A population is a group of organisms of the same species, living in the same area (or habitat) at the same time 

• The size of a population can be increased by natality (births) or immigration, while it is decreased by mortality (deaths) and emigration

Members of a population typically breed together, meaning reproductive isolation distinguishes one population of a species from another

• When multiple populations exist within a given area, they collectively form an ecological community
  

• A community and its abiotic environment (habitat) is collectively called an ecosystem
  

Ecological Organisation