Organisms and their Environment - Population size

A population is defined as a group of organisms of one species, living in the same area at the same time.

Population growth follows a sigmoid (S-shaped) curve consisting of four main phases: the lag phase, the log (exponential) phase, the stationary phase, and the death phase.

During the lag phase, the population size $N$ increases slowly as organisms adapt to their environment and reach reproductive age.

In the log phase, the growth rate is at its maximum because resources like food and $H_2O$ are abundant and limiting factors are minimal.

The stationary phase occurs when the birth rate equals the death rate, often because the population has reached the carrying capacity ($K$) of the environment.

The death phase (or decline phase) occurs when the death rate exceeds the birth rate due to factors like accumulation of toxic waste, depletion of food, or disease.

Limiting factors are environmental factors that restrict population growth, such as light intensity, temperature, predation, and competition for resources.

The human population has been increasing exponentially due to improvements in agriculture, sanitation, and healthcare, which have significantly reduced the death rate ($D$).