Spring 2001
Review Problems--Unit 2
1. An optimally foraging predator currently includes four prey types in its diet. P1 has the highest profitability, P4 the lowest. For each question (a - d), briefly explain why you chose your answer.
a. If handling times for P1 decrease such that P3 should be excluded from the diet, should P4 also be excluded?b. If the encounter rate for P1 decreased greatly, should P1 still be taken when encountered?
c. If the encounter rate for P1 decreased greatly, is it possible that another species (P5) should be added to the diet?
d. If the encounter rate for P1 decreased greatly, what should happen to the frequency with which P2 is eaten?
2. Hummingbirds feed on nectar from flowers or sugar water from feeders. A series of experiments was done with optimally foraging hummingbirds. In the first experiment, the concentration of sugar was varied. In the second experiment, both the concentration of sugar and the handling and search times were altered from the first experiment. In the third experiment, the concentration of sugar and the frequency of these concentrations were varied so that it took the hummingbirds more time to find some of the concentrations than others. In all experiments, for simplicity, assume that the hummingbirds always took 1 ml of nectar in each visit. These experiments are summarized in the tables below:
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Exp. 1 |
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concentration |
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time to get 1 ml (handling time) |
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search time between feeders = 5 sec |
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Exp. 2 |
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concentration |
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time to get 1 ml |
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search time between feeders = 30 sec |
(Note: If a hummingbird is using only 1 feeder type, it must be rejecting 2 out of 3 types, so in both these experiments, using only 1 feeder would result in 3X search time (15 sec or 90 sec). If the hummingbird were using two feeders, it would be rejecting 1 out of 3 types, so in both these experiments, using 2 feeders would result in 1.5X search time (7.5 sec or 45 sec).)
Assuming the hummingbirds are foraging optimally, and noting the large differences between handling and search times for each of these first two experiments, which feeder(s) should the hummingbirds use in experiment 1 and in experiment 2? Answer this overall question by answering each of these individual questions:
a. Which feeder has the highest profitability?b. What is the average rate of intake if only the highest profitability feeder is used?
c. What is the profitability of the next best feeder?
d. Should the next best feeder be included in the diet? Explain.
e. Continuing this logic, the hummingbirds in experiments 1 and 2 should feed at which of the three feeders? (The optimal diet could include one, two, or three of the feeders.) Explain your answer.
Competition (menu)
3. How are exploitative competition and interference competition different? Why might one say that exploitative competition is an indirect interaction between species whereas interference competition is a direct interaction between species?
4. In a study of subalpine wildflowers, I documented a negative correlation between lupines and blueberries. What are three possible hypotheses to explain this negative correlation?
5. Gause stated that "complete competitors cannot coexist." What did he mean by that and why is it likely to be true?
Lotka-Volterra competition model (menu)
6. Lotka-Volterra competition equation:
a. Given the logistic equation modified for interspecific competition, show how the zero net growth isocline endpoints are derived, assuming the population is at equilibrium.b. Draw arrows on the graph indicating what will happen to the population of species 1 if it is inside the ZNGI and if it is outside the ZNGI.
7. What condition is necessary for the Lotka-Volterra equations to predict a stable coexistence of species? Show how this condition is derived.
8. In order to test which of the Lotka-Volterra competition equation outcomes applies to salamanders, you have set up replicate pairs of cages, one in each of two habitats (A and B). From previous experiments with these two salamanders, you know that their r's are equal. In Habitat A, you start your experiment with lots of red-backed salamanders and only a few blue-spotted salamanders. The outcome is shown in Figure 1. In Habitat B, you start with lots of blue-spotted salamanders and only a few red-backs. The outcome is shown in Figure 2.
a. Which two Lotka-Volterra outcomes are consistent with the results shown in graphs 1 and 2? Briefly explain your answer.
In a second experiment, you switched which species was initially abundant and which was initially scarce in the two habitats (i.e., in Habitat A, blue-spots abundant and red-backs scarce; in Habitat B, red-backs abundant and blue-spots scarce). The results are shown in Figures 3 and 4.
b. What is the most likely Lotka-Volterra outcome to explain all four graphs? Why?
9. How could niche partitioning explain the coexistence of several bird species that all eat insects?
10. What is character displacement? What pattern of traits is associated with character displacement? Why is it difficult to demonstrate that character displacement has occurred? What other mechanisms could lead to the same pattern?
11. Resource partitioning has been accepted as the likely explanation for most coexistence among animal species but not among plant species. Why is there this difference between plants and animals?
12. Why are field studies much less likely to show significant interspecific competition than lab or greenhouse studies?
13. Assume that you conducted an additive design competition experiment. The regressions resulting from the experiment are shown below.
a. State the ECOLOGICAL interpretation of the interaction term.
b. What is the intensity of competition of neighbor A on the target (include units)?
c. If neighbor A were the same species as the target, what could you say about the value of aAB?
d. How would you have to change the experiment if you wanted to measure aBA?
