Organisms interact through competition for resources and through symbiotic relationships. Competition occurs when multiple species require the same limited resources and can lead to competitive exclusion of one species. Symbiotic relationships include mutualism, commensalism, and parasitism, where species interact and one benefits while the other is helped or harmed. Population growth is determined by immigration, birth, emigration, and death rates, and is typically limited by resource availability through factors like predation, disease, and competition.