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PROBLEM SET #1A: ECOLOGY
AP Biology
Work in small groups to answer these questions. You may reference your notes or textbook and you
are welcome to discuss the answers with your group members. After discussing these questions, write
your responses in your own words.
1. Mercury is introduced into ocean water through emissions from factory smokestacks then converted by
bacteria into a form that is easily absorbed by insects and small sea organisms. Recently, the FDA has
become concerned about the levels of mercury in large fish such as swordfish and tuna—it has reached
levels that are unsafe for human consumption. Explain how the levels of mercury got so concentrated in
these large fish by considering food chains and energy pyramids.
Since only approximately 10% of the energy is transferred from one trophic level to the next, organisms
such as large fish have to eat lots of the organisms below them to get enough energy (small sea
organisms, etc). This means that even if there are very small amounts of mercury in each small organism it
will add up when you consider how many of them the large fish eats. Toxins such as mercury accumulate
in the fat of organisms (they aren’t eliminated in waste). So as you move up a food chain, the
concentration of a toxin increases. This is known as biomagnification.
2. The population of geese shown in the graph below has reached its carrying capacity. Account for the
factors that cause the numbers in the population to fluctuate even after reaching carrying capacity.
Carrying capacity is defined as the number of organisms that
an environment can support indefinitely. In reality, this
doesn’t mean that the population can never go any higher
than the carrying capacity. When it does exceed the carrying
capacity, density dependent factors such as food, shelter,
disease will work to bring the population back down. A
population at carrying capacity will fluctuate around the
carrying capacity.
3.
The acacia tree is native to North America, however it is now found all over the world. The leaves of the
acacia tree contain many acidic compounds. Acacia trees make good firewood, so they are used by
forestry departments worldwide as a source of lumber. What effect could acacia trees have on the
surrounding environment and ecosystem if they are planted in a region in which they are not native? What
biotic and abiotic factors might be affected by the acacia tree?
The main danger with an invasive species is that it disrupts the new environment it has been transplanted
into. This includes competing with the native species for resources or creating an altered environment for
other native species to survive in. If the acacia tree is planted in an area, the leaves cause the soil to
become very acidic. This can prevent other plants from growing in that area that do not usually grow in
acidic soil.
4.
Exponential growth is one of the models used to describe population growth. Why is it theoretically
impossible for a population to exhibit only exponential growth?
The environment can only support a certain number of individuals indefinitely (carrying capacity).
Eventually, growth in any population would have to level off because of the fixed number of resources
available.
5.
Over time, wetland area slowly transitions to a swampy marsh, then a grassy field. This is followed by the
growth of most shrubs, then finally trees. What is happening in this area?
This is the process of succession. Specifically the scenario described is secondary succession since
presumably the wetland already has soil.
Name ___________________________________________________________________ Period ______
PROBLEM SET #1B: ECOLOGY
AP Biology
Now that you have had the opportunity to work on some questions with your classmates, it is time for
you to see how well you understood the topics. Again, you can use your notes and textbook for
reference, but you should complete these questions individually and in your own words.
1.
As a park ranger for the National Park Services, you are monitoring the population of the bison at
Yellowstone National Park. According to previously recorded data in 1990, the carrying capacity of the bison
was 15,000 bison. After taking some data of your own, you find that the carrying capacity has dropped to
9,500 bison. What are some reasons that the carrying capacity would drop? What are some reasons that the
carrying capacity would increase?
Reasons for Drop: generally speaking, this would occur when resources become more limited
(less food, shelter, etc. available)
Reasons for Increase: generally, this would occur when resources become more available
2.
Energy is transferred from one trophic level to the next with varying degrees of efficiency. Inevitably, not
all of the energy that is produced can be passed on to the next level. Which of these organisms, a fish or a
mammal, would be more efficient at passing on energy to the next trophic level? Explain why.
Fish are more efficient in passing on energy to the next trophic level because mammals have to use more
energy to maintain their body temperature (they are warm blooded). Fish do not do this sort of temperature
regulation, so they are able to pass on more energy to the next level.
3.
Biomass is defined as the total weight of all the living organisms in a particular area. Over the course of
4 years, scientists measure the biomass of a lake. During year 2, a new golf course is built nearby which
results in a great deal of fertilizer runoff into the lake. Explain the drastic changes in biomass over the course
of these 4 years (Hint: algae in lakes like fertilizer too, also use oxygen from the water, and tend to float at the
top of the water… think about how algae population could affect other populations
Year
Biomass (g)
in the lake)
1
246
The extra nutrients from the fertilizer runoff cause the algae to overgrow (which
accounts for the rapid increase in biomass in year 2). Because of this, the algae start
2
800
to use up all of the oxygen in the water as well and cover the surface of the lake so
3
160
that nothing can grow under it. Eventually, the lake and all the organisms will begin to
4
45
die (which is why the biomass drops dramatically in year 3 and 4)
4.
Bacterial populations are important to maintain in populations.
a) The formula for population growth rate is: N1 = N0 + B – D + I – E, where N1 is the calculated population size
(at time 1), N0 is the initial population growth rate (at time 0), B is the birth rate, D is the death rate, I is the
rate of immigration, and E is the rate of emigration. If N0 = 100 bacteria/hour, B = 150 bacteria/hour, D = 20
bacteria/hour, I = 2 bacteria/hour, and E = 0 bacteria/hour, what is N 1? 232 bacteria/hour
b) Some bacteria are nitrogen-fixing bacteria. Reference the nitrogen cycle in your notes or textbook and
explain why these bacteria could be considered a keystone species.
Nitrogen fixing bacteria take nitrogen from the soil that cannot be used by plants, and change it to a
form plants can use. If they did not exist, then nitrogen might not be accessible by plants, and
subsequently the plant populations would diminish. Without plants, populations of species dependant
on them would also diminish. The entire ecosystem would be severely impacted.
5. As a population biologist, you are studying a population of wild tree squirrels. To determine the size of the
population, you first tag 300 squirrels. You then release the squirrels. 3 weeks later, you return and capture
another 300 squirrels. Of these 300, 85 are tagged while the rest are untagged. Using this information,
determine the actual size of the tree squirrel population. Show your work.
If you only see 85 (compared to the 300 you originally tagged) tagged squirrels the second time,
you can assume that the first time you tagged the squirrels. you only tagged approximately 1/3
of the actual squirrels in the population. So, 300 x 300/85 = 1059 squirrels