Communities of organisms are also affected by biotic factors, which are factors that are living.
All animals require food to live. The availability of food is a major factor in how many animals live in an ecosystem. Areas like rainforests with rich food supplies have more species of life than other areas like deserts and the Polar regions where there is less food.
The arrival of new predators in an ecosystem can have a devastating effect. In balanced ecosystems predators and prey have evolved together. Predators can catch enough prey to survive, but not enough to kill all their prey.
The arrival of a new predator can upset this balance. An example of this is the introduction of the red fox to Australia, which has caused concern over their effect on native birds and small mammals. Introducing new predators can cause a rapid decline in the numbers of prey, which then reduces the food supply for existing predators.
The introduction of a new species into an ecosystem can result in it out-competing another native species. Several hundred years ago grey squirrels were brought to England from North America by wealthy people and let free in their grounds. The smaller native red squirrel couldn't compete with the newer, larger grey squirrel. Grey squirrels are larger so they can store more fat and survive harsher winters. So the number of red squirrels and the places they live has reduced dramatically.