Quality Core Curriculum Objectives:

HS  Biology  #1 – Uses terms and processes employed in scientific research

                        #2 – Uses reference sources appropriately.

                        #13–Discriminates relationships when using a classification model to

group living things.

#19- Describes life processes for common vertebrates.

#25- Explains the structure of an ecosystem.

#26–Assesses man’s impact on the environment/explores ways to help solve ecological problems.

Vocabulary:  limiting factor, density dependent factors vs. density independent factors, invasive species, habitat fragmentation,  

Materials:  large area for students to run, chalkboard or flip chart, writing materials, access to Internet sites such as U.S. Fish & Wildlife Service and state wildlife agencies; access to recent newspaper and magazine articles

Background:

The number of living things in any population may go up or down dramatically or fluctuate only slightly, depending on what is happening in the habitat.  All living things have the capacity to keep on reproducing.  If nothing kept their numbers in check, the world would be overrun very quickly with too many plants and animals.  A female cod, for example, can produce a million eggs at a time.  If all of these grew into adults, the consequences for the environment would be grave, but in fact a whole range of factors keeps population numbers within certain limits.  These are called “density dependent” factors, because their effects change with the population density.  A varying food supply is a prime example.  As a growing population eats up its food supply, the shortage of food will eventually cause the population size to decrease.  Populations therefore tend to stay at about the same level, fluctuating slightly above and below the numbers that a stable environment can support.

Populations are also limited by random “density independent” factors – natural events such as the eruption of a volcano on an island, which may destroy certain species, regardless of the population sizes.  The following activity, however, will illustrate to students the effects that density dependent factors can have on a species’ population.   

Procedure:

1.      Review the essential components of habitat with the students: food, water, shelter, and space in a suitable arrangement.  Emphasize that even though this activity focuses on food, shelter and water, they should not forget how important it is for animals, particularly predators, to have sufficient space in which to live.

2.      Ask the students to count off in four’s.  Have the one’s go to one area while all two’s, three’s, and four’s go together to another area.  Mark two parallel lines on the ground or floor 10-20 yards apart.  Have the one’s line up along one line; the rest line up along the other line.

3.      The one’s become “deer.”  All deer need good habitat in order to survive.  For this activity, we’ll assume the deer have sufficient space in which to live.  The “deer” need to find food, water, and shelter in order to survive.  When a deer is looking for food, it should clamp its hands over its stomach.  When it is looking for water, it puts its hands over its mouth.  When it is looking for shelter, it holds its hands together over its head.  A deer can choose to look for any one of its needs during each round of the activity.  The deer cannot, however, change what it is looking for “(e.g., when it sees what is available) during that round.  It can only change what it is looking for between rounds, if it survives.

4.      The two’s three’s and four’s are food, water, and shelter – components of the habitat.  Each student gets to choose at the beginning of each round which component he/she will be during that round.  The students depict what they are in the same way the deer indicate what they are looking for (i.e. hands on stomachs, mouths, or over heads). 

5.      The game starts with all players lined up on their respective lines (deer on one side, habitat components on the other side) and with their backs to the group on the other line.

6.      The teacher begins the first round by asking all of the students to make their signs – each deer deciding what it is looking for, and each component deciding what it will be.

7.      When the students are ready, count:  “One…two…three.” At the count of three, each deer and each habitat component turn to face the opposite group, continuing to hold their signs clearly.

8.      When the deer see the habitat component they need, they are to run to it.  Each deer must hold the sign of what it is looking for until getting to the component with the same sign.  Each deer that reaches its necessary component gets to bring that person back with him or her to the original deer line.  This is to represent that deer successfully meeting its needs and reproducing as a result.  Any deer that fails to find the component it needs “dies” and becomes a habitat component, moving to the habitat component line. 

(Note:  When more than one deer reaches a habitat component, the student who gets there first survives.  The components stay in place in their line until a deer needs them.   The habitat students can change which component they want to be from round to round.)

9.      As the facilitator, you record on the chalkboard how many deer there are at the beginning of the game and at the end of each round.  Continue the game for approx. 15 rounds.  Keep the pace brisk.

10. At the end, gather the students for discussion.  Encourage them to talk about what they experienced and saw.  For example, they saw a small population of deer continue to expand until the habitat was depleted and there was not sufficient food, water and shelter for all the members of the herd.  At that point, the deer died of starvation, thirst, or lack of shelter.

11. Ask the students to use the data recorded during the game to create a representative graph of the deer population changes.  Each round will represent a subsequent year.   Deer can be posted by five’s for convenience.  The graphs should look something like:

12. The students will see a fluctuating pattern over the years.  Explain that this is a natural process, as long as the factors which limit the population do not become excessive to the point where the animals cannot successfully reproduce.  Wildlife populations tend to peak, decline, and rebuild – as long as there is good habitat and sufficient numbers of animals to successfully reproduce.

Extensions:  (Divide the class into small groups, assigning each group one of the following activities.)

1.      Predators are a natural limiting factor in an ecosystem.   Staying on the topic of the white-tail

deer, ask students to research their main predator – the cougar (mountain lion, puma, panther).   Ask the students to concentrate on the eastern United States.  What was the population of cougars like in the eastern U.S. 200 years ago, 100 years ago, and now?  What was the population of whitetail deer in the eastern U.S. 200 years ago, 100 years ago, and now?  Do you think there is any correlation between the two populations?  Find out the factors that led to population declines/increases.  What role have humans played?  What suggestions do you have for handling any current population problems (overpopulation for deer, underpopulation for cougars)

2.      Humans are often unintentional limiting factors.  Through habitat depletion and fragmentation, human development often threatens the habitat component of sufficient space that is so crucial, particularly for large predators.  Ask one group to pick a large predator, such as the black bear, and find out the following: what size territory the average bear needs, the historical numbers of black bears living in Georgia or the southeast 200 years ago, 100 years ago, and 50 years ago.  What are their current population estimates?  What are some  factors that could have influenced the change in their numbers?  What are some suggestions for preventing bear numbers from decreasing in the future?  Pay attention to habitat fragmentation (which results from building of roads through wilderness areas);  what are some solutions people have already implemented in some areas?  (For example, building highway underpasses specifically for animals to keep them off the roads).

3.      Humans can also be limiting factors by introducing exotic (non-native) invasive species to an ecosystem.  Have one group research invasive species and their impact on native wildlife.  Good current examples to target are the Snakehead fish in Maryland and wild pigs/feral hogs in Georgia and Texas.   Much information can be found on these two species via Internet sites such as www.invasivespecies.gov and www.google.com.  Explain that plants and microorganisms can also be invasive (ex.  the Japanese kudzu vine and the West Nile Virus).  Ask the group to research what is being done, if anything, to control certain invasive species.

4.      Oftentimes, it is the prey species that controls the population of predators.  This is the case with the snowshoe hare and the lynx.  Have this group try to uncover the relationship between the lynx and the snowshoe hare populations, graphing the relationship if they can.  (Researchers have found that snowshoe hare populations seem to peak every seven to nine years and then crash.  It has also been found that lynx populations do the same thing – except that they do so one year behind the hare populations.  Recently, it was found that a major winter food of the hare is a small willow.  As hare populations grow, the use of the willow plants grows too.  When the willow plant has been “hedged” or eaten back so far, the plant generates a toxin (poison) which precludes use by the hare.  That is when the hare population crashes, followed by a crash of the lynx population a year later.  Then the willow, relieved of pressure, begins to grow again.  The hare population begins to grow in response and the lynx population follows.  This cycle begins again every seven to nine years.)