Download Population Ecology

Unit 8: Population Ecology Notes
Environmental Systems
2015
What is population ecology?
Population ecology studies organisms from the point of view of the size and structure of their populations. A
population ecologist studies the interaction of organisms with their environments by measuring properties of
populations rather than behavior of individual organisms.
Features of populations include:
A. Population size – how many organisms of that species live in that area
B. Population density – the number of organisms per unit area
 The density of a population is often a reflection of the carrying capacity of the environment.
 Carrying capacity is how many organisms an environment can support and have enough
resources to go around.
C. Population distribution - how individual members of populations are spread over an area
 This helps to determine what particular aspects of the habitat are favored over others.
 One way to retrieve information from populations is to sample them. Sampling involves
collecting data about features of the population from samples of that population (since
populations are usually too large to examine in total.)
 Sampling can be carried out directly by sampling the population itself using appropriate
equipment, or indirectly by monitoring calls or looking for droppings and other signs.
D. Demographics – a population’s vital statistics (age structure, sex ratio, etc.)
 Populations have particular ratios of different age structures and sex
 ratios.
 Age structure refers to the size of an age range within a population.
 Factors taken into consideration when looking at age structure include:
o birth rate
o time it takes for each generation to be born
o death rate.
 Age structure pyramids are used to show this relationship.
Age Structure Pyramid
The data below enables ecologists to determine whether the population is declining or increasing
in size.
In a typical population:
 Younger age groups will be larger than older age groups,
all other groups being equal.
 Sex ratios refer to the percentage of males to females. Typically, the fewer females in a
population, the slower the population growth will be. There are only so many uteruses available
to raise a fetus.

Generation time is simple the average span of time between the birth of individuals and the
birth of their offspring. The shorter the generation time, the faster a population will increase in
size.

The birth rate refers to the average number of individuals born within a population. The greater
the birthrate, with all else held constant, the faster a population will grow. Birth rate, however,
typically varies with age. The more individuals that are within child bearing age, the more likely
the population will grow.

Death rate is the rate at which individuals of a certain age die. The death rate often varies with
age with either the very young or the very old displaying the greatest death rates. Note the
population growth occurs when the overall birthrate exceeds the overall death rates.
What do ecologists do with this information?
Population ecologists are able to determine many things from looking at population data.
 They can determine how a population fluctuates at certain times of year, indicating migration or mating
patterns.
 They can look at climate and environmental effects on populations, etc.
Population Density and Distribution
Distribution and density are two interrelated properties of populations.
 Population density is the number of individuals per unit area (for land organisms) or volume (for
aquatic organisms).
 Different species exist as different densities in their habitats, and the same species may be able to
achieve one density in one environment and another in a different environment.
o In low density populations, individuals are spaced well apart. There are only a few individuals
per unit area or volume. Characteristics of individuals in these populations are usually highly
territorial, solitary mammal species.
o In high density populations, individuals are crowded together. There are many individuals per
unit area or volume. Individuals in these populations are usually colonial organisms.
Population Density and Distribution (continued)

Careful observation and precise mapping can determine the distribution patterns for a species. The
three basic distribution patterns are: random, clumped and uniform.
1) Uniform distributions occur when individuals are evenly spaced within
the area. Uniform distributions result from individual organisms actively
repelling each other. The presence of one individual decreases the
probability of finding another individual very close by.
2) Random distributions occur when the spacing between individuals is
irregular. The presence of one individual does not directly affect the
location of any other individual. Random distributions are uncommon
in animals but are often seen in plants.
3) Clumped distributions occur when individuals are grouped in patches,
sometimes around a resource. Clumping may result either from individual
organisms being attracted to each other, or individual organisms being
attracted more to some resource or patch in that area, rather than elsewhere
in the area. Thus, some parts of the area will have a large number of
individuals whereas others will contain few or none. The presence of one
individual increases the probability of finding another close by. Such
distributions occur in herding and highly social species.
Note: Both clumping and uniform distributions suggest that individual organisms are either interacting with one
another by actively seeking each other out or actively avoiding each other, or are all competing with one
another form the same limited resources, regardless of the overall population density.
E. Population growth – the fluctuations of the numbers of individuals

Organisms do not generally live alone. A population is a group of organisms of the same species living
together in one geographical area.

This area may be difficult to define because populations of various species are different as territorial,
low density populations come together to mate.

The number of individuals making up a population may also fluctuate considerably over time.
o Populations can increase, decrease, or stay the same.

These changes make populations dynamic: populations gain individuals through births and
immigration (organisms moving into the area), and lose individuals through deaths and emigration
(organisms leaving the area).

For a population that is steady, or in equilibrium, these factors balance out and there is no net change in
the population size.

When losses exceed gains, the population decreases.
F. Limits on population growth – factors that keep population sizes in check
Very few species show continued exponential growth. Population size is regulated by environmental factors
called limiting factors.
 Density independent limiting factors may affect all individuals in a population equally, regardless
of the size of the population.
o Examples include physical factors of the habitat, catastrophic events, and human factors.

Density dependent limiting factors have a greater affect when the population density is higher.
They become less important when the population density is low.
o Examples include food supply, disease, parasites, competition, predation. Organisms that are
more crowded compete more for resources, are more easily found by predators, and spread
disease and parasites more readily.