Population is defined as a collective interbreeding or otherwise interacting group of organisms of the same species occupying a particular space. In studying populations, we are not just concerned with the numbers of a given species living in a given area at a given moment in time but rather with an understanding of how populations grow, are maintained and decline in response to their environments. We are interested in distribution, density, natality rate, mortality, age distribution, growth rate, genetic characteristics directly related to their ecology, adaptiveness, fitness and persistence of a population.

Human (Homo sapiens) has been presented on the Earth for about 800,000 years. Due to better food production and medical care after the Industrial Revolution, our numbers have increased dramatically and the human population reached 6 billion in 1999. The world's population is growing exponentially at a rate of about 1.33% per year. It means that the world's population of 6 billion will grow by 80 million people, an averaged increase of 219,000 people a day, or 9,100 an hour from 1999 to 2000. It is estimated that the world's population will reach 7 billion in 2013, 8 billion in 2028 and 9 billion in 2054.

Due to rapid population growth, our demand for different kinds of resources is continuously increased. Overexploitation of resources leads to many environmental problems such as food problem, soil erosion, excessive use of pesticides and chemical fertilizers, desertification, and species extinction. The environmental impact of population depends on the number of people, number of units of resources used per person and environmental impact per unit of resource used.

Environmental impact
[Source: G. Tyler Miller, JR, Environmental Science P. 20]