Figure 1: Gradual changes in diversity may obscure strong changes in
relative abundance, as shown between 1994 and 1999 (indicated by grey
bands) in Santa Genebra Forest, Brazil (Farah et al. 2014). A)
illustrates gradual changes in relative abundances for speciesIxora gardneriana (orange) and Psychotria vauthieri(yellow). B) illustrates comparatively rapid changes in relative
abundances for species Croton floribundus (orange) andPlinia cauliflora (yellow). C) The gradual changes in abundances
in A lead to relatively small changes in diversity. Here, the arrow
indicates the change in the community with the diversity change
highlighted in grey. D) The rapid changes in abundances in B also lead
to a surprisingly weak effect on diversity change. E) Partitions total
change in diversity into an effect of selection, and rarity shifts
(blue, but barely visible). This highlights the primary role of
selection in the example presented in A. F) In contrast, partitioning of
change in example B shows selection increases diversity, while rarity
shifts simultaneously decrease diversity. This interplay of selection
and rarity shifts leads to a modest change in diversity.
To better understand the importance of rarity shifts we analyse 15
terrestrial vascular plant studies. Vegetation plots are ideal for our
purposes because plants are sessile, meaning new arrivals are unlikely
to be confused with descendants (Table 1). We use partitioning to
decompose total diversity change into selection, immigration, and rarity
shifts. We then address three questions: 1) Are rarity shifts a large
component of diversity change? We address this by comparing the
distribution of measured rarity shifts to overall diversity change
across all observations. 2) Are rarity shifts equally common across
studies? We address this by contrasting the distribution of rarity
shifts across each study. 3) We then asked how the strength of rarity
shifts changes with the strength of the other mechanisms? We address
this by using a generalized additive model to predict when rarity shifts
are most common.