Population ecology is a science that deals
with measuring changes in population size, composition, and
identifying the factors that cause these changes. A
population is define as a group of
one species of organisms occupying the same general area, using
the same resources, and acted upon by the same environmental
factors. Populations cannot grow indefinitely, many populations
will become stable over a period of time while others will show
sharp increases followed by similar decreases. Population
characteristics that afford study are its
density and the
spacing of its individuals.
Population density is the number of individuals per unit area or
volume. Population dispersion is the pattern of spacing among the
individuals within the parameters of the geographical boundaries
of the population.
Due to the impracticality of capturing or
counting each individual in a given area, ecologists use a variety
of methods to determine the density of various populations of
organisms. Some of the sampling techniques used are as follows:
Counting the number of nests or burrows in a given area, examining
the number of tracks, examination of solid waste products left
behind, and an actual capture method called mark - recapture. In
the mark-recapture method, animals are trapped within their
boundaries, marked with a long lasting sign, and released. At a
later time some of the animals will be recaptured along with other
that are not marked. The data is then placed into the following
formula to calculate the population's density
(number marked) x ( total
catch the second time)
number of marked
This method assumes individuals have the same
probability of being captured as unmarked individuals. This
assumption is not always valid.
Patterns of Dispersion:
Local densities, within a population's range,
may vary substantially due to differences in the limiting factors
present. There are three general patterns of dispersion in
relationship to other individuals:
Clumped. is a pattern when individuals
are aggregated in patches. This style is caused by a heterogeneous
environment with resources concentrated in patches. Mating or
social behavior of the individuals may also contribute to this
type of dispersion.
Uniform. is a pattern of equally spaced
individuals. Competition between individuals may set up zones or
territories for feeding, nesting or breeding.
Random. is a spacing pattern based on
total unpredictability. This form of dispersal is highly uncommon
in nature. If it does it usually results from the absence of a
strong competition among individuals.
While the above patterns apply to individuals
within a population, keep in mind that populations within a
species show dispersal patterns.
Biogeography is the study of factors
that influence the distribution of a species over its
Demography is the study of the vital
statistics affecting population size. This branch of science deals
with the affects that immigration and emigration have on a given
population. Other than new organisms leaving or entering the
population birth and death rates are also studied.
Age Structure and Sex Ratio. Many
populations have overlapping generations where individuals of more
than one generation coexist. This situation produces an age
structure in most populations. Every age group has a
characteristic birth and death rate. A standard rule of thumb sets
a high mortality for the lower and upper age groups and a low
mortality rate for the intermediate age group.
Generation time refers to the
average time span between the birth of an individual and the birth
of their offspring. Body size has a major effect on generation
time. Mice,which are small,can produce a litter of individuals
ever 21 days. Their offspring usually are ready to reproduce in 3
to 4 weeks after birth. On the other hand, the Elephant takes 2
years to produce an offspring and several more years before the
offspring can successfully reproduce. The sex
ratio plays an important role in the
development of the population. Female number plays a dominant role
in a population since they are the ones to produce the offspring.
Males may mate with several females, diminishing them to a
subordinate reproductive role.
Life Tables and Survivorship Curves.
Life tables describe how birth rates and death rates vary with age
over time corresponding to maximum life span. A variety of
information about change in population size can be obtained from
life tables. In a particular species of bird fewer than 20% of the
original females survive to an age of three years, and a three
year old female has a 44% chance of surviving to age 4. Mortality
rates for females are constant through the middle years but higher
in young and older birds. The birth rate is highest for 4 year
olds and lower in very young and very old females. Survivorship
curves plot the numbers in a cohort still alive at each age. There
are three types of survivorship curves: Type
I curves flat during early and middle life and
drops suddenly as death rates increase among the older
individuals. Type II curves are
intermediate with mortality being more constant over the life
span. Type III curves show very high
death rates for the young followed by lower death rates after the
individuals have survived to a certain critical age.
Density-Dependent and Density-Independent
Density Dependent Factors. Increasing
population size reduces available
resources limiting population
growth. In restricting population growth, a density-dependent
factor intensifies as the population size increases, affecting
each individual more strongly. Population growth declines because
of death rate increase, birth rate decrease or both. There is a
reduction in the food supply which restricts reproduction
resulting in less offspring. The competition for space
to establish territories is a behavioral
mechanism that may restrict population growth.
Predators concentrate in areas where
there is a high concentration of prey. As long as the natural
resources are available, in sufficient quantity, the population
will remain constant. As the population decreases so do the
predators. The accumulation of toxic
wastes may also limit the size of a
population. Intrinsic factors may
play a role in limiting a populations size. High densities may
cause stress syndromes resulting in
hormonal changes that may delay the onset of reproduction. Immune
disorders are also reported to be related to stress in high
densely populated areas.
Density-Independent Factors. Weather,
climate, and natural disasters such as freezes, seasonal changes,
hurricanes, and fires are examples. These factors are unrelated to
population size and affect everyone in the population regardless
of population size.