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 ...

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 has c ...

activity, but does not absolutely rely on any

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

for completion of its

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...

. 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 from ...

, such as family members of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Armillaria ''Armillaria'' is a genus of fungi that includes the '' 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''. ''Armillarias'' are long-l ...

''. ''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 th ...

. 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 ''Rhinanthus'' is a genus of annual hemiparasitic herbaceous plants in the family Orobanchaceae, formerly classified in the family Scrophulariaceae. Its species are commonly known as rattles. The genus consists of about 30 to 40 species found in ...

'' and ''

Osyris ''Osyris'' is a genus of plants in the family Santalaceae, one of the many genera known as sandalwoods, but not one of the true sandalwood. The species of this genus are mostly hemiparasitic, meaning although they can survive and grow by themsel ...

'' 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 foo ...

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 be ...

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 Milichiidae are a family of flies. Most species are very small and dark. Details of their biology have not yet been properly studied, but they are best known as kleptoparasites of predatory invertebrates, and accordingly are commonly known as fr ...

. More intimately, normally free-living microbes may opportunistically live as facultative parasites in other organisms. An example of this in humans is ''

Naegleria fowleri ''Naegleria fowleri'', colloquially known as a "brain-eating amoeba", is a species of the genus ''Naegleria'', belonging to the phylum Percolozoa, which is technically not classified as true amoeba, but a shapeshifting amoeboflagellate excava ...

'' - 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