Questions: Biodiversity Patterns: Richness, Evenness, and Gradients

Question 1

Two forest plots each contain 20 bird species. In Plot A, each species makes up roughly 5% of individuals. In Plot B, one species makes up 85% of individuals and the other 19 share the remaining 15%. Which statement is most accurate?

• A. The two plots are equally diverse because species richness is the only valid measure of biodiversity
• B. Plot A is more diverse because higher evenness means the community is more functionally balanced — no single species dominates
• C. Plot B is more ecologically stable because a dominant species provides consistent ecosystem function
• D. Plot B has higher diversity because dominant species support more dependent species in their food web

Answer: (B)

Both plots have the same species richness (20 species), but their evenness is radically different. Diversity indices like Shannon and Simpson combine both components. Plot B is functionally precarious — if the dominant species collapses, most of the community's biomass disappears. Plot A's balanced distribution means the community's function is distributed across many species, making it more resilient. The classic misconception is equating richness with biodiversity; evenness captures something equally important about community structure.

Question 2

An ecologist surveys three coral reef sites. A reef with very frequent storms has few species (only fast colonizers survive). A reef undisturbed for decades has few species (a dominant coral has outcompeted others). A reef with occasional storms has the most species. What principle does this illustrate?

• A. The competitive exclusion principle — disturbance prevents any single species from monopolizing resources
• B. The intermediate disturbance hypothesis — diversity peaks at moderate disturbance because neither competitive dominants nor disturbance specialists can fully exclude the other
• C. Island biogeography — the moderately disturbed reef has the highest immigration-to-extinction ratio
• D. The latitudinal diversity gradient — storm frequency correlates with distance from the tropics

Answer: (B)

The intermediate disturbance hypothesis predicts exactly this pattern. High disturbance favors only fast-colonizing species with little competitive ability; no disturbance allows competitive dominants to exclude everything else. At intermediate disturbance, competitive dominants haven't had time to monopolize, but disturbance isn't so frequent that slow-growing species are wiped out before establishing — so a mix persists. This applies to coral reefs, forests, grasslands, and many other systems.

Question 3

A community can have identical species richness to another community but be meaningfully less diverse if one community is strongly dominated by a single species.

• T. True
• F. False

Answer: True

True. Richness counts species; evenness describes how individuals are distributed among those species. Two communities with 10 species each are very different if one is 91% one species vs. one where each is 10%. Diversity indices (Shannon, Simpson) were specifically developed to capture both dimensions. Conservation decisions based on richness alone can miss the functional fragility of dominance-skewed communities.

Question 4

The latitudinal diversity gradient — more species in the tropics than at the poles — is explained by a single well-established mechanism: higher solar energy input supporting greater productivity.

• T. True
• F. False

Answer: False

False. The latitudinal gradient is one of ecology's most robust patterns, but its explanation remains multi-causal and contested. Energy-productivity hypotheses, greater climatic stability in the tropics (allowing long-term species accumulation without mass extinction), and geometric area effects (the tropics occupy more surface area) are all supported by evidence and are not mutually exclusive. No single mechanism fully accounts for the pattern across all taxa and regions.

Question 5

Why does protecting a square kilometer of tropical forest preserve far more biodiversity than the same area of boreal forest? What does this imply for conservation prioritization?

Model answer: Tropical forests have both higher species richness (more species per area due to the latitudinal gradient — energy, stability, and area effects) and higher evenness in many groups, meaning a given area contains a disproportionate share of Earth's total biodiversity. A hectare of tropical rainforest may contain more tree species than all of temperate Europe. For conservation prioritization, this means that protecting tropical areas yields a much higher return on investment in species conserved per dollar or per hectare. It also implies that tropical deforestation is a biodiversity catastrophe disproportionate to the area lost — and that conserving high-richness, high-evenness systems should be prioritized over ecologically impoverished areas of equal size.

The practical implication connects the descriptive patterns (richness, evenness, latitudinal gradient) to conservation strategy. Understanding that diversity is non-uniformly distributed — concentrated in predictable places for identifiable reasons — transforms conservation from a uniformly distributed concern into a spatially strategic one.