The impact of species-neutral stage structure on macroecological patterns

Despite its radical assumption of ecological equivalence between species, neutralbiodiversity theory can often provide good fits to species abundance distributions observed in nature. Major criticisms of neutraltheory have focused on interspecific differences, which are in conflict with ecological equivalence. However, neutralityin nature is also broken by differences between conspecific individuals at different life stages, which in many communities may vastly exceed interspecific differences between individuals at similar stages. These within-species asymmetries have not been fully explored in species- neutralmodels, and it is not known whether demographic stage structure affects macroecologicalpatterns in neutraltheory. Here, we present a two-stage neutralmodel where fecundity and mortality change as an individual transitions from one stage to the other. We explore several qualitatively different scenarios, and compare numerically obtained species abundance distributions to the predictions of unstructured neutraltheory. We find that abundance distributions are generally robust to this kind of stage structure, but significant departures from unstructured predictions occur if adults have sufficiently low fecundity and mortality. In addition, we show that the cumulative number of births per species, which is distributed as a power law with a 3/2 exponent, is invariant even when the abundance distribution departs from unstructured model predictions. Our findings potentially explain power law-like abundance distributions in organisms with strong demographic structure, such as eusocialinsects and humans, and partially rehabilitate species abundance distributions from past criticisms as to their inability to distinguish between biological mechanisms.