1. Populations

2. Population
• Group of individuals of the same
species occupying a common
geographical area (a species is an
interbreeding group of organisms
that produces fertile offspring).
– Same species, same place, same
time!

3. Factors affecting Population
• G.1.1 Outline the factors that affect
the distribution of plant species,
including temperature, water, light,
soil pH, salinity and mineral
nutrients.
• G.1.2 Explain the factors that affect
the distribution of animal species,
including temperature, water,
breeding sites, food supply and
territory.

4. Population Dynamics
• Natality – offspring are produced and
added to the population (birth)
• Mortality – individuals die and are
lost from the population (death)
• Immigration – individuals move into
the area from somewhere else and
add to the population
• Emigration – individuals move out of
the area and are lost from the
population

5. Population Dynamics
∀∆ = N + I – M – E
• In closed populations: ∆ = N – M
• In English: Change = Births +
Newcomers – Deaths – Exiters

6. Populations
5.3.2: Draw a graph
showing the sigmoid
(S-shaped) population
growth curve.
5.3.3: Explain reasons
for the exponential
growth phase, the
plateau phase and the
transitional phase
between these two
phases.

7. Exponential Phase
• Population increases exponentially
because the natality rate is higher
than the mortality rate.
• This is because there is an
abundance of food, and disease and
predators are rare.

8. Transitional Phase
• Difference between natality and
mortality rates are not as great, but
natality is still higher so population
continues to grow, but at a slower
rate.
• Food is no longer as abundant due to
the increase in the population size.
May also be increase predation and
disease.

9. Plateau Phase
• Natality and mortality are equal so
the population size stays constant.
• Limiting Factors (5.3.4):
– shortage of food or other resources
– increase in predators
– more diseases or parasites
• If a population is limited, then it has
reached its carrying capacity

11. Other Population Curves
Growth curve for
bacteria
bacteria
#
time
• Boom and bust

12. Factors Limiting the Growth of Natural
Populations
• All populations have a limit for exponential
growth  biotic potential (intrinsic rate of
increase), r, under perfect conditions
• Dependent on - # offspring/birth, capacity for
survival, procreation rate, maturity
• Since population is mainly affected by B-D, the
factors that limit growth either  birth rate or 
death rate
• Abiotic factors – blizzards, hurricanes, floods
(weather) – density independent regulation
(death rate is independent of the population)
• Biotic factors – predators, competitors, parasites
– density dependent regulation (the proportion of
individuals that die will depend on the size of the
population – the birth rate and death rate are
affected by the size of the population)

13. Survivorship Patterns (r-
and K-)
• r- (small
letter, small
creatures)
• K- (large
letter, large
creatures)

14. r- selected
• r-selected populations (small letter – small creatures)
• variable, unpredictable climate  ability to adapt
• density independent factors  mortality
• lots of offspring
• high juvenile mortality
• population below carrying capacity
• low level of competition
• early development, reproduction, small body size, high
reproductive capacity, reproduce only one, many offspring
• short life-span (less than one year)
• little/no parental care

15. K- selected
• K-selected populations (capital letter – large creatures)
• offspring’s survival depends on ability to obtain limited
resources – produce a few highly competitive offspring
(many young die if climate changes drastically)
• fairly constant/predictable offspring
• density-dependent mortality
• low juvenile mortality
• fairly constant population – at/near carrying capacity
• high levels of competition
• slow development, greater competitive ability, large body
size, delayed & repeated reproduction, fewer, larger
offspring
• life-span longer