POPULATIONS: GROWTH & LIMITING FACTORS
Overview
Population growth is affected by:
1.
2.
3.
Exponential (J-shaped growth)
Example: bacteria under ideal conditions (correct temperature and unlimited nutrients) will
double their population every few hours!
Time
Population size:
# of bacteria
Time
initial
3 hours
6 hours
9 hours
12 hours
15 hours
18 hours
1
21 hours
24 hours
27 hours
30 hours
33 hours
36 hours
39 hours
Population size:
# of bacteria
Create a graph of the data (time vs. population size)
In ideal conditions, a population will start growing slowly, then ______________________
S-shaped Growth
Of course, populations cannot grow exponentially for long!
Factors that limit population growth are known as: ______________________
Example: Factors that limit growth of snowshoe hare population:
Predation
Carrying capacity =
Draw graph (with labels) of a typical S-shaped graph
CASE STUDY #1: Reindeer on St. Matthew Island
Essential Question: What are the factors that affect the size of a population?
Overview
As you look more closely at land and the organisms that live on it, you will notice how
interdependent life is. Resources like food and energy are limited and many organisms
are in competition for these resources. In this activity you will examine factors that
affect the size of a population.
Carrying Capacity and The Reindeer of St. Matthew Island
St. Matthew Island is located in the Bering Sea off the western coast of Alaska. It is
128 square miles of low-growing tundra grasses, with a few dwarf willow trees. High
winds and fog are common and can occur at any time of year. The sub-Arctic island
gets between 10 and 17 inches of rain and snow each year. Many years ago, the U.S.
Coast Guard maintained a station on the island. That station was abandoned in 1944,
leaving the island to its resident arctic foxes, singing voles, and a wide variety of sea
birds. In 1944, wildlife biologists decided that the island could support a population of
reindeer that was well adapted to the island’s vegetation and climate. Reindeer were
not native to the island and had never lived there. The biologists still believed that if the
species was brought to the island to live, they would thrive. In 1944, 24 reindeer were
introduced onto St. Matthew Island. The biologists believed that the carrying capacity
of the island could easily support a stable population of 1600-2300 reindeer. Carrying
capacity is the number of individuals in a population that can be supported in a given
area within natural resource limits, without degrading the natural environment for
present and future generations. There are many factors (population limiting factors)
that must be considered when determining carrying capacity for a population. These
factors include: the abundance of food and water, the availability of shelter, the
availability of areas for giving birth to young, and the presence and kind of predators.
Predators are animals that hunt and eat other animals, thus controlling their population.
Carrying capacity also considers the presence of other animals that might compete
with a particular species for available resources.
Reindeer Are Introduced to St. Matthew Island
At first, the new reindeer thrived on St. Matthew Island. High-quality food was plentiful.
There were no predators that hunted them. By 1957, the reindeer population had
climbed to 1350 animals. The animals were healthy with an average body weight 24%
higher than mainland reindeer. By 1963, the lack of controls on the reindeer population
on the island allowed their population to rise to 6000. Between 1963 and 1966, the
continually growing population of reindeer exhausted the island’s food resources.
Nearly all them died of starvation. The population crashed, leaving only 42 reindeer
alive by 1966. The graph below reveals what happened to the reindeer population on
the island between their introduction in 1944, their crash in 1966, and their slow
recovery. With so few reindeer left alive, the island’s vegetation slowly began to
recover. Sub-Arctic plants usually take much longer to grow than vegetation in warmer
climates. Over several years, the reindeer population increased slowly. By 1975, there
were 68 reindeer on the island. As the population grew, biologists considered
introducing predators to the island to keep the reindeer population under control. The
biologists decided not to do this, however. They feared the effects predators might
have on other, native animal species on the island.
Analysis Questions
1. Calculate the population density of reindeer (a) in 1944 and (b) in 1963. Show
your work (Population density can be measured in # of reindeer per square mile)
2. What are two factors in the rapid increase of the reindeer population on St.
Matthew Island?
