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.