AP Biology: Energy & Population Dynamics (Interactive with Student Bio)

Question 1

In a temperate forest ecosystem, a prolonged drought reduces net primary productivity (NPP) by 40%. Which population would most likely show the first measurable decline?

a) Top predators (e.g., wolves)
b) Primary consumers (e.g., deer)
c) Secondary consumers (e.g., snakes)
d) Decomposers (e.g., fungi)

Question 2

Which graph best illustrates the population growth of a herbivorous insect when its host plant’s energy content (kcal/g) suddenly doubles and remains high?

a) Linear decrease to extinction
b) Exponential growth followed by a plateau at a new, higher carrying capacity
c) No change in population size
d) Cyclic oscillations with increasing amplitude

Question 3

A population of zooplankton in a lake is limited by phytoplankton abundance. If nutrient pollution causes a 3× increase in phytoplankton biomass, the zooplankton population will most likely:

a) Remain unchanged because predation keeps it in check
b) Increase until oxygen depletion or another factor limits it
c) Decrease due to toxin buildup from algae
d) Switch to consuming bacteria instead

Question 4

In a grassland ecosystem, the energy transfer efficiency between trophic levels is approximately 10%. If producers fix 20,000 kcal/m²/yr, how much energy is available to support secondary consumers after a 50% reduction in primary productivity?

a) 2,000 kcal/m²/yr
b) 200 kcal/m²/yr
c) 100 kcal/m²/yr
d) 20 kcal/m²/yr

Question 5

Which of the following populations would experience the largest proportional decline when the energy input to an ecosystem is reduced by 30%?

a) Producers (trees)
b) Primary consumers (rabbits)
c) Tertiary consumers (eagles)
d) Decomposers (bacteria)

Question 6

A scientist measures the population density of a small rodent species in two adjacent fields. Field A has high plant diversity and high NPP; Field B has low NPP due to poor soil. According to the energy availability hypothesis, Field A should support:

a) Lower rodent density because of more competition
b) Higher rodent density, up to the local carrying capacity
c) The same rodent density as Field B
d) Rodents with a much slower metabolic rate

Question 7

In an aquatic mesocosm experiment, light intensity is reduced by 75% for 8 weeks. Which population parameter of the primary producer (algae) would change most immediately?

a) Birth rate
b) Death rate
c) Age structure
d) Sex ratio

Question 8

A population of deer on an island has been stable for years. After a hurricane destroys 60% of the vegetation (reducing available energy), the deer population will most likely:

a) Exhibit exponential growth
b) Show a Type II survivorship curve
c) Decline until it reaches a new, lower carrying capacity
d) Switch to carnivory

Question 9

Which statement best explains why energy availability is a density‑independent factor affecting population size?

a) Its effect per individual does not depend on population density
b) It only affects populations above carrying capacity
c) It is caused by competition among individuals
d) It alters birth rates but not death rates

Question 10

A researcher adds a limiting amino acid to the diet of an herbivorous insect population reared on low‑protein leaves. The insect population size increases by 300%. This demonstrates that:

a) Energy availability includes both caloric content and essential nutrients
b) Caloric content alone always limits population size
c) Secondary consumers are more limited by energy than primary consumers
d) Population size is never limited by energy

Question 11

A river receives a large input of organic waste from a food processing plant. Dissolved oxygen drops sharply, killing many fish. This disruption is best described as:

a) A trophic cascade initiated by increased energy availability to decomposers
b) A biomagnification event
c) A density‑dependent population cycle
d) Primary succession

Question 12

In Yellowstone National Park, the reintroduction of wolves (top predator) reduced elk populations, allowing willow and aspen to recover. This is an example of:

a) A disruption caused by decreasing energy availability to elk
b) A disruption caused by increasing energy availability to elk
c) A disruption caused by removing a keystone herbivore
d) A disruption caused by eutrophication

Question 13

A volcanic eruption blocks sunlight for two years, sharply reducing global primary productivity. Which long‑term disruption is most likely?

a) Extinction of all primary consumers
b) A permanent shift to a detritus‑based food web
c) Collapse of higher trophic levels, followed by slow recovery
d) Immediate replacement of all species by chemosynthetic organisms

