<problems.htm> Pooled References GENERAL INDEX [Navigate to
MAIN MENU ]
SPECIAL PROBLEMS IN
POPULATION DYNAMICS (Contacts) 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? |