Commensalism and mutualism among some of the organisms could be responsible for this, or it may result from a population that that preys on the competing organisms. The competing populations can be stabilized by feedback control. An interaction between organisms in which one benefits and one is harmed based on the ingestion of the smaller sized organism, the prey, by the larger organism, the predator. Predation is brought about by endocytosis and is restricted to certain protozoa and others with amoebal phases. The end result is an oscillation between the predator and the prey.
Parasitism is when the predator is smaller than the prey.
Indirect parasitism involves the the absence of cell contact during the process. In complicated mixtures of organisms, individuals usually play several roles. For example, they can prey on others while being themselves the food for larger organisms.