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Commensalism
Sometimes in nature, two organisms live in an association where one species benefits while the other derives neither benefit nor harm. True commensalism may not exist in nature because of the nature of a symbiotic relationship. Documentation of true instances of commensalism in nature has been sketchy since verifying the impact on the second species is oftentimes vague. Dissension exists in the scientific community regarding natural occurrences of commensalism because the concept seems alien in a world that selects for relationships that have clear benefits and costs. A possible instance of commensalism may occur in the human body. A certain bacterium, Streptococcus pyogenes, normally lives peacefully in the digestive system of humans. When the bacterium finds its way to the throat area, however, it causes illness, a condition called strep throat.
Competition
Competition may occur between members of the same species or of different species. In exploitative competition, two organisms vie for the same resources, as when two species of birds fight for the same seeds. With interference type competition, one organism blocks another from accessing a resource; for example, by adhering to a rock, a barnacle prevents others from using that same space to gain access to food sources that wash over the rock. The principles that govern competition originate from the niche theory, which states that no two types of organisms can occupy the same niche at the same time. All external resources an organism requires for survival defines its niche. If two species choose to occupy the same niche, then competition occurs to exclude one species from the niche. Overall, competition mandates the need for organisms to evolve because evolution allows organisms to better compete for scarce resources.
Mutualism
Mutualism assures greater survivability by both members of an alliance. Some mutualistic bonds have become so close that neither species can survive without the presence of the other. In looser associations, the two species use resources more effectively if they link with one another. To illustrate, mycorrhizhal structures form as bonds between the roots of some plants and certain fungi. Some plants cannot use minerals in the soil efficiently, relying on the relationship with the fungi attached to their roots to convert the nutrients into a more usable form. In return, the plant supplies the fungi with food. Years of mutual evolution have allowed both species to become more efficient at obtaining resources to thrive.
Parasitism
Parasites exploit other organisms to survive. Their hosts suffer some sort of detriment. Parasites can live on or inhabit the tissues within other organisms, but parasites generally do not kill their hosts. Bacteria such as Mycobacterium leprae, the organism that causes the disease leprosy, parasitize certain cells--called Schwann cells--in the human body. M. leprae causes the body to mount an attack and destroy the Schwann cells. Following a lengthy period of evolution, some parasite species have come to rely solely on their hosts for all aspects of survival. In fact, these exploitative species have lost certain structures that they no longer require while attached to a host but previously needed when they were living independently. For example, the body systems of tapeworms have degenerated to the point that they cannot see, move or digest food.
Predator-Prey
Any organism that consumes denotes a predator, whereas prey specifies an organism that is eaten. Prey may also designate a plant eaten by an herbivore. Bonds between predator and prey keep ecosystems in balance. Neither predator nor prey species exist in independent systems; rather, organisms within the ecosystem depend on a certain balance between all predator and prey interactions. To illustrate, when humans introduced the nonnative rainbow trout to the waters of New Zealand, the trout preyed more effectively on invertebrates than the native species did. This over predation caused the invertebrate species to decrease in number and increased the amount of algae--the organisms eaten by the invertebrates.