One of the most basic limits on an organism's population is the mere fact that an area does not necessarily have limitless resources but, if left unchecked, populations typically grow exponentially. The peak at which a population has reached its maximum size without exhausting resources is known as "carrying capacity." Once the population exceeds the carrying capacity, the death rate will exceed the birth rate until the organism's population is back below the carrying capacity.
Almost all biological communities involve some amount of predation, and this is a biotic factor that limits a species' growth. Organisms that feed on other organisms are called "heterotrophs" (in contrast to "autotrophs," which make their own food through photosynthesis). Carnivores feeds on animals, herbivores feed on plants and some micro-organisms survive through parasitism. While predators limit the size of a biological community, they do not normally bring any organisms to the point of extinction because predators will switch to a new prey when a population runs low.
Food sources for heterotrophs are considered a biotic factor because they rely on other organisms, however, other changes in abiotic resources may affect a biological community. This can include a change in climate (either annually due to a change in seasons, or unexpectedly, such as in the case of drought) or a change in water supply. The Law of Minimum is a theory in biology that states population growth is limited by the resource in the smallest amount.
Through out the history of earth there has occasionally been mass extinctions that caused by external causes. These extinctions are believed to have wiped out entire biological communities. Scientists count five mass extinctions in earth's history, including one 445 million years ago (caused by an overall decline in the sea level) and one 65 million years ago that wiped out the dinosaurs (likely caused by a combination of meteor impacts and volcanoes). Some scientists believe that we are currently in the midst of a sixth mass extinction, caused by the impact of human beings destroying biological communities.