Human population size is influenced by a combination of factors, including natural forces like disease, famine, and natural disasters. However, human population Read more Ecosystems +2 What are two ways an organisms joina population and two ways organisms leave a population? Asked by Wiki User Organisms can join a population through birth (natality) or immigration. Conversely, they can leave a population through death (mortality) or ello slot - l.d8.9.adl@forum.annecy-Outdoor.com, emigration.