All animals are infected with biotic agents.
An increase in the number of circulating phagocytic white blood cells (leukocytosis), mentioned above (see Maintenance of health: Defense against biotic invasion: Phagocytic cells of the body), is one of the more common manifestations of disease.
Biotic agents include life-forms that range in size from the smallest virus, measuring approximately 20 nanometres (0.000 000 8 inch) in diameter, to tapeworms that achieve lengths of 10 metres (33 feet).
Most biotic agents require a portal of entry through the intact skin or mucosal linings of the body.
The weak host is prey to many forms of biotic infection, even those of low virulence and invasiveness.
Immunosuppressive drugs are used to block the immune response in patients about to receive an organ transplant and in the treatment of the autoimmune diseases, but such treatment renders the patient vulnerable to attack by biotic agents.
Disturbances may be generated by abiotic, or nonliving, forces such as weather and wildfires, or they may occur as a result of biotic, or living, forces such as disease and invasions of exotic species.
Several in-depth examples of abiotic and biotic disturbances, including the ecological opportunities they create, are provided below.
The next major change in the systematics of lower forms came through an advancement in the concept of the composition of the biotic world.
Through a mutual benefit arrangement (symbiosis) between legumes and Rhizobium bacteria, nitrogen gas (N2) is fixed into a compound and then becomes available to the biotic world.
R-selected species, also called r-strategist, species whose populations are governed by their biotic potential (maximum reproductive capacity, r).
