A mismatch between community assembly and abundance-based diversity indices

Abstract

Long-term ecological studies have consistently reported slower than expected changes in biodiversity over time. One explanation for this phenomenon is that commonly used diversity measurements such as species richness are too coarse to detect mechanisms shaping community assembly. Theory suggests that similar phenomena may occur in abundance-based diversity measurements, but the extent of this problem is currently unclear. We confront this theoretical prediction with field data by studying temporal changes in abundance-based diversity indices across 3341 observations from 880 plots in 15 long-term vegetation plot studies. We then partition diversity change into mechanisms of interest to ecologists: selection, drift, and immigration. We show that these resulting changes in relative abundances often produce non-linear changes in diversity. These non-linearities lead counterintuitive effects that are easy to miss when directly analysing changes in diversity. To resolve this we show that explicit partitioning of diversity change leads to one further, less-studied partition: ‘rarity shifts'. Rarity shifts measure how an individual's contributions to diversity changes over time. These rarity shifts are responsible for counter-intuitive effects, for example when radical changes in community composition lead to negligible changes in diversity. We found rarity shifts are an important component of diversity change across many studies. Furthermore, rarity shifts tend to oppose selection, drift and immigration. Therefore, rarity shifts explain why changes in relative abundance do not consistently result in changes in abundance-based diversity measurements. Ultimately, using rarity shifts can lead to a more accurate understanding of the temporal rate and nature of diversity change in ecology and conservation.