Ecology — Hard Practice Quiz

A Biology cheat sheet for Ecology — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.

Practice quiz

1. An ecosystem's primary producers collectively capture $10,000 \text{ kcal}$ of energy. Assuming a typical trophic efficiency, how much energy would be available to a tertiary consumer in this food chain, and what is the primary reason for the significant energy loss at each transfer?

   • A) $10 \text{ kcal}$; Energy is lost as heat during metabolic processes and incomplete digestion.
   • B) $100 \text{ kcal}$; Energy is consumed by decomposers before reaching higher trophic levels.
   • C) $1,000 \text{ kcal}$; Most energy is stored in biomass and not transferred.
   • D) $1 \text{ kcal}$; Energy is primarily used for reproduction at lower trophic levels.

   Answer: A) $10 \text{ kcal}$; Energy is lost as heat during metabolic processes and incomplete digestion.

2. A population of deer is introduced to a new, resource-rich island. Initially, its growth follows a J-curve, but as the population increases, it transitions to an S-curve. At what point in the S-curve is the population's growth rate highest, and which factors are primarily responsible for the eventual stabilization of the population around the island's carrying capacity?

   • A) When the population size is close to the carrying capacity ($K$); Density-independent factors like severe weather.
   • B) When the population size is half of the carrying capacity ($K/2$); Density-dependent factors such as competition for food and disease.
   • C) When the population size is very small; Density-independent factors like natural disasters.
   • D) When the population size slightly exceeds the carrying capacity ($K$); Density-dependent factors like increased predation.

   Answer: B) When the population size is half of the carrying capacity ($K/2$); Density-dependent factors such as competition for food and disease.

3. Two species of closely related birds, Species A and Species B, feed on the same type of insect in the same forest. However, Species A primarily forages in the upper canopy, while Species B forages in the understory. If a new invasive predator significantly reduces the population of Species B, allowing Species A to expand its foraging range to include the understory, which ecological principles are best illustrated by the initial coexistence and the subsequent expansion of Species A?

   • A) Initial coexistence by competitive exclusion; Expansion due to mutualism.
   • B) Initial coexistence by resource partitioning; Expansion due to a shift from realized niche to fundamental niche.
   • C) Initial coexistence by commensalism; Expansion due to increased carrying capacity.
   • D) Initial coexistence by predation; Expansion due to reduced density-independent factors.

   Answer: B) Initial coexistence by resource partitioning; Expansion due to a shift from realized niche to fundamental niche.

4. Following a massive volcanic eruption that completely sterilizes a large land area, creating new rock formations, what type of ecological succession would occur, and what role do the initial colonizing organisms play in preparing the environment for later successional stages?

   • A) Secondary succession; Pioneer species like grasses quickly establish, adding organic matter to existing soil.
   • B) Primary succession; Pioneer species such as lichens and mosses break down rock and contribute to soil formation.
   • C) Primary succession; K-selected species immediately dominate, stabilizing the ecosystem.
   • D) Secondary succession; Decomposers rapidly break down volcanic ash, releasing nutrients for plant growth.

   Answer: B) Primary succession; Pioneer species such as lichens and mosses break down rock and contribute to soil formation.

5. Deforestation is a major contributor to increased atmospheric $CO_2$ levels, exacerbating climate change. Describe two distinct mechanisms by which large-scale deforestation impacts the carbon cycle, linking these to fundamental biological processes.

   • A) Reduced nitrogen fixation by trees and increased methane release from exposed soil.
   • B) Decreased photosynthesis, leading to less $CO_2$ uptake, and increased $CO_2$ release from the decomposition or burning of cleared biomass.
   • C) Enhanced transpiration, which cools the atmosphere, and increased carbon sequestration in new agricultural crops.
   • D) Accelerated water cycle, leading to more precipitation, and reduced respiration rates in remaining vegetation.

   Answer: B) Decreased photosynthesis, leading to less $CO_2$ uptake, and increased $CO_2$ release from the decomposition or burning of cleared biomass.

6. Nitrogen is often a limiting nutrient in terrestrial ecosystems. Explain how two distinct bacterial processes contribute to making atmospheric nitrogen available to plants and then returning it to the atmosphere, and how human activities can significantly alter the natural balance of these processes.

