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This paper statistically analyzes the global biomass distribution of wild terrestrial mammals across 4795 species, employing economic inequality tools like the Lorenz curve and Gini coefficient. The analysis reveals a highly unequal distribution, with a Gini coefficient of 0.944, exceeding that of global wealth inequality, and demonstrates that an extended Rayleigh-Jeans spectral model accurately captures this distribution over four orders of magnitude. These findings extend the Wealth Thermalization Hypothesis to biological systems, linking the rarity of large mammals to wealth concentration in human societies.
The extreme rarity of large mammals mirrors the oligarchic concentration of wealth, both explained by a single statistical framework.
Using the recently published global census of the biomass of wild terrestrial mammals, we perform a detailed mathematical statistical analysis of its distribution over $N_s=4795$ species, drawing on tools developed in economics to characterize wealth inequality. We show that the Lorenz curve of the mammal biomass distribution is characterized by a large Gini coefficient $G=0.944$ exceeding the inequality reported for wealth distribution among world countries. This distribution is compared to the predictions of the Wealth Thermalization Hypothesis (WTH), in which species biomass values are treated as energy levels populated according to a Rayleigh-Jeans (RJ) steady-state distribution. We show that an RJ extended spectral model reproduces the real Lorenz and Pareto curves over four orders of magnitude of biomass variation, capturing the strong condensation of biomass among the rare, heavy-bodied species and its near-absence among the vast majority of light-bodied ones. These results extend the WTH framework, previously validated on distributions of human economic origin, to a biological distribution shaped by ecological and evolutionary constraints, and place the extreme rarity of large-bodied mammal species within the same statistical mathematical description as the oligarchic concentration of wealth in human societies. We finally discuss possible ecological mechanisms that may contribute to the observed distribution, including interspecific interactions, food availability, and competition.