Question
Refer to the figure, which shows the population of wild rabbits on an island as time progresses.
Which of the following best represents a time when the rabbit
population was experiencing exponential growth?
(A) A
(B) B
(C) C
(D) D
▶️Answer/Explanation
Ans:
(B) The most rapid population growth occurs during exponential
growth, which is at point B. Choice (A) is incorrect because it
represents growth during the lag phase. At point C, the population is
starting to decline in numbers, so choice (C) is incorrect. Point D
represents the carrying capacity, so choice (D) is also not the answer.
Question
Refer to the figure, which shows the population of wild rabbits on an island as time progresses.
Which of the following points best approximates the carrying capacity
of the island for rabbits?
(A) A
(B) B
(C) C
(D) D
▶️Answer/Explanation
Ans:
(D) Point D represents the carrying capacity because the population
size is stabilizing at point D. Point A represents the lag phase, point B
represents exponential growth, and point C represents a population that
is starting to decline, so choices (A), (B), and (C) are incorrect.
Question
The graph indicates that the population of snowshoe hares is most
likely:
(A) stabilized by density-dependent factors after 1960.
(B) regulated by density-independent factors between 1960 and
2010.
(C) growing at an exponential rate from 1930−1950, 1980−1990,
and 2000−2005 because there are few limiting factors.
(D) headed for extinction.
▶️Answer/Explanation
Ans: A
Population-density-dependent factors (density-dependent
factors) regulate population size by negative feedback-like mechanisms
resulting in a fairly stable population size over time. Larger populations
undergo a reduction in the number of individuals over generations, and
smaller populations undergo increases. The small deviations in population
size hover around a mean (average).
For example, density-dependent factors include resources such as
space, water, food, nutrients, accumulation of wastes, and opportunity
for disease to spread. As population size increases, these become limiting.
As population size decreases, resources become more available or less
limiting.
Density-independent factors like natural disasters and weather don’t
typically stabilize populations. They usually result in a severe and random
reduction in population size. They are typically too unpredictable to
function as a regulator of population size.
The steep growth typically seen at the beginning of a population curve is
exponential (1930–1950). Only the steepest portion of the curve is
exponential. As soon as growth begins to slow, the growth becomes
logistic. The increases from 1980–1990 and 2000–2005 are not exponential.
The ceiling of 45,000 individuals is strong evidence of limiting factors.
Question
Which of the following statements is not a reasonable assumption based on this data?
(A) A few years after 1990, there was a fairly large increase in the
population of lynx in the area (the predator of the snowshoe
hare).
(B) A decline in the population of lynx occurred in 1982.
(C) A sustained increase in the amount of primary productivity in
the environment occurred in 1930.
(D) The average life span of the snowshoe hares decreased in 1960.
▶️Answer/Explanation
Ans: D
A decrease in the average life span of the hares would cause a
sustained decrease in the population unless a compensatory reproductive
strategy occurred simultaneously. Choices A and B describe normal
predator–prey population cycles (an increased prey population feeds
more predators who then have more offspring, which results in more hungry
predators hunting prey, which decreases the prey population, and so on). A
sustained increase in primary productivity would increase the carrying
capacity of the environment. Finally, a rapid increase in the hare population
without an increase in primary productivity could decimate the food supply,
causing a rapid reduction in the population later.
See the box after answer 6 for a strategy for answering questions asked in
the negative and using the process of elimination.
Question
Vibrio fischeri is a species of bioluminescent marine bacteria. When grown in liquid culture, the bacteria produce light only when large numbers of them are present (a high population density). It was observed that whenever bacteria from a luminescing culture seeded a new culture, they would produce light at low population densities, but only for a short time and then they stopped producing light completely. They would produce light again only once the population in that culture grew to a high enough density.
Which of the following is a reasonable mechanism for the changes in bioluminescence with bacteria density?
(A) Low-density bacterial cultures contain an inhibitor of
luminescence whose concentration increases as their population
grows.
(B) Bacterial metabolism releases excited electrons that emit
photons in solution that are visible when large numbers of bacteria are present.
(C) The bacteria carried bioluminescent particles with them during
transfer into the new culture that were either diluted or
inactivated by exposure to a new, low-density culture.
(D) Light is produced only when bacteria make physical contact
with other bacteria.
▶️Answer/Explanation
Ans: C
The nonbiological definition of a quorum is “the minimum number
of people that must be present at a meeting to make its proceedings valid.”
In bacteria, quorum sensing is mechanism of monitoring local cell density.
Many bacterial species secrete small molecules that can be sensed by
other bacteria. By detecting the concentration of the chemical in their
environmeAAAnt, bacteria can monitor local cell density. Choices A, B, and D
are incorrect because the low-density bacteria culture that seeded the new
culture could emit light for a short period of time.