Rucete ✏ AP Biology In a Nutshell
20. The Basics of Ecology — Practice Questions
This chapter explores how organisms respond to environmental changes and how energy flows through ecosystems via trophic levels and food webs.
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(Multiple Choice — Click to Reveal Answer)
1. What type of organism is responsible for converting inorganic molecules into usable organic energy forms in extreme environments?
(A) Chemoautotroph
(B) Photoautotroph
(C) Detritivore
(D) Heterotroph
Answer
(A) — Chemoautotrophs convert inorganic molecules into usable energy, often in deep-sea or geothermal environments.
2. What is the main reason smaller organisms have higher metabolic rates than larger organisms?
(A) They reproduce more frequently
(B) They move faster
(C) They have a higher surface area-to-volume ratio
(D) They have more mitochondria
Answer
(C) — Smaller organisms lose more heat to the environment and thus require faster metabolic rates to maintain internal processes.
3. What is the role of decomposers in an ecosystem?
(A) Consuming primary producers
(B) Producing energy from sunlight
(C) Recycling nutrients from dead organisms
(D) Creating food chains
Answer
(C) — Decomposers break down organic material, allowing nutrients to re-enter the ecosystem.
4. Which of the following is a behavioral response to environmental change?
(A) Producing digestive enzymes
(B) Shivering to produce heat
(C) Migrating in response to temperature drop
(D) Lowering heart rate during sleep
Answer
(C) — Migration is a behavioral adaptation to environmental stimuli such as temperature or day length.
5. Which type of organism obtains energy by consuming dead organic material?
(A) Photoautotroph
(B) Heterotroph
(C) Decomposer
(D) Detritivore
Answer
(D) — Detritivores feed on the remains and waste products of dead organisms, contributing to nutrient cycling.
6. Which of the following best describes a trophic cascade?
(A) Energy is lost at each trophic level
(B) A top predator affects the abundance of species at multiple lower trophic levels
(C) Energy flows only through producers
(D) Consumers produce their own energy
Answer
(B) — A trophic cascade occurs when changes in the population of a top predator impact multiple lower trophic levels, often altering ecosystem structure.
7. What is net primary productivity (NPP)?
(A) Total energy from sunlight captured by producers
(B) Total energy lost as heat
(C) Energy available to consumers after producers use some for metabolism
(D) Energy stored in fossil fuels
Answer
(C) — NPP is the energy remaining after producers meet their own energy needs, which becomes available to herbivores and other consumers.
8. Which organisms occupy the first trophic level in most ecosystems?
(A) Primary consumers
(B) Secondary consumers
(C) Producers
(D) Detritivores
Answer
(C) — Producers, such as plants and algae, form the base of the food chain and occupy the first trophic level.
9. What process releases carbon into the atmosphere as carbon dioxide?
(A) Photosynthesis
(B) Transpiration
(C) Cellular respiration
(D) Nitrogen fixation
Answer
(C) — Cellular respiration releases carbon dioxide into the atmosphere as organisms break down organic molecules for energy.
10. What would happen if decomposers were removed from an ecosystem?
(A) Food chains would collapse immediately
(B) Primary production would increase
(C) Nutrients would accumulate in dead organisms
(D) Predators would lose prey
Answer
(C) — Without decomposers, dead organisms and waste would not be broken down, preventing nutrient recycling in the ecosystem.
11. Which statement best explains the 10% rule in energy transfer?
(A) 10% of energy is stored in producers
(B) 10% of energy at each level is transferred to the next
(C) 10% of organisms reproduce each year
(D) 10% of energy is used in photosynthesis
Answer
(B) — Only about 10% of the energy from one trophic level is passed to the next; the rest is lost as heat or used in metabolism.
12. Why do ecosystems rarely contain more than four or five trophic levels?
(A) Predators cannot survive above level 4
(B) Producers are limited to aquatic systems
(C) Energy is insufficient to support more levels
(D) Herbivores outcompete predators
Answer
(C) — Because energy decreases significantly at each trophic level, there isn’t enough energy to sustain many higher levels.
13. Which of the following is an abiotic factor in an ecosystem?
(A) Fungi
(B) Temperature
(C) Decomposers
(D) Insects
Answer
(B) — Abiotic factors are nonliving components of the environment, like temperature, sunlight, and water.
14. What role do keystone species play in an ecosystem?
(A) They are always top predators
(B) Their removal causes major ecosystem changes
(C) They recycle nutrients
(D) They prevent extinction
Answer
(B) — Keystone species have a disproportionate impact on their ecosystem. Their loss can destabilize community structure.
15. Which organism would be most affected by biomagnification of toxins like mercury?
(A) Phytoplankton
(B) Zooplankton
(C) Small fish
(D) Top predator fish
Answer
(D) — Toxins accumulate up the food chain, so top predators receive the highest concentration through biomagnification.