3. What are the factors that account for the reindeer population crash?
4. Some native animals on the island feed on plants and some birds use the island
vegetation to build their nests. What can you infer about the likely impact of the
reindeer on these other species?
5. What do you think the biologists feared might occur from introducing predators of
reindeer to the island? Do you think their fear was justified? Why or why not?
6. Based on what you read in the text and what you have learned in class, define
“carrying capacity” in your own words.
7. Did the reindeer population exceed the carrying capacity of the island? Use
evidence to support your answer.
8. A population limiting factor is anything that restricts the number of organisms in
an area.
(a) What was the main population limiting factor for reindeer on St. Matthew Island
that lead the crash?
(b) Would this limiting factor be considered biotic or abiotic?
(c) Name some other factors that can limit the population and designate whether
they are biotic or abiotic.
Case Study #2: Does Earth have a carrying capacity for humans?
How many humans can Earth support?
Look at the human population data show below from 10,000 BC to 2000:
Part I: use the graph above to in complete sentences.
1. (a) How would you describe the rate/amount of growth until about 1000 years ago
(b) What begins to happen after 1000 AD?
2. (a) What do you notice that happens around year 1400 AD?
(b) Why do you think there was this trend?
3. Mathematically how would you describe the population growth in this graph? HINT:
what shaped curve is it?
4. Do you think human population can or will continue to grow at this rate? Why or
why not?
Part II: view: http://www.youtube.com/watch?v=4BbkQiQyaYc. Lights (dots) indicate areas
of most growth. Each dot is one million people. Answer the following questions in
complete sentences.
1. Describe the distribution and rate of population growth from 1 AD to present
(summarize after viewing).
2. What happens in the last 100 years of time (last few seconds of the video)?
3. Continued exponential human growth is not sustainable over the long-term. Discuss
three potential consequences of continued exponential human growth.
Part III: National Geographic 7 billion:
Before watching video, world population = ______________________________
see http://www.worldometers.info/world-population/
Answer questions in blanks below after viewing video second time.
http://www.youtube.com/watch?NR=1&v=sc4HxPxNrZ0
1. It would take _________ years to count to 7 billion out loud.
2. Seven billion steps would take you ___________ times around the Earth.
3. About every second ________ people are born and _________ people die.
4. People are living longer too, in 2010 the average person lived _____ years, but in 1960
the average person lived ______ years.
5. Fewer people are living in rural areas and more are living in urban areas. In 1975 there
were only 3 megacities (exceeding 10 million people): Mexico City, Tokyo, and
____________________. Now there are __________ megacities worldwide.
6. Standing shoulder to shoulder all 7 billion people could fit into an area the size of
____________________, so space along doesn’t seem to be a limiting factor for human
growth. Other resources are in short supply and are not used equally be all:
_____ % consume 23% of the world’s energy.
_____% of us don’t have clean drinking water.
_____% lack adequate sanitation.
7. After viewing video, world population = ________________________________ see
http://www.worldometers.info/world-population/
8. In your opinion, what do you think will be the limits to human population growth on
Earth?
Part IV: National Geographic: The world’s most typical person. View clip and answer the
following questions in complete sentences.
http://www.youtube.com/watch?v=4B2xOvKFFz4&feature=relmfu
1. Describe some characteristics of a ‘typical person on Earth’
2. Describe the ways you are typical and the ways you are not typical from the average
person on Earth.
3. Describe what is meant by “our choices make a big difference [when] multiplied by 7
billion’
4. Explain: How is Earth’s carrying capacity influenced by ‘our choices’?
TEACHER PAGE
Population Clock: http://www.worldometers.info/world-population/
World Population:
http://www.youtube.com/watch?v=4BbkQiQyaYc
National Geographic ‘7 Billion’:
http://www.youtube.com/watch?v=sc4HxPxNrZ0
National Geographic ‘Are you typical?’: http://www.youtube.com/watch?v=4B2xOvKFFz4&feature=relmfu
Terra Nova: http://www.fox.com/terranova/full-episodes/1455291/ (Genesis Episode)