Two warbler species occupy the same spruce forest and eat similar insects. Species A forages in the upper canopy; Species B forages in the lower branches. Both populations remain stable over many years. What best explains this coexistence?
ACompetitive exclusion — Species A is the superior competitor and Species B will eventually go extinct
BSpatial resource partitioning — by using different parts of the habitat, each species limits its own population more than it limits the other's
CCharacter displacement — both species have evolved similar traits to reduce competition
DMutualism — the two species have evolved a cooperative relationship that allows both to persist
This mirrors MacArthur's classic warbler study. Spatial partitioning (different foraging heights) reduces niche overlap enough that intraspecific competition within each species exceeds interspecific competition between them — the condition for stable coexistence. Option A is wrong because both populations are stable, so exclusion hasn't occurred and won't. Option C has the direction wrong — character displacement produces divergence, not similarity. Option D misidentifies the relationship: these are competitors, not mutualists.
Question 2 Multiple Choice
The competitive exclusion principle predicts that two species competing for exactly the same limiting resource cannot coexist. How does resource partitioning reconcile this principle with the high diversity of natural communities?
AResource partitioning shows the principle is wrong — coexistence is always possible regardless of niche overlap
BResource partitioning means competing species are not actually exploiting exactly the same limiting resource in the same way, so the principle's core assumption is never met
CResource partitioning replaces competition with cooperation, which stabilizes communities
DThe competitive exclusion principle only applies in laboratory conditions, not in natural ecosystems
The competitive exclusion principle isn't violated — it's escaped. The principle says two species can't stably coexist on exactly the same limiting resource. Resource partitioning (different times, places, or food types) means they're not actually using exactly the same resource. Each species is primarily regulated by intraspecific competition for its own particular resource subset, not by the other species. Option A incorrectly rejects the principle; Option C mischaracterizes partitioning as cooperation; Option D is false — the principle holds in nature.
Question 3 True / False
Character displacement — the evolutionary divergence of competing species in resource use — can produce stable coexistence by increasing niche partitioning, even when species initially overlapped strongly in resource use.
TTrue
FFalse
Answer: True
When competing species come into contact, natural selection favors individuals that diverge in resource use from the competitor, reducing direct competition. Over evolutionary time this produces measurable differences between sympatric populations (co-occurring) versus allopatric ones (each alone). Darwin's finches on islands where two species co-occur show greater beak size divergence than on single-species islands — direct evidence that competition drives evolutionary partitioning that reduces overlap and enables coexistence.
Question 4 True / False
For two competing species to coexist stably, interspecific competition (between species) is expected to be stronger than intraspecific competition (within species).
TTrue
FFalse
Answer: False
This has the direction exactly backwards. Stable coexistence requires that each species limits its own population growth MORE than it limits the other's — i.e., intraspecific competition must exceed interspecific competition. When this holds, a species that becomes rare faces less intraspecific competition and can recover, producing a stabilizing negative feedback. If interspecific competition exceeds intraspecific, the rarer species cannot recover and competitive exclusion follows.
Question 5 Short Answer
Explain the relationship between resource partitioning and competitive exclusion. How does partitioning allow coexistence without violating the competitive exclusion principle?
Think about your answer, then reveal below.
Model answer: The competitive exclusion principle says two species cannot coexist when competing for exactly the same limiting resource — the superior competitor will drive the inferior one extinct. Resource partitioning allows coexistence by ensuring no two species use exactly the same limiting resource: they eat at different times, in different places, or specialize on different food types. Each species' most intense competition is with conspecifics (same-species individuals) rather than heterospecifics. When intraspecific competition exceeds interspecific competition, rare individuals of each species face less competition and can recover — producing a stable equilibrium. Resource partitioning doesn't violate the principle; it shows the conditions under which the principle's key assumption of identical resource use simply isn't met.
The limiting similarity concept formalizes this: there is a maximum degree of niche overlap compatible with stable coexistence. Beyond that threshold, interspecific competition becomes too strong relative to intraspecific competition, and one species excludes the other.