A population of a single species cannot survive by itself because there is inter dependence of one form of life on another. An aggregation of populations of different species living together (in inter dependence) in a specific area, having a specific set of environmental conditions constitute a biotic community e.g., the various plants and animals in a pond or lake constitute one biotic community whereas the plants and animals in a particular forest constitute another biotic community. Community ecology is the study of the organization and functioning of communities of organisms. It also studies the relationships of the members of a community to their environment.
Species interact in various ways such as competition, predation, parasitism, mutualism, commensalism and so on.
COMPETITION: species can compete with each other for finite resources. It is considered to be an important limiting factor of population size, biomass and species richness.
The competitive exclusions principle says that two species cannot exist together if they compete for the same resources. One will either die out or migrate, or they will adapt to carve out separate resource niches.
Example: Two birds found in American forests: the nuthatch and the brown creeper. They both seek food from the same trees, but the brown creeper travels up the trunk, while the nuthatch goes down. This means that they find and eat different insects.
An ecological niche is the role and position a species has in its environment; how it meets its needs for food and shelter, how it survives, and how it reproduces. It is advantageous for a species to occupy a unique niche in an ecosystem because it reduces the amount of competition for resources that species will encounter.
A fundamental niche is the term for what an organism's niche would be in the absence of competition.
Example: The male red-winged blackbird's mating call can be heard in the marshes in early spring. At that time, they hold the prime real estate in the marsh. However, as the season progresses, the more aggressive tri-color blackbirds move in. The tri-colors take over the best territory and force the red-wings to choose the leftovers. The entire marsh represents the red-winged blackbirds' fundamental niche.
Generally, however, there are competitors for the same lifestyle. Rabbits compete with groundhogs for food. Grasses compete with shrubs for soil, and bacteria compete with mold for nutrients among the leaf litter. The niche that a species actually inhabits, taking into account interspecific competition, is its realized niche.
MUTUALISM: is a relationship between organisms in which both species involved benefit to some extent with neither species being harmed.
Example 1: Oxpeckers and zebras or rhinos - In this relationship, the oxpecker (a bird) lives on the zebra or rhino, sustaining itself by eating all of the bugs and parasites on the animal. The bird benefits by having a readily available source of food and the zebra or rhino benefits from having the bugs removed. Also, when there is a danger to the zebra or the rhino, the oxpecker flies high and makes much noise in order to alert nearby animals to the impending danger (i.e. a predator).
Example 2: Flowers and bees - Bees and flowers have a mutualistic relationship as well. Bees get the nectar they need to make honey by traveling between flowers. The bee brings pollen from one plant to another, resulting in pollination. The bee is benefiting by getting food and the plants benefit from being pollinated.
COMMENSALISM: in biology, is a relationship between individuals of two species in which one species obtains food or other benefits from the other without either harming or benefiting the latter. The commensal relation is often between a larger host and a smaller commensal. This relationship can be contrasted with mutualism, in which both species benefit.
Example 1: Pilot fish live around sharks, sea turtles and rays and eat the parasites.
Example 2: Flatworms - A flatworm attaches to a horseshoe crab and it eats the crab’s food which does not harm the crab.