Question 14

Which of the following is an example of a disruption due to an increase in energy availability in a marine ecosystem?

a) Ocean acidification from excess CO₂
b) Coral bleaching from high temperature
c) A harmful algal bloom (red tide) caused by nutrient runoff
d) Overfishing of tuna

Question 15

In a lake, zebra mussels (invasive filter feeders) remove massive amounts of phytoplankton, reducing energy available to zooplankton. The most likely disruption is:

a) Increase in native fish populations
b) A shift to a clear‑water state with fewer nutrients in the water column
c) Permanent anoxia in all depths
d) Rapid evolution of larger phytoplankton

Question 16

A farmer applies excess fertilizer to a field. Heavy rain washes the fertilizer into a nearby pond. Two weeks later, the pond experiences a fish kill. The sequence of disruptions is:

a) ↑ nutrients → ↑ algal energy → ↑ bacteria → ↓ O₂ → fish death
b) ↑ nutrients → ↓ algal energy → ↓ bacteria → ↑ O₂ → fish death
c) ↓ nutrients → ↓ algal energy → ↓ bacteria → ↓ O₂ → fish death
d) ↑ nutrients → ↑ herbivorous fish → ↓ algae → ↓ O₂

Question 17

Which disruption is most likely to occur when energy availability to a top predator is drastically reduced due to habitat fragmentation?

a) Mesopredator release – an increase in mid‑level predator populations
b) Primary producer extinction
c) Immediate increase in biodiversity
d) Permanent shift to anaerobic respiration throughout the food web

Question 18

A deep‑sea hydrothermal vent community relies on chemosynthetic bacteria. If venting stops abruptly, eliminating the energy source (H₂S), the most immediate disruption would be:

a) Migration of all organisms to nearby vents
b) Collapse of the entire vent food web starting with the primary producers
c) Conversion to a photosynthetic community
d) No disruption, because energy is not limiting in the deep sea

Question 19

Which of the following scenarios represents a disruption caused by a decrease in energy availability over a long period (e.g., climate‑driven desertification)?

a) Increased primary productivity in a wetland
b) Replacement of a forest by grassland, then shrubland, with loss of forest specialist species
c) A one‑time fish kill from an oil spill
d) Short‑term algal bloom followed by recovery

Question 20

In a grassland ecosystem, fire suppression reduces the availability of high‑energy young plant regrowth for grazers. The resulting disruption includes:

a) Increase in grazer population due to more total biomass
b) Shift from grazing herbivores to browsing herbivores, altering plant community composition
c) Immediate extinction of all primary producers
d) Permanent increase in soil organic matter

📘 Essay 1 — Energy availability & population size

Prompt: In a temperate lake ecosystem, the base of the food web consists of phytoplankton. Agricultural runoff (nitrates/phosphates) dramatically increases nutrient availability, causing an algal bloom.

A. Explain how the sudden increase in energy availability (phytoplankton biomass) initially affects the population size of zooplankton. Use the concept of carrying capacity (K) in your explanation.
B. Describe how the energy transfer efficiency (~10%) would influence the magnitude of population change in secondary consumers (small fish) compared to zooplankton.
C. After several weeks, the algal bloom collapses, dead phytoplankton sink, bacterial decomposition depletes oxygen. Predict the long‑term effect on zooplankton population size and justify using energy availability.
D. Propose one density‑dependent and one density‑independent factor that could further affect zooplankton after the initial energy increase.

🌊 Essay 2 — Energy disruptions & trophic cascades

Prompt: A coastal kelp forest ecosystem: sea urchins graze kelp, sea otters (top predator) prey on urchins. Overhunting drastically reduces sea otter population.

A. Describe how the removal of sea otters alters energy availability for sea urchins. Then explain the expected change in sea urchin population size and subsequent effect on kelp biomass.
B. This scenario is a trophic cascade. Define trophic cascade and explain how a change in energy availability at one level disrupts the entire food web here.
C. If kelp collapses into a barren area with low primary productivity, predict how this disruption affects energy availability for higher trophic levels (e.g., fish that depend on kelp).
D. Compare the disruption caused by a decrease in energy availability (otter removal) to a disruption caused by an increase in energy availability (nutrient pollution). Provide one similarity and one difference.

⚡ Energy & Population Dynamics