Organisms that inhabit a certain area make up the community of organisms for that area. Organisms living within a community have various kinds of interactions and relationships with each other. 

Some organisms live in a predator-prey relationship. For example, a common roach eats water fleas, whereas a pike eats roaches. If the population of water fleas suddenly increases, so will the population of roaches, as well. As a result of the increased roach population, the size of the water flea population begins to decrease once again. In turn, the size of the roach population is kept in check by the pike population. Therefore, the different populations of a community affect each other in terms of the size of populations. 

Food chain: water fleas → roach → pike.

The individuals of the same species compete with each other. Competition can also arise between species whose ecological niches are similar. Having a similar ecological niche means that the two species have similar adaptations and lifestyles: for example, they can have similar diets or make use of similar mating locations.

Some species live with each other so closely that they are dependent on each other. This is called symbiosis. For example, lake fungi live in symbiosis with tiny algae. This symbiosis is mutually beneficial for the two species: the fungus receives sugar from the photosynthetic alga, whereas the alga receives a sheltered and stable growing surface from the fungus.