CHAPTER 21 Test Your Knowledge
Driving Question 1
What is ecology, and what do ecologists study?
By answering the questions below and studying Infographics 21.1, 21.2, and 21.3, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
What is the difference between a community and a population?
A population is a group of interbreeding organisms of the same species living in a particular geographic area. A community includes all the organisms (and populations) in a geographic area. Populations represent a single species, communities represent multiple species.
An ecosystem ecologist might study
a. plant populations.
b. herbivores that eat the plants.
c. predators in the population.
d. the impact of precipitation patterns on the plant populations.
e. all of the above
d
Why do the researchers collect scat as part of their study on Isle Royale?
Scat reveals information about the organism that produces it. Information that can be obtained from scat analysis includes dietary preferences of the organism, as well as its DNA. The DNA can be used to identify individual organisms, as well as to examine genetic relationships with other organisms, and genetic evidence of inbreeding.
USE IT
How would you explain to a 10-year-old what ecologists do?
Ecologists are biologists who study how organisms interact with their environments. Ecologists can study the size of populations, the factors that can control the size of populations, how populations can change their environments, and how different organisms can interact with one another.
Your local environmental group wants to determine the population size of squirrels in a nearby nature preserve. What are some methods you could use to estimate the size of the squirrel population? Would the same approaches be as useful in determining the population size of maple trees in the same area? Why or why not?
As there are likely many squirrels and as they are likely on the move, it is not feasible to try and count all the squirrels in the nature preserve. Instead, one can estimate the size of the population by counting all the squirrels found in a specific set of smaller square plots. Once the number of squirrels in the smaller plots is established, it is possible to extrapolate to the entire preserve. As the trees are not mobile, it may be feasible to count all the trees in the preserve. If the preserve is very large, it may be more reasonable to count the trees in a set of plots and scale up to the entire preserve.
How would you use scat analysis to determine whether an herbivore had a preference for a particular type of vegetation? Be specific about both the type of analysis and what the analysis would reveal for herbivores with or without a preference for a particular type of vegetation.
In this case, scat analysis can determine what vegetation the herbivore is eating. If the herbivore is eating vegetation in the same proportion as the vegetation occurs in the environment, then the herbivore likely does not have a vegetation preference. On the other hand, if the majority of vegetation in the scat is of a relatively rare plant in the environment, this would suggest that the herbivore is actively seeking out this particular form of vegetation.
Driving Question 2
What are the different patterns of population growth?
By answering the questions below and studying Infographic 21.4, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
Which of the following would cause a population to grow?
a. identical increases in both the birth rate and the death rate of a population
b. a decrease in the birth rate and an increase in the death rate of a population
c. an increase in the birth rate and a decrease in the death rate of a population
d. an increase in the birth rate and a larger increase in the death rate of a population
e. an identical decrease in both the birth rate and the death rate of the population
c
Which of the following statements describes an example of population growth?
a. The average weight of Americans has increased substantially in the past decade.
b. Tropical fish have been found in waters more northerly than their usual habitat.
c. The number of people in a town has increased by 25% in the past 5 years.
d. The number of butterflies in a region has stayed the same from 1950 to 2010.
e. all of the above
c
When a population reaches its carrying capacity, what happens to its growth rate?
At carrying capacity the population growth rate is 0.
USE IT
The wolf population on Isle Royale in 2013 was eight. There was no evidence of wolf reproduction in 2013.
a. What is the current birth rate for the wolf population?
b. If the death rate for wolves is 1 per year, what will be the size of the wolf population in 2014, assuming the birth and death rates do not change?
c. If the winter of 2015 is very cold and an ice bridge linking Isle Royale and the mainland forms, what factor could contribute to growth of the wolf population?
a: The current birth rate (as of 2013) is 0. b: If birth and death rates do not change, then 1 wolf is expected to die and no wolves will be born, leaving a 2014 population of 7. c: If an ice bridge were to form, mainland wolves could come onto the island. This could increase the numbers of wolves on the island, as well as introduce new alleles into the island wolf population.
INTERPRETING DATA
Population Q has 100 members. Population R has 10,000 members. Both are growing exponentially at a 5% annual growth rate.
a. Which population will add more individuals in 1 year? Explain your answer.
b. After 5 years, what will be the size of each population?
c. If the larger population reaches its carrying capacity at the end of the third year, what will its size be after 5 years?
a: Population R will add more members. It is larger than population Q, and 5% of a larger quantity is more than 5% of a smaller quantity.
b:
| Year | Population Q | Population R |
| 0 | 100 | 10,000 |
| 1 | 105 | 10,500 |
| 2 | 110 | 11,025 |
| 3 | 116 | 11,576 |
| 4 | 121 | 12,155 |
| 5 | 127 | 12,763 |
465
Driving Question 3
What factors influence population growth and population size?
By answering the questions below and studying Infographics 21.5, 21.6, 21.7. 21.8, and 21.9, you should be able to generate an answer for the broader Driving Question above.
