  Despite its radical assumption of ecological equivalence between species,
neutral biodiversity theory can often provide good fits to species abundance
distributions observed in nature. Major criticisms of neutral theory have
focused on interspecific differences, which are in conflict with ecological
equivalence. However, neutrality in 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-neutral models, and it is not known whether demographic stage
structure affects macroecological patterns in neutral theory. Here we present a
two-stage neutral model 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 neutral theory. 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 eusocial insects and humans, and partially rehabilitate
species abundance distributions from past criticisms as to their inability to
distinguish between biological mechanisms.
