Within the science of ecology, a community is a set of organisms coexisting within a defined area. Community ecology, then, is the study of the interactions that occur among groups of species coexisting within a region. For example, a community ecologist might consider the ways in which plants and animals within a forest affect one another's growth. Contrary to popular usage, the term ecology itself does not refer to recycling (conservation). Rather, it refers to the study of the distribution and abundance of living organisms and their interchange with each other and with the non-living (abiotic) world. Community ecology is concerned with the distribution, abundance, and interactions of mixtures of many different kinds of living things in a defined area.
By definition, a community is composed of two different species. Therefore, a set of birds consisting of two species of finches would constitute a very simple community. Most communities are much more complex, however, containing many coexisting plant, animal, bacterial, fungal, and protozoal (single-celled) individuals. Interestingly, a community is really defined by the person who is considering the community; the geographical boundaries of a community are arbitrary. The concept of a "community," depends on how it is defined.
Community ecology seeks to understand how species interact by studying many different kinds of relationships between organisms. Animal-animal interactions, animal-plant interactions, and plant-plant interactions are examples of community relationships considered. A plant-plant interaction might be the ways in which weeds affect growth of tomatoes in a garden, or how a tall tree blocks sunlight from smaller plants. An example of an animal-animal interaction is competition between birds for limited seeds for food, or how snakes prey upon mice in meadows. Plant-animal interactions include herbivory (eating of plants by animals) or Venus fly trap plants capturing and digesting insects.
Commuity ecology also looks to understand such concepts as niche, diversity, biogeography, diversity, species assembly, predation, competition, and symbiosis (the beneficial and parasitic coexistence of organisms). The major theme within community ecology is competition as the driving force for change in environments. This specialized field of biology is important because it helps scientists understand how communities are structured and how they change over time. Also, an understanding of community structure is vital to predict the effects of decline in, or extinction of, species (loss of diversity). Therefore, in order to fully understand the effect that humankind has upon our environment, knowledge of community ecology is needed.