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KEY
Name _________________________
Unit 5
Populations & Ecosystems
A population is a group of individuals of the same species living together in a particular area.
Populations will increase if the birthrate exceeds the death rate and if immigration (organisms
moving into the range) exceeds emigration (organisms moving out of the range).
biotic
Populations will increase rapidly if there are adequate __________
(living)
abiotic
and ____________
(nonliving) factors present in the environment. For
example, with adequate resources (nutrients, water, etc.) and space and no
predators, no disease, and little or no competition for resources, a
population has the potential to grow rapidly. This rapid growth is called
exponential growth, and all populations are capable of growing
exponentially if conditions remain ideal.
Population density is the number of individuals per unit area (for land organisms) or volume (for
increases
aquatic organisms). As a populations grow, its density __________________.
Because the amount
space
of resources and open _________
are usually limited, the increased population leads to overcrowding
which puts stress on the individuals within the population. This stress leads to competition for
survive
resources, so some members of a population may not ______________.
So, even though populations have the potential to grow exponentially if
conditions are good, most often populations will reach their
carrying capacity
__________________________(the
maximum number of individuals of a
particular species that a particular environment can support).
Once a population reaches the carrying capacity, there are many factors that
will help it to remain near that size.
limiting
Any factor that controls the size of a population is called a ____________________
factor. Density
independent limiting factors may affect all individuals in a population equally, although some
individuals may be better able to adjust to them than others. Density dependent limiting factors
have a greater affect when the population density is higher; they become less important when the
population density is low.
The size of one population in an ecosystem can dramatically impact the size of another population.
Imagine a population in which rabbits eat plants and coyotes eat rabbits. Complete the table below by
identifying the impact of the recorded “change” on the other populations in the ecosystem
Effect on Plant
Population
Effect on Coyote
Population
Plants decrease
Very high reproductive year;
population rises above the
carrying capacity
Plants increase
Rabbits decrease
With abundant food, the
population rises above
carrying capacity
Plants increase
A disease nearly wipes out the
entire rabbit population
Coyotes decrease
Drought/no rain
Rabbits decrease
Coyotes decrease
Plants decrease
Rabbits decrease
Coyotes decrease
People move into area
and reduce the size of
the habitat
Forest Fire
Effect on Rabbit
Population
Plants decrease
Rabbits decrease
Coyotes increase
Coyotes decrease
Review:
14. Populations
The Columbia Basin Pygmy Rabbit is an endangered species. The Pigmy
Rabbit is the smallest native rabbit in America; it weighs only one pound
when fully grown. This rabbit is the only one in the United States that
burrows into the ground and climbs and feeds on sagebrush.
Horned Owl
Coyote
Pygmy Rabbit
Bitterbrush
Deer
Sagebrush
Describe a factor that limits the growth of the Pigmy Rabbit population other than the presence of
coyotes or owls.
1)
2)
3)
4)
5)
Disease : If rabbits are too sick to survive and reproduce, the population decreases.
Food: If a fire or flood wipes out plants that the rabbits eat, the rabbit population declines.
Water: If drought dries up watering sources, the rabbit population shrinks.
Other predators: If a population of snakes moves into the area, the rabbit population decreases.
Loss of habitat: People moving into the area and removing bitterbrush and sagebrush that
rabbits eat.
6) Decrease in deer: This will any cause predators of deer to feed on rabbits.
Other than a reduction in predators, describe a condition that allows the population of Pigmy Rabbits to
increase rapidly in its environment.
1) If the deer population is reduced by a disease then the pygmy rabbit would have less
competition for sagebrush and as a result they would have more food so the population
would increase.
2) If a mutation helped pygmy rabbits better adapt to their environment or avoid predation
then the population of rabbits would increase.
In 1993 Pygmy Rabbits were placed on the endangered species list.


