How pulse disturbances shape size-abundance pyramids

Ecological pyramids represent the distribution of abundance and biomass of
living organisms across body-sizes. Our understanding of their expected
shape relies on the assumption of invariant steady-state conditions.
However, most of the world’s ecosystems experience disturbances that keep
them far from such a steady state. Here, using the allometric scaling
between population growth rate and body-size, we predict the response of
size-abundance pyramids within a trophic guild to any combination of
disturbance frequency and intensity affecting all species in a similar
way. We show that disturbances narrow the base of size-abundance pyramids,
lower their height and decrease total community biomass in a nonlinear
way. An experimental test using microbial communities demonstrates that
the model captures well the effect of disturbances on empirical pyramids.
Overall, we demonstrate both theoretically and experimentally how
disturbances that are not size-selective can nonetheless have
disproportionate impacts on large species.