Microbial Ecosystems Reveal a Universal Signature of Ecological Assembly

Microbial communities obey universal macroecological scaling laws, but which sub-processes generate them remains debated across competing theoretical frameworks. Here we calibrate a modified Yule-Simon model to metagenomic data from 11 distinct microbial environments, revealing that microbial ecosystems occupy a qualitatively distinct region of a two-parameter mechanistic space, characterized by near-neutral recruitment (i.e., linear preferential attachment) and broad diversification strategies, unlike any previously studied complex system, including prokaryotic proteomes and urban economies. This distinctive position, confirmed analytically and validated against sparse-data robustness tests, provides both a mechanistic explanation for observed self-similarities in microbial rank-frequency distributions and a quantitative signature of ecological assembly.