As surely as individuals are born and die, populations grow and decrease. At
any time in its history, the size of a population is the product of births and
deaths, and immigration and emigration. The rate at which individuals enter the
population (births and immigration) and individuals leave (deaths and
emigration) will determine whether the population is growing, shrinking, or is
stable, and if it is changing, how fast.
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 are studied 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 parameters of the
geographical boundaries of the population.
DENSITY AND DISPERSION
Every population has geographical barriers and a population size
Population density is the number of individuals per unit area or volume
Since it is impractical to capture and/or count each individual in a
given area, ecologists use a variety of methods to determine the density of
various populations of organisms. Some of the techniques used are as
follows: Counting the number of nests or burrows in a given area, analyzing
the number of tracks, examination of solid waste products left behind by a
species, and an actual capture method used to tag and release the specimen
Mark-recapture method- traps are placed within the boundaries of the
population being studied and the captured animals are marked and released.
After a few weeks, traps are set again. The proportions of marked to
unmarked that are captured the second time give an estimate of the size of
the entire population.
N= (number of marked)(total catch second time) / number
Patterns of Dispersion:
Dispersion is the pattern of spacing among individuals within the
geographical boundaries of the population. Local densities, within a
population's range, may vary considerably due to differences in the limiting
factors present. There are three types of patterns of dispersion in
relationship to other individuals: clumped, uniform, and random.
- 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
- 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.
Demography is the study of the vital statistics affecting a population size.
This branch of science deals with the influence that immigration and emigration
have on a given population, this does not usually include the new organisms
leaving or entering the population birth and death rates are also studied.
Age structure and Sex Ratio states that each age group has a
characterized birth and death rate (Juveniles and old people are more likely
to die). Birth Rate is the number of individuals produced during a certain
amount of time (is greatest for individuals of intermediate age).
Age and Sex Ratios - several populations have gone beyond 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.
Generations over time, is the average time span between the birth of
individuals and the birth of their offspring and is strongly related to the
body size over a broad range of organisms a shorter generation time will
result in faster population growth. Body size has a major effect on
For example, Elephant takes 2 years to produce an offspring and several
more years before the offspring can successfully reproduce. On the other
hand, mice can produce a litter of individuals ever 21 days. Their offspring
usually are ready to reproduce in 3 to 4 weeks after birth.
Females play a central role in a population since they are the ones to
produce the offspring, but males may mate with several females each breeding
Density-Dependent and Density-Independent Factors:
Density Dependent Factors
Population Cycles is a couple of concepts - one idea is that crowding
regulates cyclic populations and another is that population cycles are
caused by a lag time in the response to the density-dependent factors,
creating large fluctuations of population size above and below carrying
capacity. Increasing population size reduces available resources and this
eventually limits population growth. In restricting population growth, a
density-dependent factor strengthens as the population size increases,
affecting each individual more strongly. On the other hand, the population
growth declines because the death rate increases, birth rate decreases or
both. If there is a reduction in the food supply or other major resources,
could also restrict the reproduction cycle resulting in less offspring.
The animal speciesí battle to establish territories is a behavioral trait
that may restrict population growth and available food supplies. Predators
concentrate in areas where there is a high concentration of organisms. As
long as the natural resources are available in sufficient quantity, the
population will remain constant. As the population decreases so, do the
They are unrelated to population size and will affect the same # of
individuals regardless of weather and climate Examples are weather, climate,
and natural disasters such as freezes, seasonal changes such as unusually
hot or cold temperatures, hurricanes, and fires. All of these factors are
unrelated to population size but affect everyone in the population
regardless of population size or location.
As we stated in the last lesson populations do not live alone, just as
individuals do not. When individuals of the same species, or of two
different species, depend on a common important resource then competition
occurs. Competition can be defined as "interactions between individuals
brought about by a shared requirement for a resource in limited supply
leading to a reduction in survivorship, growth, and reproduction of
individuals." Interspecific competition (inter = between) is the competition
between two or more different species for a resource.
MODES OF POPULATION GROWTH
A population that begins at low levels in a favorable environment may
increase rapidly for a while, but eventually the numbers must stop growing.
Exponential Population Growth
In an ideal environment, there are no restrictions on the abilities of
individuals to harvest energy, grow and reproduce thus, the population will
grow in size with every birth and with immigration, and the population will
decrease with every death and emigration. In a population living under ideal
conditions, the population grows fast, because all the members have access
to abundant food and are free to reproduce.
Populations increase under these conditions and the size of the
population increases rapidly, resulting in the population actually
accumulating more new individuals per unit of time when it is large than
when it was small, and it will pass the threshold point (the environment
will not be able to sustain the population anymore)
Logistic Population Growth
In most populations, there is a limit to the number of individuals that can
occupy a habitat. The carrying capacity is the maximum stable population
that a particular environment can support of a relatively long period of
time. Crowding and resource limitations can have a profound effect on the
The logistic model is a model of species-specific competition: the
competition between two or members of the same species. As the population
sizes increases, the competition become more intense.
Territoriality is the defense of a well-bounded physical space. The model
is the idea that even at low populations each individual added to the
population has the same negative effect on population growth rate and that
some populations show an effect in which individuals may have a time that is
more difficult surviving and reproducing if the population size is too
Experiments for Home and Classroom
You can try these desert experiments at home. You can create a mini-pond
at home and grow your own algae, see how cyanobacteria grow mostly in the
warm, upper layers of a lake or see how warm temperatures speed up bacterial
growth. Desert plants grow better in the heat than plants imported from
colder places. See how plants require the right temperature in order to
grow. A little bit of air or water, pollution goes a long way. Find out how.
Here are 12 ecology-related experiments for you to try:
Before starting any of the experiments listed be sure to read the
Build a small ecosystem and make a mold terrarium, watch tiny blue, green
and white plants grow on leftover food.
for Students, Parents and Teachers
Now let's do Practice
Exercise 6-3 (top).
Water, Carbon and Nitrogen Cycle (top)