KNOW IT
The Mexican gray wolf has been reintroduced into parts of New Mexico and Arizona. There have been several wolf deaths due to shootings and traffic kills. Are these influences on the wolf population biotic or abiotic factors?
Shootings and traffic kills are directly related to human activities, and are thus biotic factors.
Drought causes a pond to dry up almost completely. The frog population dropped to 10% of its initial size. What kind of factor—biotic or abiotic—influenced the frog population?
The drying up of the pond due to drought is an abiotic factor.
Which of the following is a density-dependent factor influencing population growth?
a. elevated temperature
b. prolonged winters
c. a viral disease
d. a devastating forest fire
e. all of the above
c
USE IT
Why is it important for researchers to determine the cause of death of moose on Isle Royale? Can this information be used to help make predictions about moose and wolf populations? Explain your answer.
If the moose are dying primarily because of wolf predation, then one might reasonably expect the size of the wolf population to follow the size of the moose population. As the wolf prey on moose, there is less food for the wolf. This will cause the wolf population to decline, reducing the pressure on moose, and so on. If the moose are dying because of additional factors (not just wolf predation), there may be more moose deaths than would normally be expected. In this case, with fewer moose on the island, the wolf may have a much stronger (and detrimental) impact on the moose population, to the point that the moose may not be able to recover.
A group of predatory fish lives in a school in a large lake. If a parasite were introduced to the lake—for example by a vacationing fisherman—would you expect it to have a greater impact on the population if the fish were at high density or at low density? (Assume the parasite is passed from one fish to another through the water but can remain alive in the water only for a very short period of time.) What would happen to this same population if there were a severe drought and very hot summer?
The parasite will be more likely to find a new host at high population density than at low population density. Thus, its impact will be higher at high population density. In conditions of drought and high temperatures, the lake may dry up, increasing the population density of the fish, and the fish may be more stressed. This will increase the probabilities of transmission of the parasite and of the parasite-caused illness in the new host.
Classify each of the following as a biotic or an abiotic factor in an ecosystem. Then predict the impact of each factor on the moose population of Isle Royale. Explain your answers, keeping in mind possible interactions between the various factors and between the moose and wolf populations.
a. hot summer temperatures
b. ticks that parasitize moose
c. declining numbers of balsam fir trees
d. a parvovirus in wolves
e. deep winter snowfall
a: abiotic. Hot summer temperatures will increase the number of ticks on the moose, leading to moose illness and possibly death. This in turn leaves fewer prey for the wolves. b: biotic. Ticks weaken the moose, potentially leading to their deaths, and thus to a lower concentration of prey for the wolves. c: biotic. A reduction in trees would negatively affect the moose, and subsequently negatively affect the wolves, which would now have fewer prey. d: biotic. Parvovirus can weaken or kill wolves. This would cause a decline in the wolf population and a corresponding increase in the moose population. e: abiotic. A deep winter snowfall may make it harder for moose to escape wolf. If wolves can still hunt in deep snow, then moose may be more vulnerable to predation by wolves. The wolves would experience a temporary increase in their population, but there may not be enough moose to sustain this population.
Assume that a new herbivore is added to Isle Royale that is not a prey for wolves. Predict the effect of this introduction on
a. the populations of trees.
b. the moose population.
c. the wolf population.
a: A new herbivore could eat the trees, causing a decline in their population. b: The new herbivore will compete with moose for the trees; the moose are likely to suffer a reduction in their population because of this competition. c: Because the moose are declining and the new herbivores are not prey for the wolves, the wolves will be negatively impacted.
If the moose population remains stable, what other factors could influence the wolf population on Isle Royale?
Factors (in addition to moose) that could influence the wolf population include weather/climate, disease and genetic inbreeding.
MINI CASE
The wolves of Isle Royale are suffering from bone deformities, probably as a result of inbreeding in their small population.
a. Do you think that humans should intervene to save the wolves? Would your answer be different if the wolves were near human populations or agricultural centers?
b. If humans were to intervene, what kinds of strategies might help stabilize or increase the wolf population? Explain your answer.
a: Opinions will vary. b: As there is evidence for inbreeding, introducing wolves from a distinct population may introduce new alleles to compensate for the inbreeding. Efforts to protect young wolves (e.g., by confining them to a protected site) until they are old and healthy enough to be released onto the rest of the island may also help reduce wolf mortality. Ensuring that there is sufficient prey for wolves by introducing alternative prey could also be a strategy.
BRING IT HOME
Use your knowledge of ecology to plan a home saltwater aquarium. Think about the factors you need to consider in order to have a healthy community of fish and plants in your aquarium. Consider both biotic and abiotic factors and the population densities of the organisms in your aquarium.
Answers will vary. Abiotic factors include temperature, salinity of the water, lighting, and size of the tank. Biotic factors include the populations and communities of fish and plants present, their densities, and whether any have a predator–prey or a competitive relationship.