Aquatic ecology and Biodiversity

Unit 3- Population ecology

3.10. Population growth
It is the increase in size of population is called population growth. Population growth occurs when birth rates exceed death rates or immigration exceeds emigration. Population size may be regulated by physical factors (weather, water and nutrient availability) and by biological factors (food availability, predators, parasites, competitors, and diseases). Factors that affect population density can be density-dependent or density-independent. Competition for resources, parasitism, predation and diseases are example of density-dependent factors while flood, drought, fire and other climatic conditions and most pest control actions are examples of density-independent factors. Density-dependent factors are important in regulating populations and in keeping populations at equilibrium.

Population growth can be explained by using the equation: Nn = Nt + B - D + I - E.

A population may grow exponentially or logistically (see Fig.). The exponential or geometric population growth curve is described by the formula rN = dN/dt; where N is present population size, t is time, r is a constant called the instantaneous rate of population increase. The logistic growth curve dN/dt = rN (K-N)/K where, N, t, r are the same as in the exponential growth model and K is the carrying capacity, or the maximum number of individuals the environment can support. However, populations cannot grow exponentially forever. When a population is growing in a limited space, the density gradually rises until interaction reduces the rate of increase ultimately leading to a reduction in population growth. This is logistic growth and the growth curve is sigmoid or S-shaped. The S-curve differs from the geometric curve in two ways:

• it has an upper asymptote
• it approaches this asymptote smoothly, not abruptly.