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SPECIAL PROBLEMS IN POPULATION DYNAMICS

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Problem 1: The cottony-cushion scale, Icerya purchasi, kills citrus trees in the absence of its key predator the vedalia beetle, Rodolia cardinalis. The presence of Rodolia reduces the scale density to a very inconspicuous level. Cold weather is detrimental to Rodolia, causing it to slow down its searching and feeding activities on the scale.

A number of coastal citrus orchards in southern California experienced outbreaks of cottony-cushion scale during an unusually cold spell in May, which did not result in a tree kill, but interfered with fruit production. An entomologist recommended heating the orchards to control the scale outbreak. He told the growers that the physical factor "heat" was detrimental to the scale and reduced its density below the economic threshold.

a. What factors regulate the scale population?

b. What is the key factor responsible for the observed scale density?

c. What effect did heat have on the scale population density?

d. What are the limiting factors determining the density at which the scale will exist?

Problem 2: In the East African grasslands (5,000 ft elevation) there occurs a community of mammals consisting of zebras,antelopes and several predators (lions, leopards, etc.). There are also other animals such as pigs, rodents, etc. in the area. Zebras feed selectively on tall rough grass, which allows sunlight to reach tender grass species lying beneath, permitting their growth. Antelopes feed only on the tender grass species.

The antelope population densities are highest when the zebras are present, but decline markedly when zebras are removed by poaching.. The predators in the area favor the antelopes and live almost exclusively by feeding on them. The lion is the most voracious feeder, especially when antelopes are very abundant.

a. What regulates the antelope populations?

b. Name apparent common limiting factors for the antelope population, and list their hierarchy.

c. What is the apparent key factor responsible for the density of antelopes at any give time?

d. Assuming that there are five antelope species present, point out where the two types of competition exist.

Problem 3: Assume the same situation as in Problem 2, but where all predators have been poached from the area (no antelope predators).

a. What regulates the antelope populations?

b. What seems to be a common limiting factor for the antelope population?

c. What key factor is primarily responsible for the level of the antelope population densities?

d. Assuming that there are five antelope species present, point out where the two types of competition exist.

Problem 4: A lake in south California is 3 miles long and 2 miles wide. It has an average depth of 4 feet. The encephalitis virus vector mosquito Culex tarsalis breeds to enormous numbers in the lake. If one species of mosquito fish, Gambusia affinis affinis, is introduced into the lake, the mosquito population density drops by 90%.

a. What factors regulate the mosquito population with fish present?

b. What key factor affects mosquito population density?

c. What limits the mosquito population?

d. What limits the fish population?

e. What regulates the fish population?

f. What key factor affects the fish population density?

g. Where is competition (describe for all)?

Problem 5: Assume the same situation as in Problem 4, but where no mosquito predators of any kind exist.

a. What factors regulate the mosquito population?

b. What key factor affects mosquito population density?

c. What limits the mosquito population?

d. Where is competition?

Problem 6: Assume the same situation as in Problem 4, but where two subspecies of mosquito fish are introduced, Gambusia affinis affinis and Gambusia affinis holbrooki. The newly introduced Gambusia is not able to tolerate high water temperatures as well as the previous species. Culex tarsalis breeding is favored in warmer water.

a. What regulates the mosquito population?

b. What key factor affects mosquito population density?

c. What limits the mosquito population?

d. What limits the fish population?

e. What key factor affects fish population density?

f. Where is competition?

Problem 7: Assume the situation in Problem 4 with only one predatory fish species present. Emergent vegetation, in the form of sedges and other higher aquatic plants, protrudes above the surface of the water, which furnishes protection for Culex tarsalis from the ravages of Gambusia. The Culex population density then soars to a level of public health importance.

a. What regulates the mosquito population?

b. What limits the mosquito population?

c. What environmental factors determine the vitality and activity of the mosquitoes?

d. What is the role of the emergent vegetation in the system?

Problem 8: Assume the same situation as in Problem 7, except that an herbivorous fish, Tilapia zillii, is introduced in the lake. Tilapia browse out all the emergent vegetation, and the Culex tarsalis density drops to a very low level (under 0.25 larvae/400-ml dipper).

a. What regulates the mosquito population?

b. What limits the mosquito population?

c. What is the role of Tilapia in the system?

d. What regulates the Tilapia population?

e. What controlled the Culex outbreak?

Problem 9: An alfalfa field is attacked by a lepidopterous caterpillar that feeds only on alfalfa. Strip cropping (cutting alternate rows at different times to harvest) reduces the incidence of the caterpillars in the field and results in economic control. The caterpillar is attacked by several hymenopterous parasitoids and its eggs are devoured by several species of predators. The rationale behind the strip cropping is to preserve these natural enemies in the field so that they can be available for killing and lowering the caterpillar population density.

a. What factors can be suspected as involved in regulation of the caterpillar population?

b. What is the limiting factor for the caterpillar population?

c. Could a key factor exist?

d. What environmental factors influence indirectly the vitality or activity of the individuals comprising that population?

e. Where is competition?

f. Where is control?

Problem 10: The codling moth infests walnuts in California. In a hypothetical situation it infests 100% of the nuts, with usually only one larva surviving per nut. The first larva to enter the nut by chance drives out all subsequent larvae that try to get in, thereby exhibiting a kind of territoriality. There is no effective parasitoid, predator or pathogen.

a. Where is there a regulating factor?

b. Name some environmental limiting factors.

c. Is there a key factor that is primarily responsible for the level of the codling moth density?

d. Where is competition?

e. If only 50% of the nuts are infested, what is the regulating factor?

f. If 5 parasitic species are introduced, and the 3rd introduction results in a codling moth density drop to less than 1.5%

infested nuts, what is the key factor affecting the caterpillar density on walnuts?

g. What controlled the caterpillar problem?

Problem 11: The navel orangeworm, Amyelois transitella, enters walnuts that have been penetrated by the codling moth in July, and in walnuts whose husks have cracked in late August. Assuming that there are no effective parasitoids of orangeworm nor codling moth,

a. What is the regulating factor in July? In August?

b. What limits the naval orangeworm population in July? In August?

c. Where is competition?

d. If the introduction of a parsitoid against the codling moth lowers this pest's density to less than 1.5% infested nuts, what

is the key factor affecting the navel orangeworm density on walnuts?

e. What level of control would you expect of orangeworm in July? In August?

Problem 12: The Dutch elm disease kills American elm trees after the vector Scolytus multistriatus inoculates the incitant fungus.

Consequently, in eastern North America the native elms have been reduced to less than 5% of their former population density.

a. What regulates the American elm density now?

b. What are some environmental limiting factors?

c. Is there a key factor?

d. Where is competition?

e. Where is control?

f. What function has the pathogen in determining elm density?

g. What might cause an increase in the American elm population density?