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SPECIAL PROBLEMS 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 inciting 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? |