Name: ________________________
STUDYING POPULATIONS
Date: _____ Class: ____ Seat #: ____
Population density = Number of individuals
Unit Area
One way to state the size of a population is in terms of
population density—the number of individuals in a specific area.
Population density is calculated by dividing the number of
individuals in the population by the total area. The result tells how
many individuals there are per unit area. It actually looks like a
mathematical equation. (See above)
Suppose you counted 50 monarch butterflies in a garden
measuring 10 square meters. The population density would be 50
butterflies per 10 square meters, or 5 butterflies per square meter.
Some methods of determining the size of a population are direct and indirect
observations, sampling, and mark-and-recapture studies. Direct observation involves
counting the members of a population one by one. Indirect observation involves counting
tracks, nests, or other signs rather than the organisms themselves.
Sampling-Many times, it is not possible to count every member of a population because the
population is too large or spread out or the members are hard to find. Instead, ecologists make an
estimate of the population. An estimate is an approximation of a number, based on reasonable
assumptions. One type of estimating involves counting the number
of organisms in a small area (a sample), and then multiplying to find
the number in a larger area.
Ecologists sometimes use a technique called mark and
recapture. Some animals are first captured, marked, and released
into the environment. Then another group is captured. The
ecologists count the marked animals in this group. Using a mathematical formula, the ecologists
can estimate the total population of those organisms in the area.
Populations can change in size when new members enter the population or when members
leave the population. The major way in which new individuals are added to a population is
through the birth of new offspring. The birth rate of a population is the number of births in a
population in a certain amount of time. The major way that individuals leave a population is by
dying. The death rate is the number of deaths in a population in a certain amount of time. If the
birth rate is greater than the death rate, the population will generally increase in size. If the death
rate is greater than the birth rate, the population size will generally decrease.
The size of a population also can change when individuals move into or out of the population.
Immigration means moving into a population. Emigration means leaving a population.
Emigration can occur when part of a population are cut off from the rest of the population. For
instance, if food is scarce, some members of an antelope herd may wander off in search of better
grassland. If they are permanently separated, they will no longer be part of that population.
Graphs are useful to show changes in the size of a population over time.
When conditions are good, a population will
generally increase. But if that continues,
problems will arise. Eventually, some factor in
its environment cause the population to stop
growing. A decrease in population may occur
due to changes in either biotic or abiotic
factors. A limiting factor is an environmental
factor that prevents a population from
increasing. These limiting factors may be
density-dependent, or density-independent,
Limiting factors that are density-dependent
occur more often in large populations than in
small ones. Density-dependent limiting factors
include competition (for food, water, shelter &
space), predation, parasitism, and disease.
These limiting factors are triggered by
increases in population density (crowding).For
example, when cougars where captured and
removed from communities in the mid-west US,
the deer population increased. This created a
competition for food and many died from starvation.
Limiting factors that are density-independent are those that occur
regardless of how large the population is and reduce the size of all
Draw 2 density
independent factors here.
populations in the area in which they occur by the same proportion. Densityindependent factors are mostly abiotic. These can be caused by human
activities, such as polluting water systems, damming a river, deforestation.
Populations can also be affected by natural disasters, such as fires, drought,
floods, as well as changes in the weather. The population of oranges growing
in a Florida orange grove will be greatly limited by if there is a frost, a change
in the abiotic factor of temperature. Whether the population is small or large,
it will be affected by these density independent limiting factors.
The largest population that an environment can
support is called the carrying capacity.
Carrying capacity is the maximum number of
individuals of a particular species that can be
supported in an ecosystem at a particular time.
This is determined by the amount of space in
an area, the availability of food, presence of
predators, disease and other resources. If a
population exceeds the carrying capacity, there
will be a decline in number until the population
re-stabilizes. The following is an example of how space is a limiting factor for a population.
If there is not enough room for a pair of gannet birds to build a nest on a rocky shore where most
of the birds go to have their young, that pair will not reproduce and will not contribute the increase
of the gannet population. If the shore was bigger, the population would increase.