In nature, we rarely find isolated, single individuals. They live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus from a population.
Although, it is an individual organism that has to cope with changes in the environment, it is at the population level that natural selection operates to evolve the desired traits. Population ecology is, therefore, an important area of ecology because it links ecology to population dynamics, genetics and evolution.
A population has certain attributes of its own, which are different from those of an individual. Basic physical characteristics of population are - its size and density. Size speaks for the number of individuals in a population, while density tell us the number of individuals present per unit space, in a given time. Besides size and density, the other characteristics include nasality, mortality, immigration, emigration, age pyramids, expanding population, population growth forms, and biotic potential. An individual has birth and death, but a population ha birth rate and death rate.
Nasality is the birth rate of a population. It has the greatest influence on a population's growth. Nasality is a crude birth rate is used when calculating population size, whereas a specific birth rate is used relative to a specific criterion such as age.
• Absolute Nasality : the number of births under ideal conditions.
• Realized Nasality : the number of births when environmental pressure come into play.
• Absolute Nasality : the number of deaths under ideal conditions.
• Realized Nasality : the number of deaths when environmental pressure come into play.
Population growth
The size of population for any species is a dynamic parameter. It keeps changing with time, depending on various factors including food, predation pressure and adverse weather. In fact, these changes in population density that give us some idea whether it is flourishing or declining.
Growth models: Does the growth of a population with the time show any specific and predictable pattern? We have been concerned about unbridled human population growth and problems created by it in our country, and it is therefore natural for us to be curious if other animal populations in nature behave the same way or show some restraint on growth.
Exponential growth:
Resources like food, space are essential for any growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to fully realize it's innate potential to grow in numbers. Then the population grows in an exponential or geometric proportion.
Every species is capable of growing exponentially under unlimited resource conditions, and reach enormous population densities in a short period. Drawn showed how even a slow growing animal like elephant could reach enormous numbers.
But resources like food and space are not always unlimited. They may be in the beginning; but as the population density increases, so does the competition for those resources, resulting in a slowdown in the rate at which the original population was growing. This results in logistics or in sigmoid growth curve.
There is no natural habitat, which has only one species or animals of plants
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