Population size is affected by both the presence of resistance factors (things that reduce population size) and growth factors (the availability of resources that allow the population to grow), but ecologists have long debated which one has the greatest influence on population size. In most cases, both impact population size, though which plays the greater role may vary from species to species or even within a species, depending on a wide variety of factors, such as relative population sizes of a population versus its predators, competitors or prey, or seasonal climate changes.