Portfolio Effect: Biodiversity & Ecosystem Stability (2D)

Every thin line in the top chart is its own noisy, mean-reverting random walk — yet the thick line tracking their sum barely wobbles once enough of them are fluctuating independently. That gap is the portfolio effect (insurance hypothesis): statistical averaging across many imperfectly-correlated species stabilizes total ecosystem biomass even though no individual species became any less volatile. Drag species richness and the cross-species synchrony slider to watch the measured stabilization ratio converge on the closed-form prediction √[(1+(S-1)ρ)/S] — the lower chart plots that measured curve live against theory across S = 2..20, and pushing synchrony toward 1 flattens it, erasing the buffering entirely.