Notes to Instructors
Chapter 54 Community Ecology
What is the focus of this activity?
Campbell Biology, 9th edition, defines community ecology as “the study of how
that can affect the structure and organization of communities.
What is the particular activity designed to do?
Activity 54.1 What do you need to consider when analyzing communities
of organisms?
Activity 54.2 What effects can disturbance have on a community?
Activity 54.3 How can distance from the mainland and island size affect species
richness?
This activity allows students to explore the theory of island biogeography and the effects
distance from the mainland and island size have on species numbers.
All of these activities give students practice in data analysis and graphical representation
and interpretation.
Answers
Activity 54.1 What do you need to consider when
analyzing communities of organisms?
Understanding problems in community ecology most often requires integration of a
number of ecological principles.
360 Notes to Instructors
Ecological principles:
Activity 54.1 361
coevolution
realized niche
fundamental niche
tolerance
competitive exclusion
resource partitioning
character displacement
top-down vs. Bottom-up controls
trophic structure or trophic levels
keystone species
competition
disturbance
1. A small clan of hyenas killed an antelope. While they were feeding on the carcass,
two female lions approached, growled at the hyenas, and chased them away from the
carcass.
The kill is an example of predation by the hyenas. The action of the lionesses is an
2. Two species of closely related swallows live in England. The black swallow lives in
coniferous forests, and the yellow swallow lives in deciduous forests. In Ireland,
where the black swallow has never been introduced, only the yellow swallow is
present and it lives in both coniferous and deciduous forests.
This is an example of how a population’s realized (actual) niche can differ from its
fundamental or ecological niche. When the black swallow is not present, the yellow
3. In a woodland community, three species of rodents coexist: voles, field mice and
shrews. All three species eat seeds and nuts. Each species has a preference for seeds
of the most appropriate sizes for their teeth and mouths. However, all three species
compete for the same kinds of nuts. An owl species also lives in this woodland
community. The owl preys on all three rodent species. During one particular year,
a parasite that causes pneumonia in birds is introduced into the community. This
dramatically reduces the owl population, which remains low for several years as
a result. Following the initial reduction in the owl population, there is a dramatic
increase in the population of field mice and a dramatic decrease in the populations
of both voles and shrews.
The owl is a keystone species. Because the vole and shrew populations decline and
4. In 1962, five mute swans escaped from captivity and began a breeding population in
Chesapeake Bay. Today, there are over 4,000 mute swans living in the bay. Each
year they eat approximately 10.5 million pounds of aquatic grasses. These grasses
provide habitat for waterfowl and crustaceans, improve water quality, decrease
erosion, and increase dissolved oxygen concentrations in the bay. The birds are also
aggressively territorial, and have been known to trample nests of other birds (e.g.,
least terns and black skimmers) and drive away native birds like tundra swans and
black ducks from feeding and roosting areas.
This is an example of competition for territory and resources as well as an example
5. Known as the “Hawaiian woodpecker,” the `akiapola`au (aki-a-pul-a-ow) is found
only in montane mesic old-growth koa/`ohi` forests, and only on the Big Island
(Hawaii). It has a distinctive beak that is like a multiple-use tool.
362 Activity 54.1
The short straight lower mandible is used to peck holes in the wood and the long
curved upper mandible is used to probe for insects and larvae. Males have larger
beaks than females and feed on the trunks of trees. Females feed higher on branches
and twigs. ‘Akiapola`au are thought to have the lowest reproduction rate for a small
bird—only one chick per year, which is cared for by the parents for 6 months or
more. The decline in their numbers appears to correspond with the introduction of
rats, cats and logging on the island.
This species has a fairly specialized niche in old-growth forest. As a result, it is
6. A researcher collected data on an experiment she conducted on two desert islands.
The islands were of similar size, climate, and species composition and richness, and
were the same distance from the mainland. Originally, the same species of snake was
present on both islands (A and B). In her experiment, the researcher removed the
species of snake from island A. For comparison, the species of snake was NOT
removed from island B. She then recorded the number of animal species on each
island over a period of 24 months. Her data are presented in the table below.
a. Graph the data.
Activity 54.1 363
# Species
Time
(months) A B
1 36 36
2 38 37
3 35 34
4 33 35
5 31 36
6 32 38
7 29 40
b. Construct a hypothesis that explains the difference between the numbers of
species present on island A versus island B over the 24 month period.
One reasonable hypothesis would be that the snake was acting as a keystone
to extinction.
c. Propose an experimental design to test your hypothesis. Explain the reasoning
behind your design.
One possible design would be to remove the snake from island B, reintroduce
d. What would you expect to find as a result of your experiment? Describe your
expected results and draw them on the graph below.
Over time you would expect to see the numbers of species on island A increase
364 Activity 54.1
45
40
35
30
Time (months)
A
B
Activity 54.2 What effects can a disturbance have
on a community?
