More-Individuals Hypothesis

More-Individuals Hypothesis (abbreviated MIH, also known as the species–energy abundance mechanism or productivity–abundance–diversity hypothesis) is a foundational theory in macroecology and biogeography formulated by the American ecologist David H. Wright in 1983 (Oikos)—building on earlier conceptual insights by G. Evelyn Hutchinson (1959), LaMont C. Cole (1960), Joseph H. Connell and Eduardo Orias (1964), and James H. Brown (1981)—to explain the positive relationship between available environmental energy (primary productivity) and species richness (S), as well as the global latitudinal diversity gradient. Generalizing Robert H. MacArthur and Edward O. Wilson's (1967) equilibrium theory of island biogeography by replacing physical island area (A) with total available trophic energy (E, such as net primary production or actual evapotranspiration), Wright proposed a three-step causal pathway: (1) Energy → Total Community Abundance (E → N_total): environments with higher available energy flux support a larger total number of individual organisms summed across all co-occurring species within a trophic guild; (2) Total Abundance → Species Population Sizes (N_total → N_i): larger community-wide abundance translates into larger mean population sizes per resident species or allows more populations to exceed the minimum viable population threshold; and (3) Population Size → Extinction Resistance → Species Richness (N_i → ↓E_ext → S): because larger populations experience lower rates of stochastic demographic extinction, communities with more total individuals accumulate and retain a higher steady-state species richness S.

MacArthur and Wilson's Area-Mediated Extinction Model

In the classical equilibrium theory of island biogeography, the number of species S inhabiting an island is governed by the dynamic balance between the immigration rate I(S) of new species from the mainland pool P and the local extinction rate E_ext(S) of resident species on the island. Larger islands (A_large > A_small) harbor more species at equilibrium (Ŝ_large > Ŝ_small) primarily because total community abundance N_total = ρ_ind · A scales linearly with area A at constant individual density ρ_ind. With a larger total pool of individuals N_total partitioned among S species, the average population size per species N̄ = N_total / S is larger on a bigger island, buffering populations against demographic stochasticity, inbreeding depression, and environmental catastrophes and thereby lowering the per-species extinction curve E_ext(S).

Wright's (1983) Species–Energy Generalization

In 1983, David H. Wright observed that physical area A is merely an indirect proxy for the total flow of solar and biochemical energy sustaining a trophic level. Two islands or continental regions of identical area A—such as a tropical rainforest plot in Amazonia and a polar tundra plot of the same size in northern Canada—differ by more than an order of magnitude in net primary production (NPP, measured in g C m⁻² yr⁻¹ or J m⁻² yr⁻¹). Wright defined the total available energy E of a habitat patch or island of area A as the product of area and per-unit-area productivity p_E:

E = A · p_E

Substituting total energy E for area A in Preston's (1962) canonical lognormal species-abundance distribution and MacArthur–Wilson equilibrium theory yields the species–energy power law, directly analogous to the Arrhenius species–area relationship (S = c A^z):

S = k E^z = k ( A · p_E )^z

Passing empirical data for angiosperm plants and breeding landbirds across 24 islands Worldwide (spanning tropical archipelagos to high-Arctic islands), Wright (1983) demonstrated that total island energy E = A × AET (where AET is annual actual evapotranspiration, a direct proxy for vascular plant NPP) explained 70% to 84% of the variance in island species richness—substantially outperforming island area A alone (which explained only 19% to 54% of the variance across latitudes).

Following the quantitative syntheses of Kaustuv Roy, David Jablonski, and James W. Valentine (1998), Diane S. Srivastava and John H. Lawton (1998), Allen H. Hurlbert (2004), and David Storch, Bohumil Sizling, and Kevin J. Gaston (2005, 2018), the More-Individuals Hypothesis can be written as a linked system of three functional relationships relating energy flux E, total community abundance N_total, the species abundance distribution P(N), and equilibrium species richness S:

Step 1: Energy Constraint on Total Community Abundance (E → N_total)

Within a defined trophic guild of organisms sharing an average per-individual metabolic demand B̄ (in watts per individual), the total standing number of individuals N_total = ∑_i=1^S N_i that can be supported by an energy supply rate E_guild (the fraction of NPP assimilated by the guild) is:

N_total = E_guild / B̄ ∝ E^b

Under strict energetic equivalence where mean individual metabolic rate B̄ is invariant across the productivity gradient, b = 1 and N_total scales linearly with available energy E.

Step 2: Partitioning of N_total Across the Species Abundance Distribution

If the total community abundance N_total is partitioned among S species according to a relative abundance distribution f(n) (such as Fisher's log-series S = α ln(1 + N_total/α), Preston's canonical lognormal distribution, or Hubbell's zero-sum multinomial), the mean population size per species is:

N̄ = N_total / S

Furthermore, the number of species whose population size N_i falls below a critical minimum viable population threshold N_min (below which demographic birth–death fluctuations rapidly cause local extinction) is determined by the lower tail ∫₁^N_min S f(n) dn.

Step 3: Abundance-Mediated Extinction Rate and Equilibrium Richness (N_total → S)

From stochastic birth–death branching theory, the expected per-capita demographic extinction rate e(N_i) of a population of size N_i decreases rapidly with N_i (scaling as e(N_i) ∝ N_i^−γ or e^−cN_i under demographic stochasticity). Summing across all S resident species gives the community extinction rate E_ext(S, N_total):

E_ext(S, N_total) = ∑_i=1^S e(N_i) ≈ S · e₀ ( S / N_total )^γ = e₀ S^γ+1 N_total^−γ

Setting the community extinction rate equal to the regional immigration or speciation supply rate I(S) = I₀ (1 − S / P) (or constant speciation/colonization influx I₀ when S ≪ P) yields the equilibrium species richness Ŝ:

e₀ Ŝ^γ+1 N_total^−γ = I₀ ⇒ Ŝ = ( I₀ / e₀ )^1/(γ+1) N_total^γ/(γ+1) ∝ E^z

where z = bγ / (γ + 1) ∈ (0.15, 0.40). Thus, increasing energy E increases N_total, which shifts the community extinction curve downward and drives equilibrium richness Ŝ to a higher intersection point.

