Questions: Rainfall, Productivity, and Biogeographic Diversity Gradients

Question 1

A researcher fertilizes a low-productivity prairie to increase plant species richness. After several years, species richness has actually declined. The most likely ecological explanation is:

• A. Fertilizer is toxic to most native plant species, directly killing them
• B. Higher productivity allowed one or a few dominant competitor species to monopolize resources, excluding less competitive species
• C. The prairie needed more rainfall, not nutrients, to support greater diversity
• D. Species richness always declines when any disturbance is introduced to an ecosystem

Answer: (B)

This experiment illustrates the hump-shaped productivity-diversity relationship. At low productivity, adding resources initially increases diversity by supporting more species with viable populations. But at high productivity, competitive exclusion intensifies: dominant species that grow fast in resource-rich conditions crowd out slower-growing specialists. The prairie shifts from a diverse community with many coexisting strategies to one dominated by a few aggressive competitors. This is why the diversity-productivity curve peaks at intermediate productivity rather than rising indefinitely.

Question 2

The species-energy hypothesis predicts that the most productive ecosystem will always contain the greatest species diversity. Which statement best characterizes why this prediction is incomplete?

• A. Energy availability is irrelevant to species diversity; it is climate stability that matters
• B. At very high productivity, competitive dominants can monopolize resources, reducing diversity — making the relationship hump-shaped rather than linear
• C. Very productive ecosystems have too many individuals per species, causing density-dependent die-offs
• D. The species-energy hypothesis only applies to aquatic ecosystems, not terrestrial ones

Answer: (B)

The species-energy hypothesis captures an important pattern — more energy supports more individuals and therefore more species — but it assumes resource partitioning remains relatively even. At very high productivity, a few fast-growing competitive dominants often preempt resources before other species can establish, driving diversity down. Many field datasets show a unimodal (hump-shaped) diversity-productivity curve, with peak diversity at intermediate levels. The most productive ecosystems (like hyper-eutrophic lakes or highly fertilized grasslands) can actually be species-poor.

Question 3

According to the species-energy hypothesis, the primary reason tropical regions support higher diversity than temperate regions is that they receive more total solar energy annually.

• T. True
• F. False

Answer: False

The species-energy hypothesis is one important explanation, but habitat heterogeneity is an independent contributing factor. High rainfall in the tropics creates structural complexity — canopy layers, understory, forest floor — each with distinct microclimates and niches. This three-dimensional architecture provides more opportunities for specialization and niche differentiation than a structurally simple grassland. Rainfall also drives spatial heterogeneity via river networks and moisture gradients, promoting speciation by isolating populations. Both energy availability and habitat structure explain the high tropical diversity.

Question 4

Higher annual rainfall contributes to greater species diversity not only by increasing total primary productivity but also by creating structural habitat complexity that provides more ecological niches.

• T. True
• F. False

Answer: True

Rainfall drives both pathways independently. First, it increases primary productivity (energy availability), which according to the species-energy hypothesis supports more species with viable populations. Second, it creates structural and spatial heterogeneity: layered forest canopies, river networks, moisture gradients across slopes, and seasonal flooding patterns each subdivide the environment into distinct microhabitats. This heterogeneity increases beta diversity (species turnover across habitats) and promotes specialization. Both mechanisms together explain why tropical rainforests are far more diverse than simple energy-availability models would predict.

Question 5

Why does the relationship between productivity and species diversity often show a hump-shaped (unimodal) pattern rather than a simple linear increase? What ecological mechanism drives the decline in diversity at very high productivity levels?

Model answer: At low productivity, diversity is limited by the small energy pool — few species can maintain viable populations when total resources are scarce. As productivity increases, more species can be supported, so diversity rises. But at very high productivity, competitive exclusion becomes intense: dominant species that thrive in resource-rich conditions grow rapidly and preempt space, light, and nutrients before less competitive species can establish. The diversity-enabling mechanism at low to intermediate productivity (more resources to partition) becomes a diversity-reducing force at high productivity (asymmetric competition). The peak of diversity thus occurs at an intermediate level where resource supply is high enough to support many species but not so high that competitive dominants exclude them.

Classic examples include the diversity decline in hyper-eutrophic lakes (dominated by algal blooms), fertilized grasslands (dominated by fast-growing grasses), and nutrient-enriched coral reefs (dominated by macroalgae). In each case, adding resources that seem beneficial initially shifts competitive balance toward a few dominant organisms, reducing the diversity that lower-resource conditions maintained through niche differentiation.