Disturbances rarely affect only one species of a community. More often disturbances that
have direct effects on a single species produce a cascade of additional effects on other
species.
For questions 1 and 2, analyze the ecological situations described. Then answer the
questions that follow each situation.
1. A particularly popular island vacation site is home to many species of orchids. The
primary income generating businesses on the island are tourism and orchid sales.
Disturbance: To make the island more attractive to visitors, a local politician
suggested that the island be periodically “fogged” with insecticides.
Response: This fogging reduced the insect numbers. It also appeared to reduce the
number of birds and new growths of orchids.
(Hint: Examine the ideas of limits of tolerance, coevolution, predation, and
bioaccumulation.)
a. Is it likely that the disturbance directly caused the response?
Fogging with insecticides is likely to have directly reduced the number of insects.
b. What other factors might be involved?
Many pesticides are fat-soluble. The amount of pesticide directly encountered by
the birds may have been within their limits of tolerance. As insect eaters,
Activity 54.2 365
c. How could you test different factors for their effect on the response? For
example, what experiments could you set up?
Different types of experiments could be set up. One possible experiment would
d. What would you expect to find if the other factors you proposed affected
the response?
If direct spraying caused the deaths in the birds, you would expect to see equal death
2. Australia and New Zealand are home to a very wide range of marsupials (for
example, kangaroos and other pouched mammals). Until colonization by foreign
traders and other developments, placental mammals were not found in these areas.
Disturbance: Following colonization, the rabbit was introduced to Australia.
Response: The rabbit multiplied rapidly and ultimately became a pest species, doing
considerable damage to both crop and natural plants.
(Hint: Examine the ideas of competition, competitive exclusion, predation, and
coevolution.)
a. Is it likely that the disturbance directly caused the response?
The introduction of the rabbit is the most likely disturbance. The response is the
366 Activity 54.2
b. What other factors might be involved?
In areas where the rabbit is a native species, it is generally not a pest because the
c. How could you test different factors for their effect on the response? For
example, what experiments could you set up?
The government of Australia tested the hypothesis that lack of disease organisms
d. What would you expect to find if the other factors you proposed affected
the response?
If lack of disease organisms was a factor in the overpopulation of rabbits, you
3. In the boreal forest of Canada, wildfires are important disturbance factors. A single
wild fire seldom burns the whole forest. Instead it burns large patches or stands and
leaves others untouched. Following a wildfire in a black spruce forest, there is
usually a predictable regrowth of the vegetation, starting with ground lichens and
small spruce seedlings. As the spruce trees grow and form a closed-crown canopy,
feather mosses (Bryophytes) are found in an increasing proportion on the forest
floor. In some cases, the peat moss Sphagnum outcompetes the feather mosses and
eventually dominates the ground cover. Because wildfires occur naturally about
every 10 years, a forest stand can sometimes burn before Sphagnum dominance is
reached, and the whole process repeats.
a.Graph “Tree Biomass vs. Time” over a 100-year period:
i. at the stand scale (the stand is a particular part of the forest that burned) and
ii. at the landscape scale (composed of many forest stands).
• Assume, at the stand scale, that when a stand burns, all trees in that stand die.
• Assume, at the landscape scale, that there is one fire every 10 years, and
that each fire burns a different stand.
Activity 54.1 367
b. Explain how (and why) the graphs differ.
In the scenario presented, even though there are large variations in tree biomass
Activity 54.3 How can distance from the mainland and
island size affect species richness?
1. Island biogeography theory attempts to explain the patterns of species richness and
turnover on islands as a function of the size of the island and its distance from the
mainland.
a. What are the basic tenets of this theory? (Refer to Figure 54.27 in Campbell
Biology, 9th edition.)
The theory states that species richness (number of species) on islands is
b. In the graph below, curves A and A’ represent the historic immigration and
extinction rates, respectively, for an island off the coast of South America. Given
these data, what is the equilibrium number of species for this island?
368 Activity 54.3
The equilibrium species number would be approximately 8.
Activity 54.3 369
c. If a land bridge or an isthmus forms and connects the island to the mainland,
which two curves would best represent the resulting immigration and extinction
rates for the island?
Line C would represent the new rate of immigration and line A’ would remain the
d. What would be the new equilibrium number of species?
2. A research scientist comes back from a study of a group of islands in the South
Pacific. He says that his data show that in some cases, the smaller islands he
investigated had higher numbers of lizard species on them than did the larger
islands. He suggests these data indicate that we should reexamine the whole theory
of island biogeography.
a. What additional information would you like to have about the group of islands
he studied?
Some of the questions that could be asked include the following.
b. Is there any way his data could be accounted for using the current theory of
island biogeography? Explain your reasoning.
Given the theory of island biogeography, it is possible that a smaller island may
370 Activity 54.3