Populations of all organisms on planet earth are dynamic as they are constantly changing due to their interactions with various biotic and abiotic factors. At any point in time, in a population, some organisms are born, some die, some immigrate while some emigrate. These four processes are known as natality, mortality, immigration and emigration respectively. Natality is the number of births in the population that are added to the initial density during a given period while mortality is the number of deaths in the population during a given period. Immigration is the number of individuals of the same species that come into a particular habitat from elsewhere during a given period while emigration is the number of individuals of a population who leave a particular habitat and go elsewhere during a given period. In a population, natality and immigration result in an increase in population density, while mortality and emigration result in a decrease in population density.
Growth is the most fundamental, dynamic feature exhibited by populations. Interestingly, population growth exhibits two patterns: exponential growth and logistic growth. When resources available to the individuals in a population are unlimited, there is a tendency of the individuals to grow exponentially. This behaviour was observed by Charles Darwin while he was developing his theory of natural selection. He also showed that a slow-growing animal, such as the elephant, could also reach enormous numbers if resources such as food and space were inexhaustible.
The difference between the birth and death rates provides the value of ‘r’ or the ‘intrinsic rate of natural increase’. This is a very important parameter for measuring the impact of biotic or abiotic factors on population growth. The intrinsic rate of natural increase is a measure of the inherent potential of a population to grow. There is always competition between individuals for limited resources and, finally, the fittest individuals survive and reproduce. Keeping this fact in mind, the governments of many countries have introduced various restraints to limit human population growth. In fact, in nature, a given habitat has sufficient resources to support a maximum number of individuals, beyond which no further growth is possible. This maximum capacity or limit is called the carrying capacity of the species in that habitat, which is denoted by the letter ‘K’. Thus, we can say that population density is dynamic and the growth of the population is dependent on the resources available.