A facultative parasite is an

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...

that may resort to

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

activity, but does not absolutely rely on any host for completion of its life cycle. Examples of facultative parasitism occur among many species of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

, such as family members of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Armillaria ''Armillaria'' is a genus of fungi that includes the ''Armillaria mellea, A. mellea'' species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armill ...

''. ''Armillaria'' species do parasitise living trees, but if the tree dies, whether as a consequence of the fungal infection or not, the fungus continues to eat the wood without further need for parasitic activity; some species even can ingest dead wood without any parasitic activity at all. As such, although they also are important

ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

agents in the process of nutrient recycling by microbial decomposition, the fungi become pests in their role as destructive agents of

wood rot Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin tha ...

. Similarly, green plants in

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

such as '' Rhinanthus'' and '' Osyris'' can grow independently of any host, but they also act opportunistically as facultative root parasites of neighboring green plants. Among animals, facultatively

kleptoparasitic Kleptoparasitism (etymologically, parasitism by theft) is a form of feeding in which one animal deliberately takes food from another. The strategy is evolutionarily stable when stealing is less costly than direct feeding, which can mean when f ...

species generally can survive by hunting or

scavenging Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding ...

for themselves, but it often is more profitable for them to rob food from other animals kleptoparasitically, whether their hosts are of the same species or not. Such behavior occurs in lions and hyenas for example, and also among insects such as "Jackal flies" in the family Milichiidae. More intimately, normally free-living microbes may opportunistically live as facultative parasites in other organisms. An example of this in humans is '' Naegleria fowleri'' - this excavate amoeba species is a free-living bacterivore, but occasionally it successfully infects humans with an often fatal result.

References

{{Reflist Ecology Parasitology

See also

References