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Quiz: Species Interactions

Test your understanding of ecological relationships between organisms and how they shape communities with these review questions.

1. Which type of species interaction is correctly paired with its effect on each species?

  1. Mutualism: one species benefits while the other is harmed
  2. Commensalism: both species benefit equally from the interaction
  3. Parasitism: one species benefits while the other is harmed but usually not killed immediately
  4. Competition: one species benefits while the other is unaffected
Show Answer

The correct answer is C. In parasitism, the parasite benefits by obtaining nutrients from the host, while the host is harmed. Unlike predation, parasites usually do not kill their hosts immediately because a dead host means a homeless parasite. Examples include tapeworms in dogs, ticks on deer, and malaria parasites in humans. Mutualism benefits both species, commensalism benefits one with no effect on the other, and competition harms both species.

Concept Tested: Parasitism

2. What is the principle of competitive exclusion?

  1. Two species can share an identical niche indefinitely if resources are abundant
  2. Two species cannot occupy exactly the same niche in the same habitat indefinitely
  3. Competition always results in the extinction of both competing species
  4. Larger species always outcompete smaller species for the same resources
Show Answer

The correct answer is B. Competitive exclusion, demonstrated by G.F. Gause in his experiments with Paramecium, states that two species cannot occupy exactly the same niche in the same habitat indefinitely. One species will always outcompete the other for shared resources. In nature, species avoid this through resource partitioning, evolving to use slightly different resources, forage in different locations, or be active at different times.

Concept Tested: Competitive Exclusion

3. How do keystone species differ from foundation species?

  1. Keystone species are always predators while foundation species are always plants
  2. Keystone species are found only in marine ecosystems while foundation species are terrestrial
  3. Keystone species have disproportionate influence relative to their abundance while foundation species physically define the habitat
  4. Keystone species are always the most abundant organism while foundation species are always rare
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The correct answer is C. A keystone species has a disproportionately large effect on its community relative to its abundance -- like the sea star Pisaster that maintains intertidal biodiversity despite being just one species. A foundation species is often the dominant organism that creates or defines the physical habitat structure, such as coral building reef ecosystems or large trees forming forest canopy. The key distinction is influence relative to numbers versus physical habitat creation.

Concept Tested: Keystone Species

4. What is coevolution and which example best illustrates it?

  1. One species evolving without affecting others, such as a fish developing larger fins
  2. Two species exerting selective pressure on each other, such as cheetahs and gazelles both evolving greater speed
  3. A species adapting to abiotic conditions, such as cacti evolving water storage in deserts
  4. Multiple species going extinct simultaneously due to a catastrophic event
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The correct answer is B. Coevolution occurs when two species exert reciprocal selective pressure on each other, driving evolutionary changes in both over time. Cheetahs and gazelles exemplify this -- as cheetahs evolved greater speed to catch prey, gazelles evolved greater speed to escape. Other examples include the Madagascar star orchid and its long-tongued moth pollinator, and milkweed toxin resistance in monarch butterflies.

Concept Tested: Coevolution

5. Why do invasive species often thrive in their new ecosystems?

  1. They always have higher genetic diversity than native species
  2. They arrive without the predators, parasites, and competitors that controlled them in their native range
  3. They can photosynthesize more efficiently than native plant species
  4. They are always larger and more aggressive than native species
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The correct answer is B. Invasive species often succeed because they arrive in new ecosystems without the natural enemies -- predators, parasites, and competitors -- that kept their populations in check in their native range. This allows them to exploit resources and reproduce rapidly. Native species that lack evolutionary experience with the invader often have no appropriate defenses or competitive strategies against them.

Concept Tested: Invasive Species

6. What is the difference between Batesian mimicry and camouflage?

  1. Batesian mimicry involves resembling a dangerous species while camouflage involves blending into the environment
  2. Batesian mimicry is used only by predators while camouflage is used only by prey
  3. Batesian mimicry changes with seasons while camouflage remains constant year-round
  4. Batesian mimicry involves chemical defenses while camouflage involves physical armor
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The correct answer is A. Batesian mimicry is when a harmless species resembles a dangerous or toxic species to deter predators -- like harmless hoverflies with yellow-and-black stripes resembling wasps. Camouflage is when an organism blends in with its environment to avoid detection entirely, such as stick insects resembling twigs or Arctic hares turning white in winter. Both are adaptations driven by natural selection from predator-prey interactions.

Concept Tested: Mimicry

7. In predator-prey dynamics, what typically happens to prey populations after predator populations decline?

  1. Prey populations decline at the same rate as predators
  2. Prey populations remain stable because other factors limit their growth
  3. Prey populations increase because predation pressure is reduced
  4. Prey populations immediately go extinct without predator regulation
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The correct answer is C. In classic predator-prey dynamics, when predator populations decline, prey populations increase because there is less predation pressure. This was documented in the lynx and snowshoe hare populations of northern Canada, where populations oscillate in linked cycles. However, without predators, prey can overgraze their food supply and eventually crash from starvation, showing that predators actually stabilize ecosystems.

Concept Tested: Predator-Prey Dynamics

8. What role do indicator species play in ecology?

  1. They control the population sizes of other species through predation
  2. They build the physical structure of the habitat for other species
  3. They reflect the health of their environment through their presence, absence, or condition
  4. They introduce genetic diversity into populations through cross-breeding
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The correct answer is C. Indicator species are organisms whose presence, absence, or health reflects the condition of their environment, serving as biological early warning systems. Amphibians indicate water quality because their permeable skin makes them sensitive to pollution. Lichens indicate air quality because they are extremely sensitive to air pollution. Salmon indicate ecosystem health across both freshwater and marine environments.

Concept Tested: Indicator Species

9. How does resource partitioning allow competing species to coexist?

  1. Competing species agree to share resources equally through behavioral signals
  2. One species eliminates the other so that only the stronger survives
  3. Species evolve to use slightly different parts of the available resources, reducing niche overlap
  4. Species migrate to different ecosystems to avoid competition entirely
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The correct answer is C. Resource partitioning occurs when competing species evolve to use slightly different portions of available resources, reducing direct competition. Robert MacArthur's warbler study showed five species feeding in the same spruce trees but foraging in different zones -- treetop, mid-canopy, base of branches, trunk, and ground. This division of resources explains how biodiversity persists despite competition.

Concept Tested: Resource Partitioning

10. Why is high genetic diversity important for a population's long-term survival?

  1. It ensures all individuals are identical and equally adapted to current conditions
  2. It provides variation so that some individuals may survive when environmental conditions change
  3. It prevents natural selection from occurring within the population
  4. It guarantees that the population will always increase in size over time
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The correct answer is B. High genetic diversity means a population contains many different gene variants. When conditions change -- a new disease, climate shift, or invasive predator -- some individuals may carry genes that confer resistance or tolerance. Those individuals survive and reproduce through natural selection, allowing the population to adapt. Populations with low genetic diversity have fewer options and face greater extinction risk when conditions change.

Concept Tested: Genetic Diversity