Questions: Biodiversity and Ecosystem Function Relationships

Question 1

A grassland experiment compares monocultures to an 8-species mixture. The mixture produces more total biomass than the average monoculture — but also more than the single best-performing species grown alone. This result most directly supports:

• A. The selection effect — the mixture contains the most productive species, which drives high yields
• B. Complementarity — species use different resources so the community exploits the environment more completely than any single species could
• C. Functional redundancy — many species contribute equally to productivity, diluting variance
• D. The portfolio effect — diverse communities are more stable, and stability itself produces higher average yields

Answer: (B)

The selection effect predicts the mixture performs *as well as* the best monoculture (because it samples from a larger pool), not *better than* it. When the mixture outperforms even the best monoculture, this 'overyielding' is the signature of complementarity: species are occupying different resource niches (root depths, light levels, nutrient preferences) and the community collectively exploits more of the total resource pool than any single species alone can access.

Question 2

A prairie ecosystem with 30 plant species experiences a severe drought. Several grass species decline sharply in cover. Compared to a 5-species pasture under the same drought, the most likely outcome in the diverse community is:

• A. Greater collapse, because more species are present to be harmed by the drought
• B. Identical decline, because drought affects all plants regardless of community composition
• C. More stable total biomass, because drought-tolerant species compensate for declining drought-sensitive species
• D. More rapid collapse, because competition among species is disrupted under stress

Answer: (C)

This is the portfolio effect: a diverse community contains species with different drought tolerances, phenologies, and root strategies. When drought-sensitive species decline, drought-tolerant ones compensate, maintaining total ecosystem function. A species-poor community or monoculture has no such insurance — if its dominant species is drought-sensitive, total productivity collapses. Biodiversity stabilizes ecosystem function precisely because different species respond differently to the same environmental perturbation.

Question 3

The relationship between species richness and ecosystem productivity is linear — each species added to an ecosystem contributes equally to total function.

• T. True
• F. False

Answer: False

The relationship is typically saturating, not linear. The first few species added to a barren system each make large contributions by filling distinct ecological roles. As more species accumulate, new additions increasingly overlap functionally with those already present, and each additional species contributes diminishing marginal function. This pattern of diminishing returns reflects functional redundancy — extra species provide insurance (stability over time) rather than immediate proportional productivity gains. The curve rises steeply, then levels off.

Question 4

Diverse ecosystems tend to be more resistant to invasion by non-native species than species-poor ecosystems.

• T. True
• F. False

Answer: True

One mechanism for invasion resistance in diverse communities is more complete resource use: complementarity among resident species means available light, water, and nutrients are more thoroughly exploited, leaving fewer unfilled ecological niches for invaders to colonize. Species-poor systems and monocultures leave more unused resources, creating openings for invaders. This is one of the practical ecosystem services of biodiversity with direct implications for restoration ecology and sustainable agriculture.

Question 5

What is the 'portfolio effect,' and how does it explain why biodiversity promotes ecosystem stability over time?

Model answer: The portfolio effect is the ecological analog of financial diversification: just as a diversified investment portfolio is less volatile than any single stock (because different assets respond differently to market conditions), a diverse community is more stable than a monoculture because different species respond differently to environmental perturbations. When drought, disease, or temperature extremes reduce some species, others with different tolerances compensate, and total ecosystem function remains relatively stable. A monoculture's fate is tied entirely to one species under all conditions — there is no compensatory mechanism.

The analogy to financial portfolios is mathematically precise: the variance of a sum of imperfectly correlated variables is less than the sum of their individual variances. Because species do not all respond identically to environmental variation, total community biomass or productivity fluctuates less than any individual species' abundance — provided sufficient diversity is maintained.