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Science 20 Unit D – Living Systems

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1 Science 20 Unit D – Living Systems

2 Populations Populations can grow so fast that they exhaust their resources. Examples? Why is this a problem? Where do we see this happening today?

3 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.

4 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.

5 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.


7 World Populations: March 2006

8 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.

9 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.

10 Thinking back in S20…what could this similar graph model?
This is an exponential graph of population density of a bacteria vs. time.

11 Factors that affect populations
4 major factors: Number of births (natality). Number of deaths (mortality). Immigration (movement in). Emigration (movement out).

12 Above are two population graphs: what can be said of the natality/mortality rates in these countries?

13 Natality Rates, March 2006

14 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?

15 Two types of populations:
Closed: No movement in/out due to natural/artificial settings. Only death/births affect population (ex: N Korea) Open: exist in natural setting where all 4 factors affect population size (ex: Canada)

16 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.

17 Rabbits: Public Enemy #1

18 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.


20 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…


22 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."

23 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?

24 Assignment Skim pg Questions 1-6 on page 487

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