PCB3044 Principles of Ecology Exam 3 2026 – UCF
1. Which process describes the colonisation of a habitat that has not previously
supported a biological community, such as a receding glacier or a new volcanic
island?
A. Primary succession
B. Secondary succession
C. Climax community formation
D. Facilitation succession
Answer: A
Rationale: Primary succession occurs on newly exposed substrates that have no soil or
previous life.
2. According to the MacArthur-Wilson Equilibrium Theory of Island
Biogeography, which type of island is expected to have the highest species
richness?
A. Large islands near the mainland
B. Small islands near the mainland
C. Large islands far from the mainland
D. Small islands far from the mainland
Answer: A
Rationale: Large islands have lower extinction rates and those near the mainland have
higher immigration rates, leading to higher richness.
,3. What is the term for the total amount of carbon fixed by autotrophs in an
ecosystem?
A. Net Primary Production
B. Secondary Production
C. Net Ecosystem Exchange
D. Gross Primary Production
Answer: D
Rationale: Gross Primary Production (GPP) is the total rate of photosynthesis or carbon
fixation by autotrophs.
4. In the nitrogen cycle, which process converts atmospheric nitrogen (N2) into a
biologically usable form like ammonia (NH3)?
A. Nitrogen fixation
B. Denitrification
C. Nitrification
D. Ammonification
Answer: A
Rationale: Nitrogen fixation is the conversion of N2 into NH3, performed by specific
bacteria and cyanobacteria.
5. Which ecological index accounts for both the number of species and their
relative abundance (evenness)?
A. Species richness
B. Shannon-Wiener Index
C. Z-value
D. Redfield Ratio
Answer: B
Rationale: The Shannon-Wiener Index is a common measure of diversity that incorporates
richness and evenness.
, 6. The ‘Intermediate Disturbance Hypothesis’ suggests that species diversity is
highest when:
A. Disturbances are very frequent and intense
B. Disturbances are rare and low intensity
C. There are no disturbances at all
D. Disturbances occur at moderate levels
Answer: D
Rationale: Intermediate levels of disturbance prevent competitive exclusion while
allowing species other than pioneers to persist.
7. Which mechanism of succession suggests that early colonizers make the
environment less suitable for themselves but more suitable for later species?
A. Tolerance model
B. Inhibition model
C. Facilitation model
D. Random model
Answer: C
Rationale: In facilitation, early species modify the environment in ways that benefit later
successional species.
8. Which of the following describes ‘top-down’ control in an ecosystem?
A. Predators limit the abundance of herbivores, which in turn affects plants
B. Nutrient availability limits the biomass of the entire food web
C. Primary producers limit the growth of herbivores
D. The climate determines the length of the growing season
Answer: A
Rationale: Top-down control, or a trophic cascade, occurs when predators at high trophic
levels regulate lower levels.
1. Which process describes the colonisation of a habitat that has not previously
supported a biological community, such as a receding glacier or a new volcanic
island?
A. Primary succession
B. Secondary succession
C. Climax community formation
D. Facilitation succession
Answer: A
Rationale: Primary succession occurs on newly exposed substrates that have no soil or
previous life.
2. According to the MacArthur-Wilson Equilibrium Theory of Island
Biogeography, which type of island is expected to have the highest species
richness?
A. Large islands near the mainland
B. Small islands near the mainland
C. Large islands far from the mainland
D. Small islands far from the mainland
Answer: A
Rationale: Large islands have lower extinction rates and those near the mainland have
higher immigration rates, leading to higher richness.
,3. What is the term for the total amount of carbon fixed by autotrophs in an
ecosystem?
A. Net Primary Production
B. Secondary Production
C. Net Ecosystem Exchange
D. Gross Primary Production
Answer: D
Rationale: Gross Primary Production (GPP) is the total rate of photosynthesis or carbon
fixation by autotrophs.
4. In the nitrogen cycle, which process converts atmospheric nitrogen (N2) into a
biologically usable form like ammonia (NH3)?
A. Nitrogen fixation
B. Denitrification
C. Nitrification
D. Ammonification
Answer: A
Rationale: Nitrogen fixation is the conversion of N2 into NH3, performed by specific
bacteria and cyanobacteria.
5. Which ecological index accounts for both the number of species and their
relative abundance (evenness)?
A. Species richness
B. Shannon-Wiener Index
C. Z-value
D. Redfield Ratio
Answer: B
Rationale: The Shannon-Wiener Index is a common measure of diversity that incorporates
richness and evenness.
, 6. The ‘Intermediate Disturbance Hypothesis’ suggests that species diversity is
highest when:
A. Disturbances are very frequent and intense
B. Disturbances are rare and low intensity
C. There are no disturbances at all
D. Disturbances occur at moderate levels
Answer: D
Rationale: Intermediate levels of disturbance prevent competitive exclusion while
allowing species other than pioneers to persist.
7. Which mechanism of succession suggests that early colonizers make the
environment less suitable for themselves but more suitable for later species?
A. Tolerance model
B. Inhibition model
C. Facilitation model
D. Random model
Answer: C
Rationale: In facilitation, early species modify the environment in ways that benefit later
successional species.
8. Which of the following describes ‘top-down’ control in an ecosystem?
A. Predators limit the abundance of herbivores, which in turn affects plants
B. Nutrient availability limits the biomass of the entire food web
C. Primary producers limit the growth of herbivores
D. The climate determines the length of the growing season
Answer: A
Rationale: Top-down control, or a trophic cascade, occurs when predators at high trophic
levels regulate lower levels.
