Symbiosis, commen­salism, parasitism: Relation­ships between two different species

Symbiosis, commensalism and parasitism – all of them are types of relationships between two different animal species. In our blog you can learn more about their ecological function.


The terms

  • symbiosis
  • commensalism
  • parasitism

refer to forms of socialization between two different animal species. Each of these forms has its right to exist and fulfils an important ecological function.

What is a symbiosis?

In US American research, the term symbiosis generally refers to the coevolutionary coexistence of organisms, regardless of the way in which they live. In Europe, the term is specified more precisely. Symbiosis means the socialization of two different species, in which each species benefits from the other.

An example of this is the red-billed oxpecker (tax. Buphagus erythrorhynchus) which inhabits the savannahs of Central and Southern Africa. It preferably lives on large wild and domestic animals to feed on larvae and insects in their fur.

This cleaning symbiosis offers the oxpecker a food source as well as protection from predators and the host animals a liberation from fur parasites and, in this way, a reduced risk of an epidemic outbreak within the group. In addition, the oxpecker warns his hosts of approaching danger.


Forms of symbiosis: Mutualism and obligate symbiosis

Within symbiosis, a distinction is made between the two terms mutualism and obligate symbiosis.

Mutualism has a regular but non-vital correlation. Clownfish, for example, live together with sea anemones from whose tentacles they are protected. In return, the clownfish chase away the anemone's predators. Both animals benefit from each other but can also survive without each other.

In an obligate symbiosis, however, there is a vital socialization. An example of this is lichen, a symbiosis of algae and fungi. The carbohydrates photosynthetically produced by the algae serve the fungi as carbon source, the fungi in turn supply the algae with nutrient salts and water.

What is commensalism?

If the interaction between two species is positive for one species and neutral for the other, then one speaks of commensalism. An example of this are vultures that follow larger predators such as lions to feed on the remnants of their prey. When food is scarce, there may be competition between different commensals.


What is parasitism?

The most unpopular form of interaction between two organisms for many people is parasitism, an ancient, from a biological point of view highly interesting and ecologically important form of socialization of living beings. A host organism is attacked by a parasite which is usually much smaller. This parasite feeds on resources that the host provides. A classic example of this is mosquitos, which feed on the blood of humans and other mammals.

The acquisition of resources is a disadvantage or even harmful for the host. Usually the host stays alive, but in rare cases death can occur, especially if the parasite is not yet optimally adapted to its host or only uses it as an intermediate host, i.e. only needs the host temporarily to pass through a certain stage of development.

Forms of parasitism: Brood parasitism and kleptoparasitism

As with symbiosis, parasitism also takes various forms. When it comes to brood parasitism, birds use the clutches of other birds in order to let their own offspring brood. A famous example of this is the cuckoo. Kleptoparasitism – when food is stolen by other birds – is also widespread in the bird-world.

Why does parasitism exist?

As cruel as parasitism may be from a human point of view, it fulfils important ecological functions such as regulating the number of animals used as hosts. Old and sick animals are subject to a higher selection pressure because they are more susceptible to infestation and can die as a result.

In addition, all parasitically living animals together constitute a large proportion of biomass in various ecosystems and thus serve as an important food source for predatory animals.


By the way: the latest scientific studies show that the oxpecker, mentioned in the beginning of this article, has a rather parasitic lifestyle, since he spends only about 15 percent of his time on parasite eradication and is otherwise busy eating the earwax of his host animals or keeping their wounds open in order to feed on their blood or small pieces from the wound edges.

Learn more with Natucate

With a little bit of luck you can observe the oxpecker during one of the trips offered by Natucate to Central or Eastern Africa (South Africa, Botswana, Zimbabwe, Kenya).


Richard Lucius, Brigitte Loos-Frank: Biology of parasites. Springer textbook, 2008

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