Lesson 10: Stability of an Ecosystem (2.1.4)
PREDATOR/PREY RELATIONSHIPS
Ecosystems have built in mechanisms that help them maintain stability over thousands to
hundreds of thousands of years. Predator/prey relations are one way populations in an ecosystem
remain stable and at or below carrying capacity.
Figure 10.1 Predator/Prey Relationships
Figure 10.1 shows these predator/prey relationships. Let’s assume the predator is a fox and
the prey is a rabbit. As the prey population increases, so does the predator population. Why? A
larger population of prey means there is more food available for the predators. Because more
predators are able to find food and eat, less die of starvation and thus the predator population
increases. Eventually the fox population will get large enough that the rabbits are being heavily
preyed upon. This causes the rabbit population to decrease. As the rabbit population decreases,
there is less food available for the foxes. Some foxes might have a more difficult time finding
rabbits to eat, and therefore die of starvation. In turn, the fox population decreases. As the fox
population decreases, less rabbits are being eaten, and their population size will once again
increase. Populations naturally fluctuate up and down in response to one another. (Refer to
Figure 10.1)
The same concept holds true with shelter availability. Shelters
provide a source of protection from predators and the weather.
Each ecosystem has a limited number of shelters. If there are
not enough shelters for the rabbits, those not being protected
are more likely to be eaten. A population can only be as big as
the number of shelters available, and amount of available food.
Another way population sizes are kept at or below carrying
capacity is through diseases. When a population is too big,
more organisms are living close together, or sharing a shelter.
This close contact makes it easier for disease to spread amongst the population. Between 1850
and 1900, many people came over to the United States from Ireland during the Ireland potato
famine. These people did not have much money and lived in very tight quarters. Many diseases,
such as cholera, spread rapidly through these populations killing thousands of people. As you
can see, population sizes naturally fluctuate up and down due to food and shelter availability,
predator/prey interactions, and diseases. However, population sizes remain fairly constant over
time.
POPULATION DYNAMICS
The study of characteristics of a population include growth rate, density and distribution of a
population is call population dynamics. Growth rate of a population is the change in size per
unit time. Growth rates are typically reported as the increase in the number of organisms per unit
time per number of organisms present. Organisms can enter or leave a population through birth
or immigration. Organisms can leave a population by death or emigration. Growth rate
graphs often plot the number of individuals against time.
A population can grow exponentially if the birth and death rates are constant, and the birth rate
is greater than the death rate. Exponential growth occurs when the population growth starts out
slowly and then increases rapidly as the number of reproducing individuals increase.
Exponential growth is sometimes called a J-shaped curve as seen in Figure 10.3 (A). A
population cannot continue to grow exponentially without reaching some environmental limit
such as a lack of nutrients, energy, living space and other resources. These environmental limits
will cause the populating size to stabilize. Limiting factors such as abiotic (wind, water,
sunlight) and biotic (disease and overcrowding) factors tend to slow the growth of the
population.
LIMITING FACTORS AFFECT CARRYING CAPACITY
Carrying capacity is the number of organisms an ecosystem can support in terms of food,
water, shelter, reproduction, and available energy for survival. Carrying capacity is controlled
by predator/prey interactions and limiting factors, things that do not allow a population to get
any larger. If a forest only has enough food for 20 deer, then the population of deer will not get
any bigger than 20. Any deer over 20 will most likely starve to death. Competition amongst
organisms also helps keep populations in check. A rabbit may be hunted by a snake, a bird, and
a wolf. Each predator is competing with one another to “win” the rabbit. Because not every
organism in an ecosystem is able to catch and eat food on a daily basis, some organisms die.
Weather can affect population size as well. Harsh winters bring snow that covers food and
shelters. During this time, many organisms do not have enough food to eat, or shelter to protect
them from predation, temperature, and wind. These organisms may also die. Figure 10.2 (B)
shows how carrying capacity fluctuates as limiting factors change over time.
Lesson Review 10: Ecosystem Stability
A. Define the following terms.
1. The area in which certain types of plants or animals can be found living in close proximity to
each other is called a
A. habitat
C. niche
B. community
D. kingdom
2. A British ecologist stated; “When an ecologist sees a dadger, they should include in their
thoughts some definitive idea of the animal’s place in the community to which it belongs.”
How would you describe the animal’s place in the community?
A. habitat
C. niche
B. carrying capacity
D. biosphere
3. The giant noctule bat preys upon insects during the summer months and migrating songbirds
during the autumn and spring. The bat attacks the birds at night from several hundred meters
in the air. During the day, the bat roosts in the trees. What do these sentences describe?
A. community
C. biome
B. habitat
D. niche
Use the food chain to answer questions 4 and 5
grass
rabbit
snake
hawk
4. What would you expect to happen to the rabbit if the snake population decreased?
A. decrease in rabbits
B. increase in rabbits
C. rabbit population stays the same
D. unable to determine
5. If there was a drought that caused most of the grass to die, what would you expect to happen
in this food chain?
A. The carrying capacity for the rabbit, snake, and hawk would increase.
B. The carrying capacity for the rabbit, snake, and hawk would decrease.
C. The carrying capacity for the rabbit, snake, and hawk would remain the same.
D. What would happen to the carrying capacity for the rabbit, snake, and hawk is unable to be
determined.
C. Complete the following exercises.
1. Discuss how predator and prey interactions affect population sizes.
2. Describe what controls a carrying capacity and give an example.
3. What type of ecological relationship is shown in the graph below?