Symbiosis Definition and Examples

Symbiosis Definition and Examples
Symbiosis is an ecological relationship between organisms of different species. It can be beneficial or harmful.

Symbiosis is an ecological relationship between organisms of different species that is mutualistic, commensalistic, or parasitic. The symbiotic species are called symbionts. In all forms of symbiosis, one symbiont benefits, but the other may benefit, be harmed, or feel no effect. Here is a look at the different types of symbiosis, with examples.

Obligate vs Facultative Symbiosis

Symbiosis is either obligate or facultative. In obligate symbiosis, the symbiosis depend on each other for survival. In facultative symbiosis, the relationship is optional.

For example, the fungal and photosynthetic symbionts in a lichen depend on each other, so their relationship is obligate. In contrast, the relationship between cattle and cattle egrets is facultative. When cattle disturb grass, it stirs up insects that the egrets feed upon. But, they can find food without the help of the larger animals.

Endosymbiosis and Ectosymbiosis

Another way of classifying symbiosis is as endosymbiosis or ectosymbiosis. In endo symbiosis, one organism lives on the surface of another. For example, ticks are parasites that live on the surface of larger animals. In endosymbiosis, one symbiont lives inside the tissues of the other symbiont. For example, human gut bacteria live inside the human digestive tract.

Types of Symbiosis

Types of symbiosis describe the nature of the relationship between the symbionts:

  • Mutualism: Both symbionts benefit in mutualism. For example, algae living within coral have a mutualistic relationship. The algae give the coral additional nutrients, while the coral protects the algae and holds it in close proximity to sunlight.
  • Commensalism: Commensalism is a type of symbiosis where one symbiont benefits, while the other is unaffected. For example, an orchid is an epiphyte that gains support and nutrients from its host plant, yet neither benefits nor harms the host.
  • Parasitism: In parasitism, one symbiont benefits at the expense of the other. For example, a liver fluke (a type of parasitic worm) gains shelter and nutrients from its host, but its host suffers from the loss of liver function.


Amensalism is a non-symbiotic relationship between members of two species in which one species is harmed while the other is unaffected. There are two types of amensalism:

  • Antagonism (Antibiosis): In antagonism, one organism damages another via a chemical secretion. For example, the black walnut (Juglans nigra) secretes juglone, an organic compound that kills herbaceous plants in the vicinity.
  • Competition: In competition, one organism deprives another organism of a resource. For example, a mature tree takes sunlight, water, and nutrients at the expense of saplings. Meanwhile the larger tree is unaffected by the sapling.


  • Douglas, Angela (1994). Symbiotic Interactions. Oxford University Press. ISBN 978-0-19-854294-0.
  • Moran, N.A. (2006). “Symbiosis.” Current Biology. 16(20): 866–871. doi:10.1016/j.cub.2006.09.019
  • Paracer, Surindar; Ahmadjian, Vernon (2000). Symbiosis: An Introduction to Biological Associations. Oxford University Press. ISBN 978-0-19-511806-3.
  • Sapp, Jan (1994). Evolution by Association: A History of Symbiosis. Oxford University Press. ISBN 978-0-19-508821-2.
  • Wernegreen, J.J. (2004). “Endosymbiosis: lessons in conflict resolution.” PLOS Biology. 2(3): e68. doi:10.1371/journal.pbio.0020068