STUDYING POPULATIONS
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 densitydependent 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
populations in the area in which they occur by the same proportion. Density-
Draw 2 density
independent factors here.
independent 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.
Name_________________________
Studying Populations
1. What is one way to refer to the number of individuals in a specific area? ______________________________
2. How is population density calculated? __________________________________________________________
__________________________________________________________________________________________
3. Name 4 methods for determining the population density of an area.
1. ______________________________
3.________________________________
2., ______________________________
4.________________________________
4. Which method mentioned in question 3 involves making an estimate of a population by counting the number of
organisms in a small area (a sample), and then multiplying to find the number in a larger area?
_________________________________________
5. If a scientist traps and writes numbers on turtles’ shells , and then repeats it after a certain time, this method is called
_______________________________ that helps determine the size of a population.
6. When a scientist counts tracks, nests, or other signs of a population instead of the organisms themselves this is called
__________________________________.
7. If a scientist counts each organism one by one, this is called ___________________________________________.
8. Describe how a population can increase in terms of birth rate and death rate .___________________________
___________________________________________________________________________________________.
9. Compare immigration and emigration and tell how each changes the population. .___________________________
_____________________________________________________________________________________________.
_____________________________________________________________________________________________.
10. What is a limiting factor? ______________________________________________________________________
11. Give an example of a density-dependent limiting factor. ______________________________________________
12. Density dependent limiting factors affect __________________ populations.
13. Density independent factors are mostly ______________________.
14. Give 3 examples of density independent limiting factors. _____________________, _________________________,
and _______________________
15, True or False? Density independent limiting factors only affect small populations.
16. What is carrying capacity? __________________________________________________________________
17. What are some factors that determine carrying capacity? Name several. ___________________________________
________________________________________________________________________________________________
http://puzzling.caret.cam.ac.uk/game.php?game=foodchain
Name________________________________ Studying Populations
1. A vegetable garden is 12 meters long by 7 meters wide. In one square meter, you count two toads.
Estimate the population of toads in the garden.
The graph below shows how the size of the squirrel population in a city park changed over time. Use the
graph to answer questions 2—5.
2. Over which time period(s) did the squirrel population decrease? ____________________________
3. Over which time period(s) did the population increase? ____________________________
4. In which year did the population reach its lowest point? ____________________________
5. What was the size of the population that year? ____________________________
6. What causes a population to change in size?
7. Identify three limiting factors that can prevent a population from increasing. Explain how each factor
limits a population’s size.