In nature, a variety of physical and biological factors influence an organism's ability to grow and reproduce in any one place. There is variability in the way that different species respond to the environment, and that causes them to have different ranges. However, a species' actual presence on a site depends upon its ability to disperse onto that site, as well as its ability to survive its environment. Organisms of a given species that occur together comprise a population. Populations can grow, but various environmental factors prevent populations from growing indefinitely.
Populations can also decline to the point of extinction.
Species that have similar responses to the environment are typically found together in assemblages called communities. Each community is unique, but those occurring in similar environments are generally similar to one another. Adjacent communities often blend into each other. On any site, the organisms interact in various ways. Some help each other while others are harmful.