A custodial species is one that plays a vital role in maintaining the integrity of the environment, either through actions such as pollinating plants or through keeping the soil or water free of dangerous pollutants. This species is usually considered to have an irreplaceable role in the environment, and it is essential to protect this species in order to ensure that the environment is safe and healthy. Custodial species can often span multiple trophic levels, serving as both prey and predator to other species, and can also be an important component of food chains, forming energy pathways and playing an integral role in nutrient cycling.

See also: evolution, emergence, proto b, crypto, edge of chaos