The study of interactions among organisms and between
organisms and their environments in the laboratory or in
nature is called ecology. A population consist of the individuals
of a given species that occur together at one place and at one
time. Population ecologists study how populations grow and
interact. The growth rate of any population is the difference
between the birth rate and the death rate. The actual change
in a population may also be affected by emigration from the
population and immigration into it, but these factors are only
considered in human populations. Most populations exhibit a
sigmoid (logistic) growth curve, which implies a relatively
slow growth, a rapid increase (exponential growth), and then
a leveling off when the carrying capacity of the species’
environment is reached. A pattern of exponential growth with
a sudden die off of the population is called a boom and bust cycle.
In addition to a population’s size, its density the number of organisms
per unit of area influences its survival. Individuals in a population may
be dispersed (dispersion) in uniform, clumped, or random manner.
Each population grows in size until it eventually reaches the limits
of its environment to support it; resources are always limiting. Some
of the limits to the growth of a population are related to the density of t
hat population, but others are not. Factors that result from the growth
of a population and regulate its subsequent growth are density dependent
factors. Three density dependent limiting factors are competition, predation,
and parasitism. Factors such as weather conditions that regulate the
growth of a population and that operate regardless of its density are
density independent factors. When a population lives in a constant
environment for a few generations, its age distribution the proportion
of individuals that survive to the different age categories becomes stable.
Mortality is the death...