Populations of organisms change from one time period to the next. When resources (food, nesting sites, or refuges) were limited, populations would decline as individuals competed for access to the limiting resources. Helped to regulate the population around carrying capacity. Predation can also influence the size of the prey population by acting as a top-down control. Interaction between these two forms of population control work together to drive changes in populations over time (additional factors such as parasites and disease can further influence population dynamics) Most notable examples of population changes occur in species that experience large, cyclic swings in population size. Most often cycles co-occur with other species population cycles in same location. Preferred foods of predator influence population size of each, availability of food acts as bottom-up control that affects population size. When preferred foods are scarce, individuals turn to other food, they grow more slowly, reproduce less and population decline.