   • A) Nitrification converts $N_2$ to $NH_4^+$, and ammonification converts $NO_3^-$ to $N_2$; Human use of pesticides inhibits these bacteria.
   • B) Nitrogen fixation converts $N_2$ to $NH_3/NH_4^+$, and denitrification converts $NO_3^-$ to $N_2$; Human activities like fertilizer use and fossil fuel combustion add excess reactive nitrogen to ecosystems.
   • C) Assimilation converts $N_2$ to organic compounds, and decomposition converts organic compounds to $N_2$; Human deforestation reduces plant assimilation.
   • D) Weathering releases $N_2$ from rocks, and runoff carries it to oceans; Human mining of nitrogen-rich rocks accelerates this.

   Answer: B) Nitrogen fixation converts $N_2$ to $NH_3/NH_4^+$, and denitrification converts $NO_3^-$ to $N_2$; Human activities like fertilizer use and fossil fuel combustion add excess reactive nitrogen to ecosystems.

7. A persistent, fat-soluble toxin is introduced into an aquatic ecosystem. If primary producers in this ecosystem accumulate the toxin at a concentration of $0.005 \text{ ppm}$, what would be the approximate concentration of this toxin in a tertiary consumer, and why are top predators particularly susceptible to the harmful effects of such toxins?

   • A) $0.05 \text{ ppm}$; Top predators have faster metabolic rates, leading to quicker toxin breakdown.
   • B) $0.5 \text{ ppm}$; Top predators consume a wider variety of food sources, diluting the toxin.
   • C) $5 \text{ ppm}$; Top predators accumulate toxins through biomagnification, as they consume large quantities of contaminated prey over their lifespan.
   • D) $0.0005 \text{ ppm}$; Top predators excrete toxins more efficiently due to their advanced physiology.

   Answer: C) $5 \text{ ppm}$; Top predators accumulate toxins through biomagnification, as they consume large quantities of contaminated prey over their lifespan.

8. Compare the Net Primary Productivity (NPP) of a tropical rainforest to that of a desert biome. Explain the primary climatic factors responsible for this difference and how they directly influence the rate of photosynthesis in these two biomes.

   • A) Desert NPP is higher due to extreme temperatures; high temperatures increase enzyme activity for photosynthesis.
   • B) Tropical rainforest NPP is higher due to abundant sunlight and high precipitation; these conditions optimize water availability and light for photosynthesis.
   • C) Desert NPP is higher due to low competition for resources; less competition allows individual plants to photosynthesize more efficiently.
   • D) Tropical rainforest NPP is lower due to constant cloud cover reducing light; high humidity inhibits $CO_2$ uptake.

   Answer: B) Tropical rainforest NPP is higher due to abundant sunlight and high precipitation; these conditions optimize water availability and light for photosynthesis.

9. The human population has experienced significant growth, leading to discussions about Earth's carrying capacity for humans. How does the concept of carrying capacity apply to the human population, and what role do sustainable resource use and the ecological footprint play in determining our long-term ability to thrive?

   • A) Human carrying capacity is fixed and determined solely by food production; sustainable resource use is irrelevant as technology will always overcome limits.
   • B) Human carrying capacity is dynamic, influenced by technology and resource consumption patterns; sustainable resource use aims to reduce the ecological footprint, allowing more people to thrive within Earth's regenerative capacity.
   • C) Human carrying capacity is irrelevant due to continuous exponential growth; ecological footprint measures only carbon emissions, not overall resource use.
   • D) Human carrying capacity is determined by the number of individuals, not their resource use; sustainable resource use is a concept only applicable to non-human populations.

   Answer: B) Human carrying capacity is dynamic, influenced by technology and resource consumption patterns; sustainable resource use aims to reduce the ecological footprint, allowing more people to thrive within Earth's regenerative capacity.

10. Biodiversity hotspots are crucial for global conservation. Explain why these areas are prioritized for conservation efforts, and how habitat fragmentation, a prevalent form of habitat destruction, specifically threatens the long-term viability of species, including potential keystone species, within these hotspots.

    • A) Hotspots have low species richness but high relative abundance; fragmentation increases gene flow between isolated populations.
    • B) Hotspots possess high species richness and endemism but face significant threat; fragmentation reduces habitat size, increases edge effects, and isolates populations, making them more vulnerable to extinction and disrupting keystone species' roles.
    • C) Hotspots are primarily marine environments with stable climates; fragmentation only affects terrestrial species, not aquatic ones.
    • D) Hotspots are areas where human population density is low; fragmentation leads to increased resource availability for remaining species.

    Answer: B) Hotspots possess high species richness and endemism but face significant threat; fragmentation reduces habitat size, increases edge effects, and isolates populations, making them more vulnerable to extinction and disrupting keystone species' roles.