Density and Dispersion
Population ecology is the study of population fluctuations as well as of the factors that regulate population size. Population density is the number of individuals per unit area or volume. It is impractical to count all individuals in a population, a variety of sampling techniques are used to estimate densities and total population sizes. Dispersion is a pattern of spacing for individuals within the boundaries of the population.
Models of Population Growth
Population size and density reflect the relative rates of processes that add individuals to the population and processes that eliminate individuals from the population. Mortality and natality are essential factors in population dynamics. Generation time is the average age when females of a population begin reproducing which greatly impacts on the intrinsic rate of increase. Carrying capacity is the maximum population size that can be supported by the available resources, symbolized as K. No population can grow indefinitely.
Regulation of Populations
Population size is regulated through density-dependent and density-independent factors. Any factor influencing population regulation that has a greater impact as population density increases is a density-dependent factor. Any population influencing population regulation that acts to reduce population size by the same fraction whether the population is large or small is called density-independent factor. Population density may change with temperature. Some populations of birds, mammals and insects fluctuate in density with remarkable regularity which is called population cycles. Stress resulting from high population density may alter hormonal balance and reduce fertility.
Demography is the study of the vital statistics that affect population growth. It is concerned with births and deaths: Events that most directly determine population density. Age structure is a relative number of individuals of each age in a population. Mortality is one of the factors that determine population density. Life tables can be constructed by following the fate of a group of new born organisms throughout their lives until all are dead. Survivorship curve is the plot of the number of the number of members of a cohort those are still alive at each age which is one way to represent age specific mortality. Birth rates also vary with age. Net reproductive rate is the expected number of female offspring that will be produced during the average lifetime of a female member of a population.
Human Population Growth
Different countries have different rates of growth for various reasons. Environmental, cultural and historical reasons are some of them. Although technology has increased our carrying capacity, the human population can not grow indefinitely.
Evolution of Life Histories
Life history contains birth, reproduction and death – the personal episodes of organisms. Three life history characteristics influence birth rates and death rates: Clutch size, age at first reproduction and number of reproductive episodes per life time. R-selected population refers to a type of population which produces large quantities of offspring with a relatively low probability of individual success. K-selection population refers to those whose life history is centered on producing relatively few offspring that have a good chance of survival.