Download Symbiosis and Nutrition

Symbiosis and Nutrition
An IRSC Live Virtual Lesson
By
Diana Lenartiene, Ed. S.
KEY TERMS
COMMENSALISM:
A symbiotic relationship in which one organism, the commensal, benefits without causing any
detriment to the other organism, the host.
FACULTATIVE:
A term for a symbiotic relationship in which partners are capable of living apart.
HOST:
The term for an organism that provides a benefit or benefits for another organism in a symbiotic
relationship of commensalism or parasitism.
INQUILINISM:
A type of symbiosis in which one species, the inquiline, makes use of a host's nest or habitat without
causing any detriment to the host. Inquilinism is considered a variety of commensalism.
OBLIGATIVE:
A term for a symbiotic relationship in which the partners, if they were separated, would be incapable of
continuing to live.
PARASITISM:
A symbiotic relation ship in which one organism, the parasite, benefits at the expense of the other
organism, the host.
SYMBIOSIS:
A biological relationship in which (usually) two species live in close proximity to one another and
interact regularly in such a way as to benefit one or both of the organisms. Symbiosis may exist
between two or more individuals of the same species as well as between two or more individuals
representing two different species. The three principal varieties of symbiosis are mutualism,
commensalism, and parasitism
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Symbiosis Concept
Symbiosis is a biological relationship in which two species live in
close proximity to each other and interact regularly in such a way
as to benefit one or both of the organisms. When both partners
benefit, this variety of symbiosis is known as mutualism. The name
for a situation in which only one of the partners benefits is far
more well known. Such an arrangement is known as parasitism,
and a parasite is an organism that obtains nourishment or other life
support from a host, usually without killing it. By their very nature,
parasites are never beneficial, and sometimes they can be
downright deadly. In addition to the extremes of mutualism and
parasitism, there is a third variety of symbiosis, called
commensalism. As with parasitism, in a relationship characterized
by commensalism only one of the two organisms or species derives
benefit, but in this case it manages to do so without causing harm
to the host.
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HOW IT WORKS Varieties of Symbiosis
When two species—that is, at least two individuals representing
two different species—live and interact closely in such a way that
either or both species benefit, it is symbiosis. It is also possible for
a symbiotic relationship to exist between two organisms of the
same species. Organisms engaging in symbiotic relationships are
called symbionts.
There are three basic types of symbiosis, differentiated as to how
the benefits (and the detriments, if any) are distributed. These are
commensalism, parasitism, and mutualism. In the first two
varieties, only one of the two creatures benefits from the
symbiotic relationship, and in both instances the creature who
does not benefit—who provides a benefit to the other creature—
is called the host. In commensalism the organism known as the
commensal benefits from the host without the host's suffering any
detriment. By contrast, in parasitism the parasite benefits at the
expense of the host. http://www.encyclopedia.com/topic/symbiosis.aspx#11G2:3408600169-full
MUTUALISM: HUMAN AND DOG.
Mutualism is distinguished from the other two types of symbiosis,
because in this variety both creatures benefit. Thus, there is no host,
and theoretically the partners are equal, though in practice one
usually holds dominance over the other. An example of this inequality
is the relationship between humans and dogs. In this relationship,
both human and dog clearly benefit: the dog by receiving food,
shelter, and care and the human by receiving protection and loving
companionship—the last two being benefits the dog also receives
from the human. Additionally, some dogs perform specific tasks, such
as fetching slippers, assisting blind or disabled persons, or tracking
prey for hunting or crime-solving purposes.
For all this exchange of benefits, one of the two animals, the human,
clearly holds the upper hand. There might be exceptions in a few
unusual circumstances, such as dog lovers who are so obsessive that
they would buy food for their dogs before feeding themselves. Such
exceptions, however, are rare indeed, and it can be said that in almost
all cases the human is dominant.
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Obligate and Facultative Relationships
Most forms of mutualism are facultative, meaning that the partners
can live apart successfully. Some relationships of mutualism are so
close that the interacting species are unable to live without each
other. A symbiotic relationship in which the partners, if separated,
would be unable to continue living is known as an obligate
relationship. In commensalism or parasitism, the relationship is
usually obligate for the commensal or the parasite, since by definition
they depend on the host. At the same time, and also by definition,
the host is in a facultative relationship, since it does not need the
commensal or parasite—indeed, in the case of the parasite, would be
much better off without it. It is possible, however, for an organism to
become so adjusted to the parasite attached to its body that the
sudden removal of the parasite could cause at least a short-term
shock to the system.
