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.
It remains unclear whether abundance based diversity measurements are
susceptible to the same problem. To test this, we study temporal changes
in abundance based diversity indices across 3341 observations from 880
plots from 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 mechanisms are an imperfect
predictor of temporal diversity change, creating a mismatch between
changes in species abundances and changes in diversity, particularly
when shifts in diversity are rapid. To resolve this mismatch, we
quantify a less studied mechanism “rarity shifts”, which measure how
an individual’s contributions to diversity changes over time. We found
rarity shifts are an important component of diversity change across many
studies. Furthermore, rarity shifts tend to oppose other mechanisms,
particularly selection. Therefore, rarity shifts obscure changes in
relative abundance from abundance based diversity measurements,
revealing why diversity changes appear slower than expected. Ultimately,
understanding rarity shifts can lead to a more accurate understanding of
the rate and nature of temporal diversity change in ecology and
conservation.