Populations of animals exist in cooperative and competitive relationships with each other. For any species to thrive it must find access to food resources and be able to successfully reproduce. Ecologists have identified many methods for survival among the species of animals and have, consequently, described many of the survival techniques. Three fascinating relationships exist that intrigue both the scientist and the layperson. They are mutualism, parasitism, and commensalism.
Mutualistic relationships may be the most fascinating because of the cooperation that exists between species. Competitive relationships in which two or more species compete for the same resource are quite common. However, when two species evolve a pattern of survival from which both benefit, it is an interesting scenario for biologists to consider.
In a mutualistic relationship two organisms from entirely different species behave in a way in which both benefit. Most often, the two organisms are from species with very different lifestyles and nutritional needs. Often the need is not even nutritional, but rather one in which the offspring are assured a better chance for survival. No matter the reason or outcome, the dependency of the two organisms continues to grow with each passing generation until the two life cycles are dependent on each other for survival.
Complete and total dependency is termed obligative mutualism . Obligative mutualists are found in the association between termites and their intestinal bacteria. The termites cannot digest the tough plant material cellulose. The bacteria can. Consequently the termite gut is full of these cellulose-digesting bacteria. The bacteria benefit by having a relatively safe place to live and reproduce, and the termite benefits by gaining access to the nutrition of the plant after the cellulose has been digested.
Another example of mutualism is the relationship between some large grazing animals like the rhinoceros and the small tick birds. The birds feed on the insects that cling to the skin of the rhinoceros. In turn, the size and reaction of the rhinoceros to predators helps the tick birds to remain safe and live a longer life.
Another interesting relationship between species is that of parasitism. In a parasitic relationship one species lives at the expense of another. There is a clear distinction between predation and parasitism. A predator will kill the prey outright and receive its benefit directly. A parasite will not kill its host (the organism on which it lives) outright. It is better for the parasite to have its host live as long as possible to ensure continued survival and reproduction for the parasite.
One of the most familiar parasitic relationships is that of a tapeworm. The tapeworm has no digestive or sensory (eyes, ears, nose) system. In fact, it does not even have a circulatory system (heart and blood vessels). The fact that tapeworms live in the digestive system of a host means they do not need to digest their own food. It is already done for them. They do not need to sense their environment because they are protected in the gut of the host. They are small, so nutrients are passed from one cell to another by simple diffusion . Unfortunately for the host, the nutrients it ingests are being absorbed instead by the tapeworm. Over a long enough period, the host will become malnourished, lose weight, and eventually die.
Another example of parasitism is less deadly on the host, but may eventually affect populations. Interspecific (between species) and intraspecific (within the species) brood parasitism is often seen in birds. One female will lay her eggs in the nest of another. The unsuspecting nesting parents may not recognize the odd egg and will continue to sit and hatch all the eggs in the nest. After the eggs hatch, the parents will spend more resources feeding and caring for the unrelated offspring, sometimes at the expense of their own.
Cuckoo birds have been known to lay their eggs and leave their young for others to care for. Often the baby cuckoo is larger and more demanding than the other babies. More food will go to raising the cuckoo than to the other nestlings. Sometimes the larger cuckoo will even kick the competing young out of the nest, duping the parents into raising the one single chick.
Purple martins are another species of birds in which brood parasitism occurs. The main difference is that the parasitism occurs within the species. This may not be as harmful a situation for the nestlings as it is with the cuckoo. In fact, some researchers suggest that this form of parasitism may even promote colonization and population growth.
Commensalism is a newer and more inclusive term for an older concept called symbiosis . The term "symbiosis" now more broadly describes the three types of relationships discussed in this article. Commensalism is a relationship in which one organism benefits and the other is unaffected. There is a very fine line in identifying a relationship as commensal . Some relationships may actually be long-term parasitism. Much study is needed before a relationship can be truly described as commensal.
In one such relationship between a small crab and an oyster, the crab enters the oyster as a small larva. The inside of the shell provides a safe place for the crab to grow and develop. Once the crab has reached maturity, however, it is too large to leave through the oyster's narrow valve opening. It remains inside the oyster, feeding off floating particles of food that are siphoned by the oyster from the surrounding water. Neither animal is harmed, and the crab has a safe place to live and receive food.
Another commensal relationship exists between some species of ants and aphids, the sucking parasites of plants. The aphids secrete a sweet liquid substance that is consumed by the ants for food. The ants tend the aphids as a farmer would tend livestock. The aphids are not harmed and the constant attention by the ants keeps the aphid colonies fairly clean.
Scientists still do not completely understand why and how mutualism, parasitism, and commensalim evolved. Each of these types of relationships are distinct and specialized. They have evolved over time, and the survival of individual species is dependent on these highly evolved relationships. There is a great deal to be learned about each of them.
see also Endosymbiosis; Parasitism.
Brook Ellen Hall
Cody, Martin L., and Jeffrey A. Smallwood, eds. Long-Term Studies of Vertebrate Communities. San Diego: Academic Press, 1996.
Levin, Simon A. Fragile Dominion: Complexity and the Commons. Reading, MA: Perseus Books, 1999.
Tokeshi, M. Species Coexistence: Ecological and Evolutionary Perspectives. Oxford: Blackwell Science, 1999.