Free Ecology Community Species Interactions Questions and Answers — Questions and Answers
Question 1: In a rocky intertidal zone, the presence of the sea star *Pisaster ochraceus* prevents the mussel *Mytilus californianus* from dominating the habitat and excluding other species like barnacles and algae. Which ecological concept does the sea star BEST exemplify?
- Foundation species
- Keystone species (Correct answer)
- Indicator species
- Pioneer species
Correct answer: Keystone species
A keystone species is one that has a disproportionately large effect on its environment relative to its abundance. The sea star *Pisaster ochraceus* is a classic example; by preying on the competitively superior mussels, it allows other, less competitive species to persist, thus maintaining high biodiversity. Without the sea star, the mussels would outcompete other organisms and drastically reduce species diversity.
Question 2: The reintroduction of gray wolves into Yellowstone National Park led to a decrease in the elk population. This, in turn, allowed for the recovery of willow and aspen trees, which then supported an increase in the beaver population. This chain of effects is a clear example of:
- Competitive exclusion
- Resource partitioning
- A trophic cascade (Correct answer)
- Character displacement
Correct answer: A trophic cascade
A trophic cascade describes the indirect effects that a predator can have on the rest of an ecosystem by controlling the population of its prey. In this scenario, the top predator (wolf) influences the herbivore (elk), which in turn affects the primary producers (willow and aspen trees) and other associated species (beavers). This top-down effect is the defining characteristic of a trophic cascade.
Question 3: Which of the following scenarios describes a commensal relationship?
- A tick feeding on the blood of a deer, harming the deer in the process.
- Bees gathering nectar from a flower, which helps the flower with pollination.
- A cattle egret feeding on insects stirred up by a grazing cow, where the cow is unaffected. (Correct answer)
- Two species of barnacles competing for limited space on a rock.
Correct answer: A cattle egret feeding on insects stirred up by a grazing cow, where the cow is unaffected.
Commensalism is a type of symbiotic relationship where one organism benefits and the other is neither helped nor harmed (+/0 interaction). The cattle egret benefits by getting an easy meal, while the cow is not significantly affected by the egret's presence. The other options describe parasitism (-/+), mutualism (+/+), and competition (-/-), respectively.
Question 4: Two species of finches live on the same island. Where their territories overlap, their beak sizes show greater differences than where they live separately. This allows them to specialize on different seed sizes, reducing direct competition. This evolutionary divergence is known as:
- Competitive release
- Resource partitioning
- Niche generalization
- Character displacement (Correct answer)
Correct answer: Character displacement
Character displacement is an evolutionary divergence that occurs when two similar species inhabit the same environment. Traits differ more in sympatric (geographically overlapping) populations than in allopatric (geographically separate) populations. This phenomenon, often seen in Galapagos finches, is a direct result of interspecific competition leading to the selection for traits that minimize niche overlap.
Question 5: The harmless hoverfly has black and yellow banding that closely resembles that of a stinging wasp. Predators that have learned to avoid wasps will also avoid the hoverfly. This is an example of which type of mimicry?
- Aggressive mimicry
- Müllerian mimicry
- Batesian mimicry (Correct answer)
- Automimicry
Correct answer: Batesian mimicry
Batesian mimicry occurs when a harmless species (the mimic) evolves to imitate the warning signals of a harmful species (the model) to deter predators. The hoverfly (harmless mimic) gains protection by looking like the wasp (harmful model). Müllerian mimicry, in contrast, involves two or more harmful species that resemble each other.
Question 6: When two species utilize the same limited resource in different ways, at different times, or in different locations to minimize competition, they are engaging in:
- Competitive exclusion
- Resource partitioning (Correct answer)
- Amensalism
- Mutualism
Correct answer: Resource partitioning
Resource partitioning is the division of limited resources by species to help avoid competition in an ecological niche. By dividing a niche, species can coexist because there is less direct competition between them. This contrasts with the competitive exclusion principle, which states that two species competing for the exact same limited resources cannot stably coexist.
Free Ecology Community Species Interactions Questions and Answers
In a rocky intertidal zone, the presence of the sea star *Pisaster ochraceus* prevents the mussel *Mytilus californianus* from dominating the habitat and excluding other species like barnacles and algae.
Which ecological concept does the sea star BEST exemplify?