Spring 2001
Review Problem ANSWERS--Unit 2
1. Optimal foraging predator
a. Yes. If a more profitable species should be excluded from the diet then a less profitable species should also be excluded.b. Yes. The most profitable species should always be taken when encountered.
c. Yes. If the search time increased enough, a species with relatively low profitability might be included in the optimal diet.
d. If P2 was included in the diet previously, it should still be in the diet. If it wasn't included in the diet previously, it might be that it should now be included. If it is included in the diet, it should always be eaten when encountered.
2. Optimally foraging hummingbirds
In experiment 1, the hummingbirds should feed ONLY at feeder type C. Here's how to come to that conclusion:a. Feeder C has the highest profitability (0.005 g/sec)
b. Rate of energy intake using only feeder C: E/(h+s) = 0.3/(60+15) = 0.004
c. Next best feeder is B; profitability = 0.0033
d.The rate of energy intake using feeder C only is greater than the profitability of the next best feeder B, so do not include B (or A) in the diet. Note that once you've declined to include a next best food item in the diet, there's no need to check any others since they will be lower profitability than the item just rejected.Notice that the hummingbirds are being picky when search times are small relative to handling times.
In experiment 2, the optimal diet includes all three feeders. Here's how to come to that conclusion:
a. Start with feeder C because it's the most profitable.
b. Rate of energy intake using only feeder C: 0.3/(5+90) = 0.0032 g/min
c. Profitability of feeder B: 0.2/5 sec = 0.04 g/sec
d. Profitability of feeder B is higher than the average rate of energy intake using only feeder C so add feeder B to diet.
e. Continuing in the same wayNew rate of energy intake using both feeders B and C: 0.25/(5+45) = 0.005 g/sec
Profitability of feeder A: 0.1/5 = 0.02
Add feeder A to diet because profitability of A is higher than the average rate of energy intake using feeders B and C only.Notice that the birds are generalists when search time is long compared to handling time.
Competition (menu)
3. Exploitative competition is an interaction between species based on shared resource depletion and limitation. One species is affected by the other only through the shared resource and it is therefore an indirect interaction. Interference competition is a direct negative interaction between species because the species physically (or chemically) interact with each other.
4. A negative correlation between the abundances of two species could be caused by differences in habitat requirements, by the two species being alternative hosts of a single herbivore, or by competition. There could be other explanations as well.
5. Complete competitors cannot partition resources because their resources are identical. One of the species is likely to be better at depleting the resources or at surviving at low resource levels and be a better competitor. If one species is a better competitor for any reason, it will drive the other to extinction. Even if the two species were identical in their resource use, one would exclude the others through random processes (as you saw in the rabbit competition simulation).
Lotka-Voterra model (menu)
6. The endpoints of the ZNGI are K1/a12 on the N2 axis and K1 on the N1 axis. The derivations are done by recognizing that the (K1- N1-a12N2) term can be set equal to zero at equilibrium. First set N1 = 0 and solve for N2, and then set N2 = 0 and solve for N1.
7. For the Lotka-Volterra equations to predict a stable equilibrium (at equal carrying capacities), intraspecific competition must be stronger than interspecific competition or both a12 and a21 must be less than 1 (each species limits its own growth more than the other species limits it). This comes about when K1/a12 > K2 and K2/a21 > K1 and K1 = K2.
8. Salamander competition
a) Either competitive exclusion or an unstable equilibrium would explain the data in graphs 1 and 2. In habitat A, red-backs exclude blue-spots. In habitat B, blue-spots exclude red-backs. This could be due either to habitat differences favoring each species (competitive exclusion) or to the initial advantage of abundance (unstable equilibrium).b) The results presented in graphs 3 an 4 show that the habitat difference hypothesis cannot be correct. Now the only difference consistent with all four graphs is that the species with the initial abundance advantage will win the competition. This is the main characteristic of the unstable equilibrium outcome.
9. Niche partitioning could explain the coexistence of several insectivorous bird species if each species foraged for insects in a different place (part of a tree, ground vs. tree, deciduous vs coniferous trees, etc.), a different time (night vs. day), or foraged for different insect types.
10. Character displacement is the process by which competition causes species to become more different (in some feature related to their resource use) when in sympatry than when in allopatry. It is difficult to demonstrate this process because it occurs over evolutionary time. All we usually have is the pattern of bigger differences in sympatry than in allopatry, and this pattern could arise through convergence in allopatry, environmental gradients, or other mechanisms.