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Inquilinism
A special variety of commensalism is inquilinism, in which the
commensal species makes use of the host's nest or habitat, without
causing any inconvenience or detriment to the host. Inquilinism (the
beneficiary is known as an inquiline) often occurs in an aquatic
environment, though not always. In your own yard, which is your
habitat or nest, there may be a bird nesting in a tree. Supposing you
benefit from the bird, through the aesthetic enjoyment of its song or
the pretty colors of its feathers—in this case the relationship could
be said to be a mutualism. In any case, the bird still benefits more,
inasmuch as it uses your habitat as a place of shelter.
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HUMANS AND OTHER SPECIES.
Humans engage in a wide variety of symbiotic relationships with
plants, animals, and bacteria. Bacteria may be parasitic on humans,
but far from all microorganisms are parasites: without the
functioning of "good" bacteria in our intestines, we would not be
able to process and eliminate food wastes properly. The relationship
of humans to animals that provide a source of meat might be
characterized as predation (i.e., the relationship of predator to prey),
which is technically a form of symbiosis, though usually it is not
considered in the same context. In any case, our relationship to the
animals we have domesticated, which are raised on farms to provide
food, is a mixture of predation and mutualism. For example, cows
(Bos taurus ) benefit by receiving food, veterinary services, and
other forms of care and by protection from other predators, which
might end the cows' lives in a much more unpleasant way than a
rancher will.
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All important agricultural plants exist in tight bonds of mutualism
with humans, because human farmers have bred species so
selectively that they require assistance in reproducing. For example,
over time, agricultural corn, or maize (Zea mays ), has been selected
in such a way as to favor those varieties whose fruiting structure is
enclosed in a leafy sheath that does not open and whose seeds do
not separate easily from the supporting tissue. In other words,
thanks to selective breeding, the corn that grows on farms is
enclosed in a husk, and the kernels do not come off of the cob
readily. Such corn may be desirable as a crop, but because of these
characteristics, it is incapable of spreading its own seeds and
thereby reproducing on its own. Obviously, agricultural corn is not
on any endangered species list, the reason being that farmers
continue to propagate the species through breeding and planting
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Another example of human-animal mutualism, to which we alluded
earlier, is the relationship between people and their pets, most
notably dogs (Canis familiaris ) and house cats (Felis catus ). Fed and
kept safe in domestication, these animals benefit tremendously
from their interaction with humans. Humans, in turn, gain from
their pets' companionship, which might be regarded as a mutual
benefit—at least in the case of dogs. (And even cats, though they
pretend not to care much for their humans, have been known to
indulge in at least a touch of sentimentality.) In addition, humans
receive other services from pets: dogs protect against burglars, and
cats eradicate rodents.
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COMMENSALISM IN NATURE.
Plants that grow on the sides of other plants without being
parasitic are known as epiphytic plants.
Among these plants are certain species of orchids, ferns, and
moss. By "standing on the shoulders of giants," these plants
receive enormous ecological benefits: the height of their hosts
gives them an opportunity to reach a higher level in the canopy
(the upper layer of trees in the forest) than they would normally
attain, and this provides them with much greater access to
sunlight. At the same time, the hosts are not affected either
negatively or positively by this relationship
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WHERE TO LEARN MORE
"Biology 160, Animal Behavior: Symbiosis and Social Parasitism." Department of Biology,
University of California at Riverside (Web site).
<http://www.biology.ucr.edu/Bio160/lecture25.html>.
Knutson, Roger M. Furtive Fauna: A Field Guide to the Creatures Who Live on You. New York:
Penguin Books, 1992.
Lanner, Ronald M. Made for Each Other: A Symbiosis of Birds and Pines. New York: Oxford
University Press, 1996.
Lembke, Janet. Despicable Species: On Cowbirds, Kudzu, Hornworms, and Other Scourges.
New York: Lyons Press, 1999.
Margulis, Lynn. Symbiotic Planet: A New Look at Evolu tion. New York: Basic Books, 1998.
Mutualism and Commensalism. Neartica: The Natural World of North America (Web site).
<http://www.nearctica.com/ecology/pops/symbiote.htm>.
"Parasites and Parasitism." University of Wales, Aberystwyth (Web site).
<http://www.aber.ac.uk/parasitology/Edu/Para_ism/PaIsmTxt.html>.
Sapp, Jan. Evolution by Association: A History of Symbiosis. New York: Oxford University
Press, 1994.
Symbiosis and Commensalism. The Sea Slug Forum (Web site).
<http://www.seaslugforum.net/symbio.htm>.
Trager, William. Living Together: The Biology of Animal Parasitism. New York: Plenum Press,
1986
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