Which plot(s) show evidence of density-dependent mortality? Circle one answer.

EXAM #2                                                                  

Multiple choice (2 pt)

1- 2. Chromis cyanea is a coral reef fish. Hixon and Carr (1997) hypothesized that predation on recently-settled Chromis was a key factor regulating the population density of Chromis. They tested their hypothesis by allowing predators to access some reefs (A) and excluding them from others (D). The plots show the per-capita mortality of Chromis versus the number of settlers at the start of the experiment.

1. Which plot(s) show evidence of density-dependent mortality? Circle one answer.

Plot A              Plot D              Both                Neither

2. Which option is the most reasonable interpretation of these plots?
3. Predation is density-dependent in a way that regulates the Chromis population.
4. Predation is density-dependent in a way that works against regulating the Chromis population.
5. Predation isn’t density-dependent. Some other factor must be regulating the Chromis population.
6. Predation isn’t density-dependent. No factor appears to be regulating the Chromis population.

3. On the plot below, the logistic model predicts per-capita birth rates to fall along the solid line. The points show your actual data for a songbird.

What ecological factor could account for the difference between your data and the logistic model’s prediction?

(a) lack of energy resources at low density

(b) the bird is a cooperative breeder, so it has low reproductive success at low density

(c) lack of nesting sites limits reproduction at high density

4. In which real situation would an exponential model be a better choice for predicting population growth than the logistic model?
5. A deer population that is declining because its density is much higher than its resources can support.
6. A bacteria population that has already covered most of the growth medium in its petri dish.

C A population of Black-footed Ferrets that has been newly founded in favorable habitat.

5. I conducted a batch mark-recapture study of frogs living along 50m of stream. I used the Lincoln-Peterson model to estimate the population size at 80 frogs. I later find out that the actual abundance during the sample period (120 frogs) was significantly higher than my estimate. Abundance estimates can be biased if the procedure violated one or more assumptions of the method. Which biological explanation would be consistent with these data (estimate < actual) ?

(a) Several marked frogs died between the first and second visits.

(b) Several new unmarked frogs immigrated into of the site between the first and second visit.

(c) Frogs marked in the first visit were more likely to be recaught in the second visit than others.

6. Batch mark-recapture and depletion are the primary methods to estimate population size that we considered. They share several assumptions. Which assumption is only true for one of the two?
7. All individuals have an equal probability of capture at the start of the study.
8. Each attempt at capturing animals used the same level of effort.
9. No immigration or deaths during the sample.

7. All things being equal, a life history with lots of reproduction early in life should always be favored. Yet many animals wait decades to reproduce. That means all things AREN’T equal. In which situation would selection favor later reproduction?
8. Environments in which juveniles experience intense resource competition.
9. Environments with plentiful resources, and selection favoring rapid growth.
10. Environments with little predation on juveniles, and selection favoring low parental care.

8. The Lotka-Volterra competition predicts that, when two competitors share a single resource, one will drive the other to extinction most of the time. Yet we see many many examples of competitors coexisting in nature. Suppose grasshopper B. tough is competitively dominant to another grasshopper B. wimpy. Its populations can grow when resources are too low for B. wimpy to maintain its population. Which additional detail would promote coexistence of these two species?
9. The resident bird predator preferentially feeds on B. tough.
10. B. tough is an intraguild predator—it eats the eggs of B. wimpy.
11. B. wimpy individuals are larger than B. tough individuals.

9- 10. The tables and figures below are all from Crowder et al. (1994).

9. As precisely as you can, explain what the two numbers in the box in Table 2 mean. What information from Table 1 accounts for those values?

10. Why do hatchlings have such low reproductive value (Table 3) even though they have all their reproduction ahead of them?

(a) They represent a large portion of the population.

(b) They are only in that stage for one year.

(c) They are likely to die before reaching reproductive age.

11. Source-sink models add a bit of complexity to classic metapopulation models. What complexity do they add?
12. Allow the size of patches to vary.
13. Allow the distance between patches to vary.
14. Allow the rate of population growth within a patch to vary.

12. A metapopulation of frogs lives in a network of patches of wetlands. The plot shows the change in the fraction of occupied patches over time. From your understanding of metapopulations, which event would be most likely to cause the shift in the pre-1990 vs. post-1990 data?
13. New species introduced in 1990 that provides an additional food source for the frogs.
14. Increased rainfall each year since 1990, increasing the size of each wetland.
15. New road built through the middle of the network in 1990, decreasing the connectivity of the patches.

13. Cleaner shrimp sometimes “cheat”, eating bits of host flesh rather than parasites. In which situation would selection favor “cheating” more often?
14. Alternative foods for cleaner shrimp are abundant.
15. Hosts that visit are agile predators.
16. Hosts have few parasites on them.

14. It is rare for a predator to drive its prey to extinction in mainland areas. Which of the following is most important for the coexistence of predator and prey in most places?

(a) Prey can survive in some habitats that predators cannot access.

(b) Most predators are specialists.

(c) Prey populations have very high birth rates, leading to a high r_max.

15. In which direction (A, B, or C) would you expect this predator-prey system to move next?
16. The plot below has isoclines for two competitors, Species 1 (solid line) and Species 2 (dashed line).

Suppose we manipulate the populations so that N₁ = 15 and N₂=20. Use arrows to indicate how you expect the population sizes to change over time. Based on this, what do you expect at equilibrium? (circle one)

STABLE COEXISTENCE                UNSTABLE COEXISTENCE          ONE SPECIES EXTINCT

17. Same interaction and plot as #14, what type of competition is stronger in this interaction?

INTERSPECIFIC                  INTRASPECIFIC                  NEITHER

Short answer (6 pt unless noted)

18. (4 pts) Hatcher et al. (2012) proposed that parasites are essential components of ecosystems. Ticks are common parasites in wild areas, affecting many species of conservation concern. Give one argument why Hatcher et al.(2012) would say eliminating ticks from Shenandoah National Park would be a bad idea.

19. Gilg et al. (2003) asserted that lemming populations were regulated by density-dependent predation from stoats, and averaged about 250 individuals at this site. (Don’t worry about the cycles here.)

1. On the plot, add per-capita birth (b) and death (d) rates that would be consistent with that result.
2. Schmidt et al. (2012) found that changes in climate, a density-independent factor, caused the lemming populations at this site to decline to about 50 individuals. Assuming the decline was due to a change in birth rates, add a dashed line for a new per-capita birth rate that would be consistent with the decline.

20. The Central Stoneroller population of Hazel Run has grown rapidly in the time our class has monitored it. The number caught in the Upper site by electrofishing each year is shown below.

Based on these data, and your knowledge of exponential and logistic growth models, how many Stonerollers do you think we’ll catch next year? (circle one)

75                    118                  138                  250

Explain the reasoning for your choice above. (Good justifications exist for each option.)

21. A reserve manager says they will focus on protecting just the highest quality habitat for native species, so they will no longer manage properties that can’t support themselves. You disagree. Based on your knowledge of source-sink models, what value might those properties (sink habitats) have for preserving native species?

22. Oxpecker birds roam over the bodies of water buffaloes, removing ticks and other parasites and eating them. Some research indicates that, in addition to pulling ticks, oxpeckers also use their bills to open new wounds in the buffaloes and drink blood from them.

(a) Design an experiment that would strongly test whether this is a mutualist interaction. Be sure to name the dependent variable(s) and control groups (if any).

(b) Draw a plot/ table of data that would support mutualism. Remember to label your axes/ columns.

23. (2 pt) What idea/ skill from this section of the course will you remember/ use five years from now?