Empirical abundance distributions are more uneven than expected given their statistical baseline

Exploring and accounting for the emergent properties of ecosystems as complex systems is a promising horizon in the search for general processes to explain common ecological patterns. For example the ubiquitous hollow-curve form of the species abundance distribution is frequently assumed to reflect ecological processes structuring communities, but can also emerge as a statistical phenomenon from the mathematical definition of an abundance distribution. Although the hollow curve may be a statistical artefact, ecological processes may induce subtle deviations between empirical species abundance distributions and their statistically most probable forms. These deviations may reflect biological processes operating on top of mathematical constraints and provide new avenues for advancing ecological theory. Examining similar to 22,000 communities, we found that empirical SADs are highly uneven and dominated by rare species compared to their statistical baselines. Efforts to detect deviations may be less informative in small communities-those with few species or individuals-because these communities have poorly resolved statistical baselines. The uneven nature of many empirical SADs demonstrates a path forward for leveraging complexity to understand ecological processes governing the distribution of abundance, while the issues posed by small communities illustrate the limitations of using this approach to study ecological patterns in small samples.

Automated summary

Exploring and accounting for emergent properties of ecosystems as complex systems is a promising horizon in understanding common ecological patterns. While the ubiquitous hollow-curve form of species abundance distribution may arise as a statistical phenomenon, deviations between empirical distributions and statistical baselines can reflect biological processes and offer new avenues for advancing ecological theory. Empirical abundance distributions are often uneven and dominated by rare species, demonstrating the potential of leveraging complexity to understand ecological processes, but limitations may arise in studying small communities due to poorly resolved statistical baselines.