Despite its radical assumption of ecological equivalence between species, neutral biodiversity theory provides good fits to species abundance distributions (SADs) observed in nature. However, individual-level neutrality in nature is broken by differences in fecundity and mortality of conspecific individuals at different life stages, which may vastly exceed interspecific differences between individuals at similar stages. These individual-level asymmetries have not been fully explored in species-neutral models, and it is not known whether demographic stage structure affects macroecological patterns in neutral theory. Here we present a simple two-stage neutral model, where both fecundity and mortality are allowed to change as an individual moves from a stage to the other. We explore different qualitative scenarios, and compare numerically obtained SADs to predictions of unstructured neutral theory.
We find that abundance distributions are resilient to structure as long as subpopulations at different stages fluctuate in synchrony, but they are impacted if adults have sufficiently low fecundity and mortality. In addition, we show that the cumulative number of births per species, known as the progeny distribution, is distributed as a power law with a 3/2 exponent, and is invariant even when the SAD departs from unstructured model predictions. Upon examining abundance data from eight different arthropod groups, we find that a few, including Hymenoptera, fit a stage-structured neutral SAD better than the classical logseries typical of unstructured models. Eusocial insects display strong stage structure marked by sterile workers and reproductive queens, suggesting a possible explanation for these results. Our findings partially rehabilitate the SAD from past criticisms as to its inability to distinguish between biological mechanisms. On the other hand the fact that insects are not likely to undergo neutral dynamics underlines neutral theory’s notorious success in capturing the major drivers of species abundance distributions.