Tilman's model (menu)
**NOT REQUIRED SPRING 2003**
14. Consider the following diagram of the ZNGI's for two species, A and B.
a. What do the axes represent? Label the axes.b. Fill in the boxes on each axis consistent with how you labeled the axes
c. What are the possible outcomes of competition?
d. What will likely be the outcome of competition if the resource supply is at the point "o" on the graph? Why?
15. To be able to predict the outcome of competition between two species before we run the competition experiment, what must we know …
a. according to the Lotka-Volterra model?
b. according to Tilman's model?
Predator-Prey Interactions (menu)
15a. What allowed Huffaker's experiments with herbivorous and predatory mites to result in stable cycles rather than extinction of one of the species?
a. increased sample sizes
b. increased environmental complexity
c. repeated immigration from outside the system
d. repeated insecticide applications
16. Consider the following diagram of predator-prey isoclines:
a. Why does the ZNGIV go up at the left?
b. Why does the ZNGIV go down at the right?
c. How will predator and victim populations behave when the ZNGIP goes through
point A?point B?
point C?
d. At which ZNGIP, through B or through C, would average predator density be higher?
e. On the axes below, graph the victim density over time if 1) the ZNGIP goes through point B, and 2) if the ZNGIP goes through point C. Be sure to identify which line is which.
17. Laboratory experiments with predator-prey systems typically resulted in the predator going extinct or both the predator and prey going extinct. Why did that happen? What has been done with those lab experiments that keeps both in the system longer? What does that suggest about why these systems are more stable in the field?
18. Consider the victim and predator ZNGIs shown below.
a. Why does the predator ZNGI bend down to the right?
b. What is the expected trajectory of these populations?
Herbivory (menu)
19. In the context of herbivory, what does compensation mean? Cite an example.
20. What are some examples of structural defenses, animal defenses, and chemical defenses against herbivory?
21. What is an induced defense against herbivory? What kinds of defenses can be induced?
22. What is a secondary chemical? What are two hypotheses for why secondary chemicals are found in plants?
23. What evidence is there that secondary chemicals are defensive?
24. How do the effects of secondary chemicals often differ between specialist and generalist herbivores?
25. Why has the evolution of insect herbivores and their hosts been called a "coevolutionary arms race?"
Mutualisms (menu)
26. Many mutualisms are obligate for both partners. Why has this mutual dependence evolved (i.e., why is it good) and why is it also risky?
27. What are three mutualisms on which the world's communities depend? Name one intracellular mutualism, one extracellular mutualism, and one behavioral mutualism. For each one, describe why essentially all communities depend on it.
28. An ecologist studying pollination in the Great Dismal Swamp recorded the blooming times of all the hummingbird-pollinated shrubs. She then plotted the bloom times on a graph and measured the time from the peak bloom of one species to the peak bloom of the next blooming species.
a. When she tested to see whether the species had aggregated, random, or uniform bloom times, she found that the bloom times were not significantly different from random. What might she conclude from this test about competition for pollinators and peak bloom time?b. Because the ecologist was out in the field every day watching plants and pollinators, she noticed that hummingbirds become much more abundant about 35 days after the first species' peak bloom. So she split the bloom sequence into two pieces, one sequence before 35 days and the other after 35 days. Testing these separately, she found that the peak bloom times were more uniform than random before 35 days and random after 35 days. What might she conclude now?
29. An ecologist compared the number of seeds produced by scarlet gilia flowers that he had hand-pollinated with the number of seeds produced during the same interval by open-pollinated scarlet gilia flowers. He did this experiment once in early May, at the beginning of the flowering season, and again in mid-June, at the end of the flowering season. The mean numbers of seeds set per flower are shown below along with the statistical analyses.
a. What limits seed production in early May? Cite data to support your claim.
b. What limits seed production in mid-June? Cite data to support your clam.
c. State the ECOLOGICAL conclusion of the pollination*fertilizer interaction in early May and mid-June.
d. State the ECOLOGICAL conclusions of the season*pollination and season*fertilizer interactions in the combined ANOVA.
e. Now explain the conclusions you've made from your experiments to your mother or your housemate.
Indirect Interactions (menu)
30. Diagram the interactions for each of the following
scenarios and indicate the sign
(+ or -) of the indirect effect between:
a. Predators that share a prey speciesb. Predators that do not share prey species, but whose prey species compete for a food plant
c. A predator and the plant fed on by the herbivore prey of the predator
d. A predator and a mutualist with the predator's prey
e. Mutualists with each of two competing species
f. Two species that both compete with the same species
31. Assume that two species in a community are not connected by any direct interaction, but they are connected by several indirect interactions pathways as shown on the diagram below.
a. What is the sign of the indirect effect between B and F by the pathway B-C-E-D-F?By the pathway B-C-F?By the pathway B-C-D-F?
By the pathway B-A-F?
b. What might you predict would be the combined indirect effect of B on F? Why?
32. Two herbivore species share a predator as well as a common food plant. When the population of herbivore A goes down, the population of herbivore B goes up.
a.How can you explain this response through competition?
b. How can you explain this reponse through predation?