Examples of facultative parasitism occur among many species of
fungi, such as family members of the
genus Armillaria.
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 agents in the process of
nutrient recycling by microbial decomposition, the fungi become pests in their role as destructive agents of
wood rot. Similarly, green plants in
genera 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 species generally can survive by hunting or
scavenging 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
percolozoan amoeboflagellate species is a free-living
bacterivore, but occasionally it successfully infects humans with an often fatal result. ==See also==