Describe how the loss of the Pygmy rabbit impacts two other populations in its ecosystem
Describe how each of these relationships affects the stability of this ecosystem
One Population:
1) The deer population would increase if the pygmy rabbit population decreases because the
deer wouldn’t have to compete with the rabbits for food (sagebrush).
2) The coyote population would also increase because of the increase in deer.
Another Population:
3) The bitterbrush and sagebrush populations would increase because fewer rabbits are
eating them.
4) The horned owl population would decrease because there will be less food (pygmy rabbits)
for the owls to survive off of.
Many plants in the temperate forests of the Pacific Northwest produce berries and other fruits. These
fruits contain seeds. When animals like bears eat these fruits, their cells use the sugar stored in the fruit
to produce ATP, and the seeds are eliminated in their waste. Many plants rely on a variety of animals
and birds to distribute their seeds.
Imagine that a bear “deposited” seeds from the berries it ate in three different locations: on a rock, in a
dry meadow, and near a stream. Determine in which environment the seeds will be most likely to
sprout and grow into a new berry bush, then describe two conditions in that ecosystem that might
allow this berry bush population to increase rapidly.
Environment:
Near a stream.
One Condition:
The water in the stream will allow the berry bush to grow and the stream can also help
distribute the berry bush seeds downstream.
Trees tend to grow along streams making the area shady. If the berry bushes thrive best in
shady areas, then the shady conditions along the stream will help the berry bush population
increase rapidly.
Another Condition:
Another condition would be the animals that go to the stream to drink the water also eat the
berries and redistribute them to other areas where they grow.
Imagine that the population of berry bush did rapidly increase in the ecosystem you selected. Describe
two ways that the increase in the berry bush population could limit the population of another species in
that ecosystem.
One Way:
If the berry bush population increased this would limit the nutrients available for other
plants to grow.
If the berry bush population increased this would limit the amount of sunlight reaching the
ground around the bushes. This may prevent smaller plants that require a lot of sunlight
from growing.
Another Way:
If the berry bush population increased this would limit the space available for other species
of plants to grow.
____ COE: “Columbia White Tailed Deer #1”
____ COE: “DIY Plant Experiment #3”
Review:
15. Population Density
Calculate the population density of pine trees in the two different 2 acre plots shown below:
= pine tree
9.5 trees/acre
19 trees/2 acres = ___
___
4 trees/2 acres = ___
2 trees/acre
___
Describe two factors that would allow one plot to have a greater density of trees than another plot
One factor that would allow one plot to have a greater density than another is a wild fire may have
affected the density in the plot with fewer trees.
Another factor would be if one plot had more resources (nutrients in the soil) then that plot would
have a greater density of trees.
I have a 5 gallon bucket full of sand and small snails. Without dumping them all out, describe a method
for estimating the population density of snails in my bucket.
If you took a one gallon sample of sand from the 5 gallon bucket and counted the number of snails
in the one gallon of sand then you would multiply that number by 5 and get an estimate for how
many snails are in the 5 gallon bucket.
In a small neighborhood pond, the frog eggs have transformed into tadpoles.
Describe how you could estimate the population density of the tadpoles
(without trying to count each one individually).
Use the mark and recapture method for estimating the tadpole population.
To do this you would dip a net into the pond in different locations to catch
some tadpoles. Mark and count the captured tadpoles with an oil-based
marker so that it will not dissolve away. After two days, dip the same areas
again and count the number of tadpoles that have a dot and don’t have a dot.
Use a proportion equation to estimate the population of tadpoles.
Describe two factors that allowed this population density to increase rapidly (& include “how” each
caused the population to increase).
One factor is a decrease in the number of predators which feed on tadpoles.
This will allow more tadpoles to survive to adulthood and reproduce, thus
increasing future tadpole populations.
Another factor is the pond shrinking in size from a drought. This would
increase the population density without increasing the tadpole population.
____ COE: Worms in Action #1 (review 5 before answering)
Population Graphs
Graphing populations allows us to
observe changes in population numbers
over time, infer why the changes are
occurring, and make predictions as to
how populations may change in the
future.
Predator-Prey relationships are
important because they keep
populations in balance in the ecosystem
(negative feedback).
Prey populations will increase if they
have adequate resources and space and
predator numbers are low.
An increase in the prey population will
be followed by an increase in predators
because there is more food for them,
which means they are healthier, compete less with each other, and have more reproductive success.
An increase in the predator population will be followed by a decrease in the prey population as more of
them are eaten and fewer of them are reproducing. When prey numbers drop, predators begin to
starve, so a drop in predator numbers will follow a drop in prey numbers.
With fewer predators, prey will have more reproductive success, and their numbers will begin to rise….
and the cycle starts again.
Removal of predators will allow the prey populations to rise above carrying capacity, which means they
consume too much vegetation and will soon run out of food and begin to starve. If prey are removed
from the ecosystem, the predators starve. So, as much as we don’t want to worry about wolves and
other large predators, they play a very important role in keeping prey populations (deer, elk, moose,
etc.) under control, which helps keep the entire ecosystem in balance.
Review:
16. Population Graphs
Study the Graph to the right. State at least 2 conditions
in the ecosystem that could cause the trend shown. For
each condition state how it affects the population.
1) The population started with a small number of
rabbits with an ample food supply. Since the
rabbits have plenty to eat, they will reproduce
exponentially until they reach carrying
capacity.
2) A steady decline in the number of predators
that feed on the rabbits. The rabbits will be
better able to survive and reproduce.
3) A mutation that enables the rabbits to become
better camouflaged to avoid predation.
The predator prey graph shows the relationship between
the coyote (predator) and the rabbit (prey) over many
years.
Using the predator-prey population graph, describe two
relationships between the populations of coyote and
rabbit as the populations change over time.
One Relationship:

