Transcript Population Growth

Populations change over time
• Why does the population dip at about 1400 CE?
• When does human population reach 1 billion? 2 billion?
 Three important characteristics of populations:
 Geographic distribution (range) – the area inhabited by
a population.
 Population density – the number of individuals per unit
area.
 Example: 30 students in one classroom.
 Growth Rate – pace at which the population grows.
 There is a population of bullfrogs living in Jacobson
Park. Let’s say there are 400 bullfrogs and the lake is 2
km2.
 What is the geographic distribution of the bullfrogs?
 What is the population density of bullfrogs in the
lake?
 Three factors can affect population size:
 The number of births
 The number of deaths
 The number of individuals that enter or leave the
population.
 Immigration – the movement of individuals into an
area  increase in population size.
 Emigration – the movement of individuals out of an
area  decrease in population size.
 If immigration and emigration remain equal,
decreased birthrate is the most important factor to
slow growth rate
 To continue the example with the bullfrogs…
 If the bullfrog population was 400 individuals at the
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beginning of the year and 500 at the end of the year,
what was the population’s growth?
Consider the same starting population of 400:
Suppose that 50 baby bullfrogs were eaten by great
blue herons, what was the population’s growth?
Suppose that 75 adult bullfrogs were eaten by great
blue herons and gigged by humans during the year,
what was the population’s growth?
So, it should be clear that populations can experience
negative as well as positive population growth.
 Under ideal circumstances with unlimited resources, a
population will grow exponentially.
 Exponential growth occurs when the individuals in a
group reproduce at a constant rate.
 The population becomes larger and larger until it
approaches an infinitely large size.
 Exponential growth is characterized by a “j-shaped”
curve.
Exponential Growth
 How are these graphs alike/different? What do these differences indicate?
 What is another major difference between the reproduction of bacteria and
of elephants?
 Are the lines identical? Are the rates of reproduction identical?
 In nature, exponential growth does not occur for very
long.
 As resources become less available, the growth of a
population slows or stops.
 Logistic growth occurs when a population’s growth
slows or stops following a period of exponential
growth.
 Logistic growth is characterized by an “s-shaped”
curve.
 The largest number of individuals of a population that
given environment can support is called the carrying
capacity.
 When a population reaches the carrying capacity of its
environment, its growth levels off. The average growth
rate is zero.
 When primary productivity of an ecosystem is reduced
when there is an insufficient amount of a particular
nutrient, what is that nutrient called?
 Limiting nutrient
 This is a specific example of a broader principle
 Limiting factor – a factor that causes population
growth to decrease.
 Density-dependent factors – a limiting factor that
depends on population size.
 Density-dependent factors become limiting only when
the population density reaches a certain level.
 Can you think of any examples?
 Examples include: competition, predation, parasitism,
and disease.
Wolf and Moose Populations on Isle Royale
Moose
Wolves
 How are the increases and decreases in the moose population related
to the changes in the wolf population?
 What other factors could affect these populations?
 In this case: wolves were affected by disease and moose by food supply.
 Density-independent factors – affect all populations in
similar ways regardless of population sizes.
 Can you think of any examples of density-independent
limiting factors on populations?
 Unusual weather
 Natural disasters
 Seasonal cycles
 Human activities –
damming rivers, cutting forests, etc.
U.S. Population
Percentage of Population
Percentage of Population
 1. Exponential growth rate occurs when individuals
reproduce:
 A. At an increasing rate
 B. At a decreasing rate
 C. At a constant rate
 2. Population density increases due to:
 A. Immigration
 B. Emigration
 C. Carrying capacity
 3. Exponential growth slows or stops as:
 A. Rate of immigration increases
 B. Rate of immigration decreases
 C. Resources become limited.