nor helped. One example of commensalisms is the
relationship between two kinds of insects, silverfish and army ants.
Certain kinds of silverfish live with army ants and share the food
caught by
position on the
tree enables it to obtain adequate light, water (rainfall dripping down
the branches), and minerals (washed out of the tree's leaves by
rainfall). Thus, the epiphyte benefits from the association, whereas
the tree remains largely unaffected.
Parasitism:
Taking at Another's Expense
Parasitism is a
symbiotic relationship in which one member, the parasite, benefits and
the other, the host, is adversely affected. The parasite obtains
nourishment from its host, and although it may weaken the host, it
rarely kills n. (A parasite would have a rough life if it kepi killing
off its hosts!) Some parasites, such as ticks, live outside the host's
body; others, such as tapeworms, live within flu-host.
When a parasite
causes disease and sometimes the death of a host, it is called a
pathogen. For example, humans sometimes gel histoplasmosis, a serious
and often fatal disease caused by a fungus. Humans are infected when
they breathe the spores of the fungus into their lungs. The spores grow
and invade the lung tissue, causing chronic coughing and fever.
Eventually the disease progresses to other organs of the body. The
spores of the fungus are common in soils that have high concentrations
of bird droppings, and the disease is more prevalent in warm tropical
regions of the world.
Crown gall
disease, which is caused by a bacterium, occurs in many different kinds
of plants and results in millions of dollars of damage to ornamental
and agricultural plants each year. The bacterium, which lives in the
soil, enters plants through small wounds such as those caused by
insects. It causes galls, or tumorlike growths, often at the crown
(between the stem and the roots) of a plant. Although plants seldom die
from crown gall disease, they are weakened, grow more slowly, and often
succumb to other pathogens.
Many parasites
do not cause disease. For example, humans can acquire the pork tapeworm
by eating poorly cooked pork that is infested with immature Tapeworms.
Once the tapeworm is inside the human digestive system, it attaches
itself to the wall of the small intestine and grows rapidly by
absorbing nutrients that pass through. Pork tapeworms that live in the
human digestive tract do not cause any noticeable symptoms.
We have seen
that each organism has its own niche within its community and that
organisms form intimate relationships among themselves. Now we examine
how the types of organisms found
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