11. Resource partitioning is typically assumed to be a likely mechanism of coexistence for animals because animals have many ways to partition resources (big seeds vs little seeds, day foraging vs night foraging, foraging in trees vs grass, etc.). For plants, it is difficult to partition the light, water, and mineral nutrients that are usually limiting resources.
12. Field studies are less likely to show significant interspecific competition (or any specific process for that matter) because many more species, interactions, resource levels, and conditions are affecting any one species in the field and all those differences increase the variation among replicates. The increased variation makes it more difficult to detect differences among the treatments as significant.
13. Additive design experiment
a. The ECOLOGICAL interpretation of the interaction is: Neighbor A has a stronger competitive effect on the target than does neighbor B.
b. The intensity of the competitive effect of neighbor A on the target is shown by the slope: an increase of 1 gram of neighbor A results in a loss of 0.8 grams of target biomass.
c. If neighbor A is the same species as the target, then the slope of its regression line represents the intensity of INTRAspecific competition and the slope of the regression line of neighbor B represents the intensity of INTERspecific competition. The ratio of those slopes is aAB = .4/.8 = 0.50. Adding a gram of biomass of neighbor B is only half as detrimental to the target as adding a gram of biomass of its own species, A.
d. To measure aBA, you would have to replace the target of species A with targets of species B.
Tilman's Resource Ratio Model (menu)
**NOT REQUIRED SPRING 2003**
14. Resource ratio diagram
a. The axes represent two different resources.
c. Competition might lead to exclusion or coexistence, depending on the position of theresource supply point.
d. If the resource supply is at *, then species B will exclude species A. (Coexistence occurs only if resources are between the two dashed lines.)
15. To predict an outcome of competition before an experiment is run,
a. according to the Lotka-Volterra model, one would have to know the effect of each species on the other (i.e.,a ). This is essentially impossible to know before the experiment.b. according to Tilman's model, one would have to know the R*s for each resource for each species. This can be measured experimentally in single species experiments.
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. Predator-victim ZNGIs
a. The ZNGIV goes up at the left because the prey have a refuge: no matter how high the predator population, the prey maintain their population at that low level because all individuals can escape from predation.b. The ZNGIV goes down at the right because even at zero predator population, the prey population would maintain its size because of resource limitation.
c. When the ZNGIP goes through point A, the populations will spiral out from near point A until the victim population is small enough to all be in protected in the refuge. At that point, the predator population will decrease (no food) and the victim population will increase until it again increases past the ZNGIP. Then the outward spiral will begin again. When the ZNGIP goes through point B, the populations will cycle continuously at the same amplitude (barring environmental changes). When the ZNGIP goes through point C, the populations will spiral in toward point C, eventually coming to a stable equilibrium with constant density of both predator and victim.
d. Average predator density would be higher if the ZNGIP goes through point B than if it goes through point C. (Average predator density is shown by the height of the equilibrium point in each case.)
The important components are that the line for B oscillates stably whereas the line for C should show damped oscillations and should have a higher mean than the line for B.
17. Laboratory tests of predator-prey systems are typically very simple. The predator has no trouble in finding all the prey, or at least all the prey out of a refuge, and then the predator starves. Adding enough environmental heterogeneity to allow the prey to remain alive long enough to reproduce keeps both prey and predator in the system longer. The field is enormously heterogeneous and that may be the key to keeping both predator and prey in the system at low numbers.
18. Predator-victim ZNGI
a. The predator ZNGI bends to the right because there is a carrying capacity in the environment, unrelated to victim population, that limits the predator population. Therefore, the predator population can only reach some maximum regardless of how big the victim population gets.b. The populations will smoothly approach the stable equilibrium point rather than cycling in.
Herbivory (menu)
19. Compensation means that a plant can make up for any loss of fitness incurred due to herbivores. The example that we used in class was increased production of 2o and 3o umbels when webworms attacked the 1o umbels of wild parsnip.
20. Structural defenses include thorns, hairs, egg mimics; animal defenses are typically ant defenders; and chemical defenses include many classes of secondary chemicals such as alkaloids, terpenoids, glycosides, as well as drugs and flavors we use in medicine and cooking.
21. An induced defense is one that arises only after herbivory has occurred. Both structural and chemical defenses have been shown to be inducible.
