In a world where competition among individual organisms drives evolution, the concept of symbiosis seems foreign. Symbiosis describes a close association of two organisms that benefits at least one of the organisms. At times, these close relationships evolve; some beneficial relationships may go sour, while destructive relationships persist to the point of benefiting both species. Changes in genes or behavior that improve reproductive chances transfers to offspring, while any trait detrimental to an organism’s survival generally decreases in frequency in descendant populations until that characteristic dies out altogether.
In mutualistic relationships, both species benefit. Evolution created an association in which each species survives better in the company of the other. Termites harbor bacteria in their digestive systems. The bacteria break down cellulose from wood into nutrients upon which both use; the termites provide the cellulose and habitat for the bacteria.
In the dry grasslands of Central America, ants nest within the acacia tree’s thorns, using the nectar and protein produced by the leaves as food. As payment, the ants defend the tree against attack by insects and secrete a noxious chemical that drives animal grazers away from the normally edible leaves. The ants only attack pests while leaving bees and other pollinators intact.
Parasitism benefits only one organism in the relationship and harms the other. Organisms that live off and damage others are the culprits responsible for diseases, from the most common infections to the deadliest of illnesses. The host is the organism that harbors the parasite. In general, parasites are much smaller than their hosts, give rise to more offspring and will kill the host if present in great numbers. Parasites can cause their hosts to evolve; for instance, female birds that choose to mate with male birds decorated with the brightest feathers since presumably, parasite-infested males would not be able to spare the resources to produce brilliant-colored plumage.
Commensalism expresses a relationship between two organisms in which one benefits and the effect on the other is neutral. In nature, commensalism often is difficult to observe. Most associations between two organisms affect both in some way, but biologists may not be able to ascertain exactly what those effects may be. A bird called the cattle egret feeds on insects rustled up by the activities of grazing cattle. The relationship does not seem to affect the cattle, but upon further investigation, a more mutualistic relationship may exist, as the egret feasts upon ticks on the cattle’s hides. Barnacles harmlessly attach to the shells of scallops, but biologists are unable to determine whether the barnacles weigh down the scallops or compete with them for food.
Ectosymbiont vs. Endosymbiont
Symbiosis can occur on the surface of an organism or within its body. Parasites such as fleas and ticks latch onto a host’s body; endoparasites such as certain disease-causing, or pathogenic, bacteria must reside within their hosts’ organs to complete their life cycles and derive nutrients. The bacteria that live within tube worms inhabiting the hot vents deep in the oceans convert the sulfur exuding from the vents to nutrients for themselves and the tube worms. Both harmful and beneficial bacteria live within the large intestines of humans.
Obligate vs. Facultative Symbiont
Obligate symbionts must live together to be able to carry out life processes, whereas facultative symbionts benefit from the association but do not need to live in symbiosis. Obligate symbionts such as tapeworms have lost most of their functions to pursue their parasitic relationship with their hosts. Two historic instances of obligate symbioses have defined modern cells; the chloroplasts essential to a plant’s food production and the mitochondria that convert nutrients in the cells to energy both evolved from relationships between primitive cells.