symbiosis and nutrition an irsc live virtual lesson by diana lenartiene, ed. s

Symbiosis and Nutrition

An IRSC Live Virtual LessonBy

Diana Lenartiene, Ed. S.

KEY TERMSCOMMENSALISM: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 ConceptSymbiosis 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.

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.


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.


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.


InquilinismA 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.


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.


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


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.


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


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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symbiosis and nutrition an irsc live virtual lesson by diana lenartiene, ed. s

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