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

Abstract

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 ~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.

Dietary abundance distributions: Dominance and diversity in vertebrate diets

“…One possibility, which we take seriously given the ubiquity of hollow‐curve abundance distributions in ecology and beyond, is that DAD shape is more of a statistical phenomenon than a biological one. In that case, foraging behaviour, food traits, and environmental factors may merely generate minor variation within fundamental mathematical constraints, as has been proposed for SADs (Diaz et al, 2021). Future work could examine the extent to which empirical DADs deviate from their statistical baselines (Diaz et al, 2021).…”

“…In that case, foraging behaviour, food traits, and environmental factors may merely generate minor variation within fundamental mathematical constraints, as has been proposed for SADs (Diaz et al, 2021). Future work could examine the extent to which empirical DADs deviate from their statistical baselines (Diaz et al, 2021).…”

Consistent patterns of common species across tropical tree communities

“…Nevertheless, consistent patterns of commonness do not necessarily imply the same causal mechanisms. The ubiquity of the broad ‘rare is common, common is rare’ pattern in ecology, which is also found in non-biological complex systems 44 , means inferences as to the cause of this broad pattern are challenging 27 , 45 . Although combinatoric methods 45 and models that maximize the entropy of information 46 , 47 both produce the ubiquitous ‘reverse lazy-J’ pattern, empirical observations show fewer common species and more rare species than expected by statistical controls alone 45 .…”

“…The ubiquity of the broad ‘rare is common, common is rare’ pattern in ecology, which is also found in non-biological complex systems 44 , means inferences as to the cause of this broad pattern are challenging 27 , 45 . Although combinatoric methods 45 and models that maximize the entropy of information 46 , 47 both produce the ubiquitous ‘reverse lazy-J’ pattern, empirical observations show fewer common species and more rare species than expected by statistical controls alone 45 . Similarly, neutral models produce the same broad pattern, but produce too few individuals of the most common Amazonian tree species 48 .…”

Unveiling global species abundance distributions

“…Despite decades of research and dozens of proposed statistical fits to describe SADs 16 , there remains little conclusive evidence for the shape of SADs (compare refs. 6 , 17 – 19 ). The debate surrounding the shape of SADs may be partly driven by the fact that the empirical data on which these distributions are fitted has historically been focused on local-scale biodiversity samples 20 .…”