16. What causes the greatest energy loss in a food chain?
(A) Digestion by herbivores
(B) Transfer between trophic levels
(C) Cellular respiration and heat loss
(D) Biomagnification
Answer
(C) — Most energy is lost as heat during cellular respiration, making energy transfer between trophic levels inefficient.
17. What does a food web represent that a food chain does not?
(A) Only producer-consumer relationships
(B) The direction of energy flow
(C) Interconnected feeding relationships in an ecosystem
(D) The trophic pyramid
Answer
(C) — A food web shows how multiple food chains are connected, representing complex feeding relationships in an ecosystem.
18. Which of the following cycles involves nitrogen-fixing bacteria converting nitrogen gas into a usable form for plants?
(A) Carbon cycle
(B) Nitrogen cycle
(C) Water cycle
(D) Oxygen cycle
Answer
(B) — The nitrogen cycle includes nitrogen fixation by bacteria that convert atmospheric N₂ into forms plants can absorb.
19. What characteristic distinguishes an ecosystem from a community?
(A) Ecosystems do not include abiotic factors
(B) Communities include only top predators
(C) Ecosystems include both living and nonliving components
(D) Communities are larger than ecosystems
Answer
(C) — Ecosystems include both biotic (living) and abiotic (nonliving) components; communities include only living organisms.
20. Which process reduces atmospheric carbon dioxide?
(A) Burning fossil fuels
(B) Cellular respiration
(C) Photosynthesis
(D) Volcanic activity
Answer
(C) — Photosynthesis removes CO₂ from the atmosphere as plants use it to build organic molecules.
21. Why is biodiversity important in ecosystems?
(A) It ensures all species can reproduce
(B) It increases photosynthesis
(C) It stabilizes ecosystems by increasing resilience to disturbances
(D) It prevents invasive species
Answer
(C) — Greater biodiversity allows ecosystems to recover from disruptions more effectively and maintain stability.
22. What determines the carrying capacity of an ecosystem?
(A) The number of predators
(B) The maximum birth rate
(C) Availability of resources like food, water, and space
(D) Reproductive isolation
Answer
(C) — Carrying capacity is limited by available resources that can sustain a population over time.
23. What happens when an ecosystem exceeds its carrying capacity?
(A) The population stabilizes
(B) Resources increase
(C) The population declines due to resource scarcity
(D) New species form
Answer
(C) — When a population exceeds carrying capacity, resources become limited, leading to competition and population decline.
24. Which organisms play a direct role in nitrogen fixation?
(A) Earthworms
(B) Fungi
(C) Legume-associated bacteria
(D) Herbivores
Answer
(C) — Certain bacteria in the root nodules of legumes convert nitrogen gas into usable forms for plants.
25. What describes an ecological niche?
(A) A species’ total population
(B) Its physical environment only
(C) The role and position a species has in its environment
(D) Its reproductive rate
Answer
(C) — An ecological niche is the sum of a species' interactions with the environment, including its role, habitat, and resource use.
26. Which of the following best explains the concept of ecological efficiency?
(A) The total energy available in an ecosystem
(B) The percentage of energy transferred from one trophic level to the next
(C) The energy stored in decomposers
(D) The speed at which nutrients cycle in an ecosystem
Answer
(B) — Ecological efficiency refers to the proportion of energy that is passed from one trophic level to the next, usually about 10%.
27. Which scenario most likely results from removal of a keystone predator from a community?
(A) No major effect on biodiversity
(B) An increase in producer diversity
(C) Collapse or drastic change in species composition
(D) Enhanced nitrogen fixation
Answer
(C) — Keystone predators maintain balance; their removal often leads to trophic cascades and disrupted species interactions.
28. In a food web, which of the following organisms would have the highest concentration of a fat-soluble pollutant like DDT?
(A) Algae
(B) Zooplankton
(C) Small fish
(D) Bald eagle
Answer
(D) — As a top predator, the eagle receives the cumulative effect of biomagnification, leading to the highest concentration of pollutants.
29. Why is energy transfer between trophic levels inefficient?
(A) Energy is used in reproduction only
(B) Consumers are inefficient at digestion
(C) Most energy is lost as heat and through metabolism
(D) Only carnivores can absorb energy
Answer
(C) — A large portion of energy is lost through heat, movement, and metabolic processes, making energy transfer inefficient.
30. What ecological impact might a sharp increase in primary productivity have on an aquatic ecosystem?
(A) Decreased biodiversity
(B) Oxygen depletion and fish kills due to algal blooms
(C) Increased transparency in the water
(D) Stabilization of trophic levels
Answer
(B) — Excess productivity, often from nutrient runoff, causes algal blooms, which can deplete oxygen when decomposed, leading to dead zones.
31. Which of the following best explains how invasive species can disrupt ecosystem balance?
(A) They produce more oxygen
(B) They increase biodiversity
(C) They outcompete native species for resources
(D) They fix nitrogen in the soil
Answer
(C) — Invasive species often lack natural predators and outcompete natives, disrupting food webs and reducing biodiversity.
