USABO - USA Biology Olympiad Population Ecology and Dynamics Questions and Answers — Questions and Answers
Question 1: An age-structure diagram for a developing country shows a classic pyramid shape, with a very broad base that narrows progressively toward the top. Which of the following is the most accurate long-term prediction for this population, assuming current trends continue?
- The population will experience rapid growth due to a large proportion of individuals in pre-reproductive and reproductive age groups. (Correct answer)
- The population size will remain stable as the high birth rate is offset by a high death rate in older age groups.
- The population will begin to decline as the large younger generation moves into post-reproductive years.
- The population will experience a 'boom and bust' cycle due to density-independent factors.
Correct answer: The population will experience rapid growth due to a large proportion of individuals in pre-reproductive and reproductive age groups.
A pyramid-shaped age-structure diagram indicates that a large percentage of the population is young (pre-reproductive) and in their reproductive years. [23, 24] This large cohort of young individuals will mature and have children, leading to a significant increase in population size over time, characteristic of rapid population growth. [25]
Question 2: A species of marine invertebrate releases millions of eggs into the water, of which very few survive to maturity. However, those that do survive the initial larval stages have a high probability of living for a long time. Which type of survivorship curve would this species most likely exhibit?
- Type I
- Type II
- Type III (Correct answer)
- A combination of Type I and Type II
Correct answer: Type III
This life history strategy is characteristic of a Type III survivorship curve. [8, 9] This curve represents high mortality rates for the very young, followed by a much lower, constant death rate for the few individuals that survive to older age. [6, 14] This is common in species that produce a large number of offspring with little to no parental care. [8, 15]
Question 3: In the logistic growth model, represented by the equation dN/dt = rN((K-N)/K), what happens to the population growth rate (dN/dt) as the population size (N) approaches the carrying capacity (K)?
- The growth rate increases exponentially.
- The growth rate approaches zero. (Correct answer)
- The growth rate becomes negative.
- The growth rate remains constant at the maximum rate (r).
Correct answer: The growth rate approaches zero.
In the logistic growth equation, the term (K-N)/K represents environmental resistance. As the population size (N) gets closer to the carrying capacity (K), the numerator (K-N) approaches zero. [3, 4] This causes the entire expression for the growth rate (dN/dt) to approach zero, meaning the population size stabilizes. [26]
Question 4: Which of the following describes a key characteristic of a K-selected species compared to an r-selected species?
- Early age of first reproduction
- High intrinsic rate of increase (r)
- Extensive parental care and few offspring (Correct answer)
- Short lifespan and small body size
Correct answer: Extensive parental care and few offspring
K-selected species are adapted to stable environments and tend to exist near their carrying capacity (K). [7, 10] Their life history strategy involves investing heavily in a small number of offspring to maximize their survival chances. This includes traits like long gestation periods, extensive parental care, and a long lifespan. [1, 13] In contrast, r-selected species produce many offspring with little care. [2]
Question 5: A sudden, severe frost kills a large portion of a monarch butterfly population overwintering in Mexico. This event is an example of:
- A density-dependent limiting factor.
- A K-selection pressure.
- Intraspecific competition.
- A density-independent limiting factor. (Correct answer)
Correct answer: A density-independent limiting factor.
A density-independent limiting factor is one that affects a population regardless of its density. Natural disasters and severe weather events, like a frost, are classic examples. [5, 17] The frost kills butterflies irrespective of how many are clustered together. In contrast, density-dependent factors, such as disease or competition, have a greater effect as population density increases. [16, 21]
Question 6: A conservation biologist is studying several fragmented populations of a rare orchid. She observes that one large, thriving population (Population A) frequently provides seeds that colonize smaller, less stable habitat patches where the orchid populations would otherwise go extinct. In metapopulation theory, what are Population A and the smaller patches called, respectively?
- Founder population and satellite populations
- Core population and peripheral populations
- Keystone population and indicator populations
- Source population and sink populations (Correct answer)
Correct answer: Source population and sink populations
This scenario describes source-sink dynamics within a metapopulation. The productive Population A, which can export individuals and rescue other populations, is a 'source'. [27, 29] The smaller, less stable populations that rely on immigration to persist and would otherwise go extinct are known as 'sinks'. [22, 27] This dynamic is crucial for the long-term persistence of the species across the fragmented landscape. [19]
USABO - USA Biology Olympiad Population Ecology and Dynamics Questions and Answers
An age-structure diagram for a developing country shows a classic pyramid shape, with a very broad base that narrows progressively toward the top.
Which of the following is the most accurate long-term prediction for this population, assuming current trends continue?