Population ecology is the study of the variables that determine the abundance and distribution of a population in time and space. The genetic makeup of the population together with the local environment and ecological factors determine its success – how well it survives and how fast it grows.
The number of individuals of a species in a defined area is a measure of population density. Population size increases and decreases over time. There are factors such as resource availability, competition, parasitism, predation, climate, etc. that usually operate in an ecosystem to keep populations within certain boundaries.
Birth and death rates, and immigration and emigration determine age distribution (proportion of individuals in each age group) of a population. Fecundity (rate at which females produce eggs), fertility (rate at which females produce zygotes), and sex ratio (proportion of male and female in the population) affect birth rate. Typically, expanding populations have a large percentage of young individuals while declining populations have a large percentage of old individuals, and stable populations have a relatively even distribution among age groups. Many of the pests in the managed systems are short-lived, with a life cycle well synchronized with the culture of the crop.
Dispersal is movement of individuals or their offspring into or out of an area. Dispersal allows individuals to colonize new areas of crop fields. Dispersal, along with birth and death rates, regulates population size, and plays an important role in evolution through mixing of genes between populations. Dispersal is accomplished through immigration (movement into a population), emigration(movement out of a population) or migration (frequent movement into or out of a population area).
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, parasitoids, competitors, 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 demographic equation: Nn = Nt + B – D + I – E.
A population may grow exponentially or logistically. 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 for ever. 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: (i) it has an upper asymptote and (ii) it approaches this asymptote smoothly, not abruptly.