22. A secondary chemical is one for which we know of no biochemical use in the plant. These chemicals could be waste products or they could have been produced for the purpose of defense.
23. The evidence that secondary chemicals are defensive includes inducibility, deposition in the plant's most valuable tissues (reproductive tissues), and patterns that suggest that there is a significant cost to the production of these chemicals.
24. Specialist herbivores often are attracted to the secondary chemicals of their host and use them to home in on the plants whereas generalists are usually deterred by secondary chemicals or suffer lowered fitness from consuming them.
25. The evolution of insect herbivores and their hosts has been called a "coevolutionary arms race" because each new defense that the plants evolve puts selection pressure on the insects to develop some counter adaptation that allows the insect to feed, which puts selection pressure on the plants to develop some novel defense, which puts selection pressure on the insects . . .
Mutualisms (menu)
26. Dependence on another species has typically evolved because specialization has increased the efficiency with which the interaction is accomplished. For example, if an orchid species evolves flowers that look like female wasps, male wasps transfer pollen very faithfully from one flower to another. But these flowers will attract only one wasp species, i.e., the plant is dependent on the wasp species for pollination. This dependence carries a risk in that disappearance of the wasp pollinator would mean disappearance of the orchid.
27. Three mutualisms on which the world's communities depend are mycorrhizae (often intracellular--you could also use mitochondria), nitrogen fixation by plant-associated bacteria (extracellular), and pollination (behavioral). Both mycorrhizae and nitrogen fixation are absolutely necessary for plant growth, and pollination is necessary for most fruit development. Since all animals need plants, or animals that eat plants, for food, either directly or indirectly, the world's communities could not exist without these mutualisms.
28. Hummingbird-pollinated shrubs
a. The ecologist might conclude that there is no competition for pollinators, but a safer conclusion is that any competition for pollinators that exists has not resulted in divergence of bloom times (i.e., uniform spacing of bloom time).b. Finding uniformity early in the season, especially combined with the observation that hummingbird pollinators were fairly scarce, suggests that competition might have caused bloom times to diverge to be more uniform than random. It does not demonstrate that competition is the cause of the pattern; she can say only that these data are consistent with that explanation. After 35 days, when hummingbirds became more abundant, there is no suggestion that there is competition for pollinators or that any competition has caused divergence in bloom times.
29. Scarlet gilia pollination problem
a. In early May, it looks like pollinators limit seed production. Hand-pollinated flowers produce more seeds then open-pollinated flowers whether the plants are fertilized or not (P=0.002).b. In mid-June, it looks like soil nutrients limit seed production. Fertilized plants produced more seeds per flower than unfertilized plants whether the plants were hand-pollinated or not (P=0.001).
c. Hand-pollination affects seed production similarly for fertilized and unfertilized plants. OR Fertilization affects seed production similarly for hand-pollinated and open-pollinated plants.
d. Season*pollination: The effect of hand-pollination depends on the season in which it is conducted. Season*fertilizer: The effect of fertilization depends on the season in which it is conducted.
e. In early May, it looks like scarlet gilia plants have plenty of mineral nutrients from the soil but lack enough pollinators (hummingbirds) to achieve maximum seed production. However, later in the season, pollinators are sufficient to fully pollinate flowers but plants lack sufficient mineral nutrients to mature the maximum number of seeds.
Indirect interactions (menu)
30. Indirect interactions diagrams:
31. Indirect interactions
a. PathwaysB-C-E-D-F is (-)(+)(-)(+) = positiveB-C-F is (-)(+) = negative
B-C-D-F is (-)(-)(+) = positive
B-A-F is (+)(-) = negative
b. The combined effect of B on F is likely to be negative because the two negative pathways (2 steps each) are shorter and likely to be stronger than the two positive pathways (4 and 3 steps).
32. The pattern could be explained as follows:
a. Competition - the food plant is in short supply relative to the demand for it from the two herbivore species. Therefore, as the population of herbivore A goes down, there is more food for the herbivore B population, so its growth rates increase and its population goes up.b. Predation - as the population of herbivore A goes down, there are fewer individuals of herbivore A to feed the predator. Although this might cause the predator population to decrease (allowing an increase in herbivore B), it might also cause the predator to shift its diet (optimal foraging!) to including more herbivore B than it did previously. This would cause the population of herbivore B to decrease. This phenomenon has been called "apparent competition" in the ecological literature.