The population of rabbits at its maximum is always larger than the population of coyotes
at its maximum. This means that coyotes must consume many rabbits in order to survive
and reproduce.

The coyote population rises and falls with fluctuations in the rabbit population.
However, this rise and fall always lags behind the rabbit population.
Another Relationship:

The rabbit is the prey of the coyote.

Both rabbits and coyote populations grow exponentially after a crash.

The coyote and rabbit population interactions are cyclic and show a pattern of negative
feedback.
Identify whether the relationship between the coyote and the rabbit is an example of positive feedback
or negative feedback. Describe the reason for your answer.
Feedback:
The relationship between coyotes and rabbits is an example of negative feedback. This is evident by the
cyclic nature of the interactions. When the rabbits are abundant, coyote populations expand, and when
the density of rabbits is reduced, the coyotes are forced to hunt ground squirrels, grouse, and mice. This
shift away from rabbits takes its toll, and the coyote populations ultimately shrink.
The Shrub Steppe Ecosystem of Washington State
Includes land surrounding the Tri-Cities
Hawks & other
Raptors
Badgers
Coyotes
Rattlesnakes &
other snakes
Pygmy Rabbits
Jackrabbits
Sagebrush & Rabbitbrush
Mt. Lions
(Cougars)
Lizards &
small reptiles
Squirrels &
small rodents
Desert wildflowers
Bats
insects
Deer &
other grazers
Grasses
(Soil Bacteria, worms, fungi, etc.)
Predict what would happen to the population of sagebrush if the coyotes and other large predators were
removed from the shrub steppe ecosystem. Describe the reason for your prediction.
Predict:
The population of sagebrush would most likely decrease. The reason is that large predators like coyotes
and mountain lions feed upon pygmy rabbits, jackrabbits, and deer. These are the same primary
consumers that feed on sagebrush. If you take away the predators of these primary consumers, then the
population of rabbits and deer will increase exponentially. This increasing population will eat up nearly
all of the sagebrush.
____ COE: “Wolves in Yellowstone #2”