A facultative parasite refers to a species, which may resort to parasitic activity, but is not completely dependent on any host for finishing its life cycle. Therefore, the growth of Armillaria fungi inside a tree's roots, making the tree to rot, but will survive even if the tree dies, is an example of facultative parasitism.
An obligate parasite refers to a parasitic species, which cannot finish its life-cycle in the absence of a suitable host. Therefore, rickettsia bacteria's tendency to surviving only in a eukaryotic cell is an example of obligate parasitism.
A long-term symbiosis or biological association in which members of one species attain benefits while those of the other species is neither harmed nor are benefited is known as commensalism. Hence, a pseudoscorpion, a kind of arthropod hiding under the wings of beetle for protection and transportation, however, it does not affect the beetle is an example of commensalism.
Predation refers to a biological association, where one species, the predator, kills and consumes another species, known as prey. A freshwater amoeba feeds on bacteria in the water is an example of predator and prey.
The muscular system of an organism controls the organism’s muscles. When a hummingbird’s muscular system is affected, the muscles will become weak from a disease and they will potentially die because they will not be able to survive without the strong support of their legs or any other muscle in its body.