Ecology
Exam 2 KEY

Spring 2004

1. Bluegills and pumpkinseeds are both species of small sunfish. Assume that pumpkinseeds are native to small lakes in an area and bluegills were added to one of the lakes (Lake 1). Both populations were monitored and the population trajectories are shown below.

a. Assume the two sunfish populations are competing. On the axes above, draw the Lotka-Volterra competition diagram that predicts this outcome. In the boxes, write the axis values at each intercept (use "b" and "p" as subscripts so I can tell to which species each term refers). (6 pts)

b. In a second lake (Lake 2) to which bluegills were introduced, the outcome was different. Below I've shown you the Lotka-Volterra diagram describing the outcome. On the set of axes on the right, draw the population trajectories expected given this Lotka-Volterra diagram. (6 pts)

 

c. Which of the following conditions must be met for bluegills to compete with pumpkinseeds? Circle ALL correct answers. (6 pts)

a. The species must use at least one resource in common.

b. The species must physically encounter each other.

c. The shared resource(s) must be necessary to pumpkinseeds.

d. The shared resource(s) must be in short supply (limit the pumpkinseed population).

e. Bluegills must use the resource(s) more efficiently than pumpkinseeds.

f. Pumpkinseeds must use the resource(s) more efficiently than bluegills.

 

2. Consider two plant species, A and B. Species A is a small, short-lived species occupying a shady, moist, high-nitrogen forest site. Species B is a large common tree species occupying a sunny, dry, low-nitrogen site.

a. What kinds of chemical defenses would be predicted for each species under Apparency theory? Use the terms that were developed in Apparency theory in your answer. (4 pts)

Species A - qualitative defenses

Species B - quantitative defenses

 

b. What kinds of chemical defenses would be predicted for each species under Resource Availability theory? Use the terms that were developed in Resource Availability theory in your answer. (4 pts)

Species A - mobile defenses

Species B - immobile defenses

 

c. What kinds of chemical defenses would be predicted for each species under the Carbon:Nutrient Ratio theory? Use the terms that were developed in C:N ratio theory in your answer. (4 pts)

Species A - nitrogen-based defenses

Species B - carbon-based defenses

 

3. Presume an owl has two possible food choices: rabbits (1500 kcal) and field mice (100 kcal). It takes an owl an hour to catch and eat a rabbit, but only 10 minutes to catch and eat a field mouse. Assume the owl saw a rabbit every 3 hours and a mouse every 30 minutes. (Therefore, if the owl eats rabbits only, search time is 3 hours; if the owl eats mice only, the search time is 30 minutes; if the owl eats both, assume the average search time is reduced to 25 minutes.) According to MacArthur and Pianka's optimal diet width model, what would be the owl's optimal diet (rabbits only, mice only, or both rabbits and mice)? Show your calculations for credit. (12 pts)

Profitabilities:

Rabbits: 1500 kcal/60 min = 25 kcal/min * higher profitability food item

Mice: ….100 kcal/10 min = 10 kcal/min

Average rate of energy intake eating only rabbits:

[E/(h+s) =?= E/h]

1500 kcal/(60 min + 180 min) = 6.25 kcal/min

Because Ave rate of energy intake eating only rabbits < profitability of mice, the optimal diet includes both rabbits and mice.

 

4. Two species of early spring wildflowers, miner's lettuce and spring beauty, both bloom at similar times. The number of seeds per plant was measured in plants of each species under two conditions: when they were intermixed with plants of the other species and when they were isolated from the other species. The results are shown below.

a. What analysis is represented by the table on the right? What are three properties of the data that made this analysis appropriate? (5 pts)

2-way ANOVA

replicated data

categorical independent variables only

two variables, each with two levels

b. What analysis is represented by the letters on the graph? (2 pts)

Fisher's PLSD test

c. What are the ecological conclusions you can draw from these results? Indicate either a p-value or the test that supports each result. (6 pts)

1) Miner's lettuce produced significantly more seeds than did spring beauty (P = 0.0341).

2) Being intermixed increased the seeds per plant of miner's lettuce whereas being intermixed decreased the seeds per plant of spring beauty (Fisher's test). [The fact that the species' responses are different is supported by P = 0.0059, but not whether individual differences between conditions are significant.]

 

d. Suggest a mechanism based on pollinator interactions that could cause this result. (5 pts)

The presence of the other species benefits miner's lettuce but hinders spring beauty. One possibility is that sufficient pollinators are attracted to spring beauty to pollinate it fully when it is by itself, but miner's lettuce competes for pollinator attentions, decreasing the pollination success of spring beauty. In contrast, miner's lettuce may not be able to attract sufficient pollinators on its own, so the presence of spring beauty increases the number of pollinator visits to miner's lettuce.

5. What are two kinds of risks inherent in biological control? (6 pts)

Answers we mentioned in class: unintended target species, interference with established control agents, release of a different species that was originally controlled by the target species

 

6. The following phase diagram shows the ZNGIs for two mutualist species, ants and bracken fern. Please use this phase diagram to answer the following questions.

a. What does the intercept at "10" on the fern axis represent for the mutualism? (5 pts)

This intercept represents the minimum density of ferns necessary to allow an ant population to be established.

 

b. Are the ferns obligately or facultatively dependent on the ants? How can you tell? (5 pts)

The ferns are facultatively dependent on the ants. Bracken fern can have a density of 5 ferns/per m2 when there are no ants present.

c. Interpret the shape of the fern ZNGI by describing how the relationship between ants and bracken fern is different with this curved line from what it would be with a straight line (shown for comparison). (5 pts)

The straight line would imply that each additional ant allows the same increase in fern density. The curved line implies that the first ants allow a larger increase in fern density, but that additional ants allow a smaller and smaller marginal increase, up to a carrying capacity limit on ferns imposed by some other resource constraint.

d. At equilibrium, what would be the population sizes of ants and bracken fern? Explain how you know. (5 pts)

Because all trajectories lead to the equilibrium point at the upper right, the equilibrium sizes of the populations of ants and ferns would be 40 and 30, respectively.

 

7. The following data were collected in an experiment to test the effects of removing Pisaster (P-predatory sea star) and/or Nucella (N-predatory snail) on the density of mussels on the outer coast of Washington.

a. Considering only the data for Nucella, are the mussels controlled top-down or bottom-up? Cite the evidence that supports your conclusion. (3 pts)

Because in both cases, when Nucella is removed, there is an increase in the mussel density (P = 0.0218), mussels must normally be kept below their carrying capacity by predation from Nucella. Therefore, the mussels are controlled top-down.

b. In the absence of Nucella, what is the effect on mussel density of removing Pisaster? Cite the evidence that supports your conclusion. (3 pts)

When Nucella is absent, there was an increase in mussel density when Pisaster was removed (Fisher's test, P<0.05).

c. In the presence of Nucella, what is the effect on mussel density of removing Pisaster? Cite the evidence that supports your conclusion. (3 pts)

When Nucella was present, there was a decrease in mussel density when Pisaster was removed (Fisher's test, P<0.05).

d. Suggest a hypothesis for the interactions among all three species that takes into account all the data. (5 pts)

Without Nucella around, Pisaster affected mussels negatively. However, with Nucella around, Pisaster affected mussels positively. This suggests that there is something going on between Pisaster and Nucella. Since Nucella's effects don't depend on Pisaster, it would seem that Pisaster is doing something to Nucella. If Pisaster were eating Nucella, and Nucella were a stronger predator on mussels than Pisaster, then removing Pisaster might allow more Nucella predation on the mussels.

Bonus: What was the fact or observation at Hylebos wetland that you found most interesting? (2 pts)