32. What is the main factor driving species turnover in ecological succession?
(A) Predation pressure
(B) Abiotic changes in the environment over time
(C) Immigration from neighboring communities
(D) Biomagnification
Answer
(B) — Succession occurs as abiotic factors change, altering which species can survive and leading to gradual shifts in community composition.
33. A grassland ecosystem receives 100,000 J of solar energy per square meter per day. If primary producers capture 1% of this energy, and 10% of that energy is transferred to primary consumers, how much energy is available to secondary consumers?
(A) 1,000 J
(B) 100 J
(C) 10 J
(D) 0.1 J
Answer
(C) — 100,000 J × 0.01 = 1,000 J (producers) → 100 J (primary consumers) → 10 J (secondary consumers).
34. Which of the following statements about ecological pyramids is most accurate?
(A) Biomass always increases at higher trophic levels
(B) Energy pyramids can be inverted
(C) Pyramid of numbers may be inverted in some ecosystems
(D) All pyramids are always upright
Answer
(C) — In some cases (e.g., one tree supports many insects), the pyramid of numbers may be inverted even if biomass and energy pyramids are upright.
35. Why does nitrogen need to be fixed before plants can use it?
(A) Nitrogen gas is toxic to roots
(B) N₂ is unreactive and must be converted to ammonium or nitrate
(C) Nitrogen is only usable in organic form
(D) N₂ is only found in oceans
Answer
(B) — Atmospheric nitrogen (N₂) is chemically stable and must be converted by bacteria into ammonium (NH₄⁺) or nitrate (NO₃⁻) before plants can absorb it.
36. Describe how the 10% rule limits the number of trophic levels in an ecosystem.
Answer
Because only about 10% of energy is transferred to the next trophic level, there’s not enough energy to support more than 4–5 levels before energy becomes insufficient to sustain top consumers.
37. Explain how primary succession differs from secondary succession.
Answer
Primary succession occurs in areas with no previous life or soil, such as lava flows or glacial retreats, while secondary succession takes place where a disturbance has cleared existing life but left the soil intact.
38. Why are decomposers essential for nutrient cycling?
Answer
Decomposers break down dead organisms and waste, releasing nutrients like nitrogen and phosphorus back into the environment for use by producers, maintaining ecosystem productivity.
39. How do keystone species influence biodiversity?
Answer
Keystone species have a disproportionately large effect on community structure. Their presence maintains species diversity by regulating populations and balancing interactions among organisms.
40. Describe the process of nitrogen fixation and its significance in ecosystems.
Answer
Nitrogen fixation is the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium (NH₄⁺) by bacteria, making nitrogen accessible to plants for building proteins and nucleic acids.
41. What impact does biomagnification have on top predators?
Answer
Top predators accumulate the highest concentrations of fat-soluble toxins like mercury or DDT, which can impair reproduction, growth, or survival due to their position at the top of the food chain.
42. How does energy flow differ from nutrient cycling in ecosystems?
Answer
Energy flows in a one-way direction, entering as sunlight and exiting as heat, whereas nutrients cycle continuously through biotic and abiotic components of the ecosystem.
43. Why does high biodiversity contribute to ecosystem stability?
Answer
High biodiversity increases resilience, allowing ecosystems to better recover from disturbances, resist invasive species, and maintain ecosystem functions like productivity and nutrient cycling.
44. What is an ecological niche, and how does it reduce competition?
Answer
An ecological niche includes an organism’s role, habitat, and resource use. When species occupy different niches, they minimize direct competition by exploiting different resources or behaviors.
45. Explain one way that invasive species can reduce native biodiversity.
Answer
Invasive species often outcompete native species for resources, lack predators, and may alter habitat conditions, leading to the decline or extinction of native species.
46. How do producers contribute to the carbon cycle?
Answer
Producers remove CO₂ from the atmosphere during photosynthesis and use it to build organic molecules. When they are consumed or decompose, carbon re-enters the atmosphere through respiration or decay.
47. Describe how eutrophication affects aquatic ecosystems.
Answer
Eutrophication occurs when excess nutrients (like nitrogen and phosphorus) promote algal blooms, which reduce oxygen levels upon decomposition, leading to dead zones and aquatic life loss.
48. Why is only a small portion of solar energy converted into biomass by producers?
Answer
Most solar energy is reflected, lost as heat, or used in plant metabolism. Only a small portion is converted into chemical energy via photosynthesis and stored in biomass.
49. What is ecological succession, and what causes it to occur?
Answer
Ecological succession is the gradual change in species composition in an area over time, caused by disturbances, changes in resources, or the establishment of new species.
50. How do abiotic factors influence the structure of an ecosystem?
Answer
Abiotic factors such as temperature, light, water, and soil determine which organisms can survive and thrive in an area, shaping species interactions and ecosystem structure.