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Quiz: Community Ecology and Species Interactions

Test your understanding of interspecific interactions, community structure, succession, and island biogeography with these review questions.

1. What does the competitive exclusion principle state?

  1. Two species with different niches will always compete for the same resources
  2. Species that compete for resources always evolve mutualistic relationships over time
  3. Competition between species always benefits one species and has no effect on the other
  4. Two species competing for the exact same ecological niche cannot coexist indefinitely; one will eventually outcompete and eliminate the other
Show Answer

The correct answer is D. The competitive exclusion principle, demonstrated experimentally by G.F. Gause using two Paramecium species, states that when two species occupy identical ecological niches and compete for exactly the same limited resources, one species will eventually outcompete and eliminate the other from that habitat. Species avoid this outcome in nature primarily through resource partitioning, which reduces niche overlap.

Concept Tested: Competitive Exclusion Principle

2. In the relationship between epiphytic orchids and the trees they grow on, the orchid gains access to sunlight while the tree experiences no measurable benefit or harm. Which type of interaction is this?

  1. Mutualism
  2. Commensalism
  3. Parasitism
  4. Competition
Show Answer

The correct answer is B. Commensalism describes an interaction in which one species benefits (here, the orchid gains elevated access to sunlight) while the other species experiences no significant positive or negative effect (the tree is essentially unaffected by providing physical support). This distinguishes commensalism from mutualism, where both species would benefit, and from parasitism, where the host would be harmed.

Concept Tested: Commensalism

3. How does a species' realized niche differ from its fundamental niche?

  1. The fundamental niche is the full range of conditions under which a species could survive without competition, while the realized niche is the narrower set of conditions the species actually occupies due to competition and other interactions
  2. The realized niche is always identical to the fundamental niche in every environment
  3. The realized niche is the full range of conditions a species could survive in the complete absence of competitors, while the fundamental niche is narrowed by interactions with other species
  4. The fundamental niche only applies to plants, while the realized niche only applies to animals
Show Answer

The correct answer is A. The fundamental niche represents the complete range of abiotic and biotic conditions under which a species could theoretically survive and reproduce if no competitors were present. In real communities, competition and other interspecific interactions typically restrict a species to a smaller subset of those conditions, known as the realized niche — the set of conditions the species actually occupies in practice.

Concept Tested: Ecological Niches

4. Rhizobium bacteria living in the root nodules of legume plants convert atmospheric nitrogen into a usable form for the plant, while the plant supplies the bacteria with sugars produced through photosynthesis. Which interaction type is this?

  1. Commensalism, because only the bacteria benefit
  2. Parasitism, because the bacteria drain resources from the plant
  3. Mutualism, because both the bacteria and the plant benefit from the exchange
  4. Competition, because both organisms need the same limited resource
Show Answer

The correct answer is C. This relationship is a classic example of mutualism because both species gain a benefit: the legume plant receives usable nitrogen it could not otherwise access, and the Rhizobium bacteria receive a steady supply of sugars from the plant's photosynthesis. Because both partners benefit rather than one benefiting at the other's expense or with no effect, this is neither commensalism, parasitism, nor competition.

Concept Tested: Mutualism

5. Two raptor species hunt the same rodent prey in the same field, but one hunts exclusively during the day while the other hunts exclusively at night. What mechanism allows these species to coexist despite using the same resource?

  1. Competitive exclusion, because one species will eventually eliminate the other
  2. Mutualism, because both species benefit from hunting the same prey
  3. A trophic cascade, because the raptors indirectly affect vegetation
  4. Temporal resource partitioning, because the two species divide access to the shared prey by hunting at different times
Show Answer

The correct answer is D. By hunting at different times of day, the two raptor species divide access to the same prey resource without directly competing for it at any given moment — a strategy called temporal resource partitioning. This reduction in niche overlap allows both species to persist in the same habitat, avoiding the outcome predicted by the competitive exclusion principle for species with fully overlapping niches.

Concept Tested: Resource Partitioning

6. According to the theory of island biogeography, which island would be predicted to have the highest species richness at equilibrium?

