2. Population Fluctuations
 Populations rarely, if ever, remain static for long periods of time.
What does static mean? What causes population sizes to change?
 Four factors cause populations to change:
Births
Deaths
Immigrations
Emigrations
 What is the difference between immigration and emigration?
 Births and Immigrations add to a population, whereas Deaths and
Emigrations take away from one.
 The equation for population change is therefore:
Pop. Change = (Births + Immigration) – (Deaths +
Emigrations)

3. Reproductive Patterns
 The reproductive patterns of
different species affects how
their populations grow.
 Some species have lots of
young as early and as often
as possible.
 Do these species tend to
take care of their young?
 This first reproductive
strategy is successful when
very few individuals are likely
to survive to reproductive
maturity (more offspring die
than survive to adulthood).

4. Reproductive Pattern
 Other organisms tend to
have long maturation
rates and few young at a
time. The parents
spend more time taking
care of their offspring
 Offspring of organisms
that follow this second
pattern are more likely
to mature to adulthood
and even reach old age.

5. Population Fluctuations
 How can you explain the population
fluctuations shown in this graph? (Why does
the mouse population increase and decrease?)

6. Amoreyou notice about the population of mice in relation to the
What do complete story
population of weasels? What does the additional information reveal
about the relationship between mice and weasels?

7.  Examine the graph. Write 3 to 5 sentences describing
the information about the rabbit population provided
by the graph.

8. Nice Curves!
 Organisms that employ the first
reproductive pattern often have
population growth that
resembles this J-curve, where
you have exponential growth
over a short period of time.
 If there were not environmental
pressures to stop this kind of
growth, some organisms would
literally cover the earth.
 Why do you think the
population increases so quickly
over such a short period of
time?

9.  In 1 to 2 sentences describe the change in the sparrow population.
Generation 1-14. In 3-5 sentences describe the change in the
sparrow population from generation 15 to24. What do you think
caused the population to decrease then increase again?

10. Even Nicer Curves!
 Fortunately, ecosystems have
environmental resistance that
prevents exponential growth (J-shape)
from continuing indefinitely.
 What happens to the population as it
reaches the carrying capacity? What
is carrying capacity?
 A population’s carrying capacity is a
measure of the maximum population
that can live in an ecosystem and
maintain a stable population.
 The population stabilizes due to a
decreased in the amount of resources
available to the increased number of
individuals in the population. This is
called logistic growth, and is
represented by an S-curve

11. This is what happened to the
sparrows!!!
 Oftentimes, you’ll see a
population fluctuate around the
carrying capacity.
 In this instance, a population
overshot the carrying capacity
(more organisms than resources
available) so the population
dropped due to starvation.
Fewer organisms in turn leads
to an “abundance” of resources,
allowing for a resurge in the
population
 This cycle can go on for
generations

12. Populations Crash!
 When a population far
overshoots its carrying
capacity, the population
crashes.
 This is due to reproductive
lag, where the death rate hasn’t
caught up with the birth rate.
The increased number of
organisms leads to all the
resources being used up.

13. Other Factors That Contribute to
Crashes
 Aside from overshooting the
carrying capacity, crashes can also
be caused by diseases.
 This little brown bat has white-
nose syndrome, a fungus that kills
hibernating bats. These bats are
expected to go extinct in less than
10 years, and their declining
numbers already cost farmers $3.7
BILLION per year, and their
complete disappearance may cost
$53 billion per year. Why?
 Invasive predators can also
contribute to population crashes,
as can habitat destruction.

14. Stability
 Long established ecosystems generally have had
more time to stabilize.
 In stable populations the number of individuals
within the population remain level or fluctuate at
regular intervals
 Human actions like habitat destruction and
introducing non-native species can throw off these
balances, sometimes with disastrous consequences.
 Do you know any examples?

15. Don’t Forget About Us…
 Humans are not
immune to these
pressures.
 What is suspiciously (or
ominously) missing
from this graph?