Global evidence of non-pyramidal and uniform ratios of animal diversity across terrestrial trophic levels.

Thermodynamics imposes a well-established pyramidal distribution of energy availability across trophic levels, but whether species richness follows the same pattern remains unclear. In this study, we examined species richness across trophic groups for all known terrestrial tetrapod and arthropod species, representing over 90% of Earth's terrestrial animal diversity. By categorizing species into fundamental trophic levels, we found that 46% are primary consumers (feeding on plants), 43% are higher-level consumers (feeding on primary consumers) and 11% are mixed consumers (generalists). Further analysis of global community trophic structures in mammals and birds uncovered a consistent ratio of species richness across trophic levels, independent of geographical location or total species richness. We propose that this non-pyramidal distribution of diversity arises from higher ecological differentiation among species at higher trophic levels, which may offset their greater extinction risk associated with smaller populations. This process could generate uniform trophic structures if primarily driven by mechanics intrinsic to trophic positions and their interactions rather than specific environmental characteristics. Such intrinsic mechanisms may ultimately influence diversity across trophic levels and the eco-evolutionary dynamics structuring ecological networks.