Transcript Science 20 Unit D – Living Systems

Science 20 Unit D – Living Systems
Populations
Populations
• Populations can grow so fast that
they exhaust their resources.
Examples?
• Why is this a problem? Where do
we see this happening today?
Some statistics: National
Geographic
• In 8000 B.C., only 5 million
people were alive.
• By comparison, 33 million
people live today in Canada.
• Today, the number is around
6.7 billion.
• http://www.census.gov/cgibin/ipc/pcwe
The world population increased from 3 billion in 1959 to 6 billion by 1999, a doubling
that occurred over 40 years. The Census Bureau's latest projections imply that
population growth will continue into the 21st century, although more slowly. The
world population is projected to grow from 6 billion in 1999 to 9 billion by 2040, an
increase of 50 percent is expected to require 41 years.
In addition to growth rates, another way to look at population growth is to consider
annual changes in the total population. The annual increase in world population
peaked at about 88 million in the late 1980s. The peak occurred then, even though
annual growth rates were past their peak in the late 1960s, because the world
population was higher in the 1980s than in the 1960s.
http://www.census.gov/ipc/www/idb/ranks.html
World Populations: March 2006
In order to study population growth, bacteria is used; it is easy to
study and can imitate human populations very well.
Plus, bacteria grow and reproduce much faster than humans,
allowing us to develop generations of data in a number of days.
Exponential Growth
• Bacteria (like people), grow
exponentially.
• When measuring exponential
growth, we don’t count by how
much the population grows by
each time…
• we count how long it takes for
the population to double.
This is an exponential graph of
population density of a bacteria
vs. time.
Thinking back in S20…what
could this similar graph model?
Factors that affect
populations
4 major factors:
1. Number of births (natality).
2. Number of deaths (mortality).
3. Immigration (movement in).
4. Emigration (movement out).
Above are two population graphs: what can
be said of the natality/mortality rates in these
countries?
Natality Rates, March 2006
Immigration
sometimes faces
criticism, even as
most economies
are struggling to
find workers.
Take a look at the
graph to the right:
how is this data
displayed?
Does anything
smell fishy to
you?
Two types of populations:
1. Closed: No movement in/out
due to natural/artificial
settings. Only death/births
affect population (ex: N Korea)
2. Open: exist in natural setting
where all 4 factors affect
population size (ex: Canada)
Population explosions
and crashes
• If an organism is introduced where
there are no predators; a population
explosion/growth occurs.
• What happens when the food is
gone? The population crashes!
• Example: Rabbits released in
Australia (1859).
• Curve shape: up, then down.
Rabbit
s:
Public
Enemy
#1
Carrying Capacity
• The maximum number of
individuals that can be sustained by
an ecosystem indefinitely; limited
by disease, competition and famine.
• Instead of crashing, the population
levels off at a stable number of
organisms.
• Curve shape: S curve.
Carrying Capacity of the Earth?
Kind of a loaded question…
Estimates peg it somewhere between one
and two billion (UN population report).
But opinions do vary…
http://www.youtube.co
m/watch?v=B6JLvIxdbj
Q
http://www.youtube.com/
watch?v=HIUyyY6L5PQ&fea
ture=fvsr
Prof. Steve Jones,
University College London:
"Humans are 10,000 times
more common than we
should be, according to the
rules of the animal
kingdom, and we have
agriculture to thank for
that. Without farming, the
world population would
probably have reached half
a million by now."
Malthus
• Thomas Malthus was
the first to hypothesize
how populations grow.
• He predicted that the
human population
would crash because of
shortage of resources.
Why did this not
happen?
• Do you think a
population crash will
occur? Why or why not?
Assignment
• Skim pg 482-487
• Questions 1-6 on page 487