  1. A large island close to the mainland
  2. A small island close to the mainland
  3. A large island far from the mainland
  4. A small island far from the mainland
Show Answer

The correct answer is A. Island biogeography theory predicts species richness as the equilibrium between immigration and extinction rates. Larger islands support lower extinction rates because they offer more habitat and resources, and islands closer to the mainland experience higher immigration rates because colonizing species have a shorter distance to travel. An island that is both large and close to the mainland therefore combines both effects, producing the highest predicted species richness.

Concept Tested: Island Biogeography

7. In Yellowstone, wolf reintroduction was followed by recovering willow and aspen vegetation along riverbanks. Which explanation best captures the full mechanistic chain responsible for this vegetation recovery?

  1. Wolves directly eat willow and aspen seedlings, allowing faster regrowth
  2. Wolves reduced elk population size and altered elk grazing behavior (elk avoided exposed riverbanks), which decreased browsing pressure on riparian vegetation and allowed it to recover
  3. Vegetation recovery was unrelated to wolf reintroduction and instead resulted from an unusually wet climate cycle
  4. Wolves compete directly with elk for the same food resources, reducing elk fitness
Show Answer

The correct answer is B. This is a top-down trophic cascade: reintroduced wolves both reduced elk numbers through predation and changed elk behavior, since elk began avoiding exposed riverbanks where they were more vulnerable to attack. With less browsing pressure, riparian vegetation such as willows and aspens recovered, which in turn improved habitat for beavers, fish, and birds. A complete answer must trace this multi-step mechanism rather than describing a single direct effect.

Concept Tested: Trophic Cascades

8. Sea otters are considered a keystone species in Pacific kelp forest ecosystems even though they are far less numerous than sea urchins or kelp. What does this best illustrate about the concept of a keystone species?

  1. Keystone species must always be the most numerous organism in a community
  2. Keystone species have no measurable effect on other species in the community
  3. A keystone species's ecological importance is measured by its effect on community structure relative to its abundance, not by how common it is
  4. Any predator automatically qualifies as a keystone species regardless of its ecological effect
Show Answer

The correct answer is C. A keystone species is defined by having a disproportionately large effect on community structure relative to its own abundance. Sea otters are relatively few in number, yet their predation on sea urchins prevents urchin populations from overgrazing kelp forests; removing otters triggers a cascade that collapses the entire kelp forest ecosystem, demonstrating that ecological importance is not simply a function of population size.

Concept Tested: Keystone Species

9. A marine reserve is considering whether to reintroduce sea otters to a kelp forest area where otters were hunted to local extinction decades ago and urchin barrens have since replaced the kelp forest. Using evidence from the classic sea otter-urchin-kelp case study, how should ecologists evaluate this proposed reintroduction?

  1. Reintroduction is unlikely to have any ecological effect, since sea urchins and kelp are unrelated to otter population size
  2. Reintroduction should be avoided because keystone species reintroductions have never succeeded in any ecosystem
  3. Reintroduction would only affect sea otters themselves and would not influence any other trophic level
  4. Reintroduction is likely to reduce urchin populations through predation, potentially allowing kelp to recover and restoring habitat for the many species that depend on kelp forests, based on documented outcomes elsewhere
Show Answer

The correct answer is D. Documented case studies of sea otter reintroduction and recovery elsewhere on the Pacific coast show that restoring otter predation reduces urchin populations, which allows overgrazed kelp forests to regenerate and, in turn, restores habitat for fish, invertebrates, and seabirds that depend on kelp. Evaluating this proposal using that evidence supports reintroduction as an ecologically sound intervention, not a neutral or ineffective one.

Concept Tested: Keystone Species (Ecological Consequences of Reintroduction)

  1. Measure only the total population size of each lizard species over one season
  2. Record the diet, perch height, and time of activity for both species across multiple sites, and statistically compare whether the two species differ significantly in resource use along at least one of these niche dimensions
  3. Remove one lizard species from the island and observe whether the other species goes extinct within a week
  4. Compare the two species' skin color patterns, since coloration is unrelated to resource use
Show Answer

The correct answer is B. Testing a resource partitioning hypothesis requires directly measuring the niche dimensions the two species might be dividing — diet, spatial use (perch height), and temporal activity — across multiple sites, then statistically comparing whether the species differ in one or more of these dimensions. Measuring only population size, forcing extinction over an unrealistically short window, or comparing an unrelated trait like coloration would not provide evidence about how the species actually divide shared resources.

Concept Tested: Resource Partitioning (Experimental Design)