Diagnostic Predictions of the More-Individuals Hypothesis vs. Alternative Species–Energy Hypotheses

Support at Regional and Continental Scales

Across broad geographic gradients—such as North American and British breeding bird surveys (Hurlbert, 2004; Evans et al., 2005) and global forest tree plot networks—both steps E → N_total (productivity increases total bird or tree density) and N_total → S (total abundance predicts species richness along a unified sampling curve) receive strong statistical support. In structural equation models of North American avifauna across 1° × 1° grid cells, NDVI/NPP strongly increases total bird abundance N_total, which in turn accounts for a major fraction of the increase in species richness S.

Experimental Microcosm Critiques and the Srivastava–Lawton Paradox

Despite macroecological correlations, fine-scale experimental manipulations—most notably Diane S. Srivastava and John H. Lawton's (1998) experiments on aquatic insect larvae inhabiting water-filled treeholes of varying detrital energy input—identified two empirical puzzles for the simple non-interactive version of MIH:

1. Uncoupled Manipulation of E and N_total: When Srivastava and Lawton experimentally decoupled energy per container from habitat volume, higher detrital energy produced higher total larval abundance N_total during colonization, but artificial manipulations of N_total did not reproduce the species richness predicted by energy alone, indicating that productivity also affected oviposition site selection by adult females.
2. Invariance of Mean Population Size (N̄ = N_total / S): Critics noted that if N_total and S both double along a productivity gradient, the observed mean population size per species N̄ = N_total / S remains nearly constant rather than increasing with E.

Resolution via the Equilibrium Theory of Biodiversity Dynamics (ETBD)

David Storch, Bohumil Sizling, and Kevin J. Gaston (2005, 2018) resolved the N̄-invariance paradox through the Equilibrium Theory of Biodiversity Dynamics (ETBD). They demonstrated mathematically that at dynamic equilibrium between extinction and colonization/speciation, S adjusts until the average per-species extinction rate balances the immigration/speciation rate. Because per-species extinction depends directly on mean population size N̄ = N_total / S, equilibrium species richness Ŝ scales nearly proportionally with N_total precisely so that N̄ returns to the equilibrium threshold dictated by the immigration-to-extinction ratio. Thus, an approximately constant observed N̄ across productivity gradients is not a refutation of the More-Individuals Hypothesis, but rather the expected steady-state signature of abundance-regulated extinction balance once S has equilibrated to N_total.

How does the more-individuals hypothesis differ from a passive statistical sampling effect?

Under a passive sampling effect (the random placement hypothesis of Coleman, 1981), drawing a larger sample of N individuals at random from a fixed regional pool automatically captures more rare species down the tail of the species-abundance distribution within a single census, without any change in population persistence over time. In contrast, the more-individuals hypothesis (Wright, 1983) is a dynamical population-persistence mechanism: higher energy flux sustains larger breeding populations across generations, lowering the real demographic extinction rate E_ext(S) and raising the long-term equilibrium number of coexisting species Ŝ.

Why doesn't the average population size per species (N̄) always increase in high-energy environments?

When available energy E and total community abundance N_total increase in a region, per-species population sizes N_i initially expand and extinction rates drop below immigration/speciation rates. Over ecological and evolutionary time, new species accumulate in the community (S increases) and partition the larger total abundance N_total among more species until mean population size N̄ = N_total / S settles back near the demographic viability threshold where community extinction again balances colonization (Storch et al., 2018).

How does body size or metabolic scaling modify the more-individuals hypothesis?

Because total community abundance scales as N_total = E_guild / B̄, where B̄ ∝ M_body^3/4 exp(−E_a/kT) is the individual metabolic rate governed by body mass M_body and ambient temperature T, any systematic shift in average body size or temperature across a latitude or productivity gradient alters the conversion of energy E into individual numbers N_total. If higher productivity favors larger-bodied species (which have higher per-capita metabolic demands B̄), total individual abundance N_total increases sublinearly with E (b < 1), dampening the MIH response.

1. Wright, D. H. (1983). "Species-energy theory: An extension of species-area theory." Oikos, 41(3), 496–506. https://doi.org/10.2307/3544109
2. Srivastava, D. S., & Lawton, J. H. (1998). "Why more productive sites have more species: An experimental test of theory using tree-hole communities." The American Naturalist, 152(4), 510–529. https://doi.org/10.1086/286187
3. Hurlbert, A. H. (2004). "Species–energy relationships and habitat complexity in bird communities." Ecology Letters, 7(8), 714–720. https://doi.org/10.1111/j.1461-0248.2004.00630.x
4. Evans, K. L., Warren, P. H., & Gaston, K. J. (2005). "Species–energy relationships at the macroecological scale: A review of the mechanisms." Biological Reviews, 80(1), 1–25. https://doi.org/10.1017/S1464793104006517
5. Storch, D., Bohdalková, E., & Okie, J. (2018). "The more-individuals hypothesis revisited: The role of community abundance in species richness regulation and the productivity–diversity relationship." Ecology Letters, 21(6), 920–937. https://doi.org/10.1111/ele.12941