POPULATIONS
AND HOW
THEY GROW
• Biosphere
• Ecosystems
ECOSYSTEM LEVEL
Eucalyptus forest
COMMUNITY LEVEL
All organisms in
eucalyptus forest
• Community
• Population
POPULATION LEVEL
Group of flying foxes
ORGANISM LEVEL
Flying fox
Brain
ORGAN SYSTEM LEVEL
Nervous system
ORGAN LEVEL
Brain
Spinal cord
Nerve
TISSUE LEVEL
Nervous
tissue
CELLULAR LEVEL
Nerve cell
MOLECULAR LEVEL
Molecule of DNA
Figure 1.1
POPULATIONS
Population-all of the individuals of a
species that live together in one place at
one time.
 Demography-the statistical study of
populations. It is used to predict how the
size of a population will change.

COUNTING POPULATIONS
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direct observation-count
indirect observation-estimate
sampling-count small number in a small area
and multiply to get the number in the large
area
mark and recapture studies-capture, mark,
release

mark-recapture method depends on likelihood
of recapturing the same individual
Figure 35.2A
KEY FEATURES OF POPULATIONS
1. Population size
 is the number of individuals in a population.
 had an important effect on the ability of the
population to survive.
A small population is more likely to become extinct:
 in the case of random events or natural disaster
 due to inbreeding where the population is more
genetically alike.
 with reduced variability it is harder to adapt to
changes.
KEY FEATURES OF POPULATIONS, CON’T
2. Population
density
– the number of
individuals in a
given area.
 – if they are too far
apart they may only
rarely encounter
one another
resulting in little
reproduction.

KEY FEATURES OF POPULATIONS, CON’T
POPULATION SIZE IS LIMITED BY:
density-independent factors
density-dependent factors
Disease
 Competition
 Predators
 Parasites
 Food
 Crowding









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The greater the population,
the greater effect these
factors have.
Ex. Black plague in the
Middle Ages – more deaths
in cities

Volcanic eruptions
Temperature
Storms
Floods
Drought
Chemical pesticides
Major habitat disruption (as in
the New Orleans flooding)
Most are abiotic factors
KEY FEATURES OF POPULATIONS, CON’T
3. Dispersion
 – the way in which the individuals are arranged.

Most common
POPULATION GROWTH
Population Growth
 Three factors can affect population size:
 number of births (natality)
 the number of deaths (mortality)
 the number of individuals that enter or
leave the population (immigration and
emigration).
* Simply put, a population will increase or
decrease in size depending on how many
individuals are added to it or removed from it

IMMIGRATION & EMMIGRATION


Immigration
 the movement of individuals into an area, is another factor that
can cause a population to grow.
Emigration
 the movement of individuals out of an area, can cause a
population to decrease in size.
PREDICTING POPULATION GROWTH, CON’T
Nearly all populations will tend to grow
exponentially as long as there are resources
available.
 Two of the most basic factors that affect the
rate of population growth are the birth rate, and
the death rate.
 r(rate of growth)=birth rate – death rate

PREDICTING POPULATION GROWTH,
CON’T
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Exponential growth curve:
population growth plotted
against time.
As a population gets larger, it
also grows at a faster rate.
Maximum population growth
under ideal circumstances.
Includes plenty of room,
unlimited resources (food,
water) and no predators.
FACT: No population exhibits this type of growth for long.
PREDICTING POPULATION GROWTH, CON’T

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Logistic model: This model accounts for the declining
resources available to populations as they grow.
It assumes the birth and death rates are not constant.
As the population grows, births decline and death rises.
Eventually birth=death so the population stops
growing.
Carrying capacity (K): The number of organisms of one
species that an environment can support indefinitely.
PREDICTING POPULATION GROWTH, CON’T
TWO MODES OF POPULATION GROWTH.
The Exponential curve (also known as a Jcurve) occurs when there is no limit to
population size.
The Logistic curve (also known as an S-curve)
shows the effect of a limiting factor (in this case
the carrying capacity of the environment).
POPULATION GROWTH STRATEGIES
There are 2 ways a population can prosper:
1. Depends on the rate of growth (r)
2. Influenced by the carrying capacity (K)
POPULATION GROWTH STRATEGIES, CON’T
r-strategists: “boom and crash” characterized by
exponential growth, which results in temporarily large
populations, followed by sudden crashes in population
size. Ex. Insects, bacteria, some plants
 live in unpredictable and rapidly changing
environments
 Reproduce quickly when
conditions are favorable
 Many offspring: small,
mature rapidly, no
parental care
 r = rate of growth
POPULATION GROWTH STRATEGIES, CON’T
K-strategists: characterized by a
high degree of specialization. Ex.
Trees, whales, tigers, etc.
 Live in stable and predictable
environments
 Can compete effectively
 Reproduce late in life
 Few offspring: large,
mature slowly, often
much parental care
 K = carrying capacity
POPULATION GROWTH STRATEGIES, CON’T

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Human Populations:
K- strategist characteristics
But, recently, humans learned to expand carrying capacity of
environment by increasing food supply, combating pests and curing
diseases.
Can Earth support this increase?
THE HUMAN POPULATION
 doubled
three times in the last three
centuries
 about 7 billion and may reach 9.3 billion by
the year 2050
 improved health and technology have lowered
death rates
 The
history of human population growth
Figure 35.8A
Figure 2.10x