Results
All mechanisms contributed substantially to the 3341 observations of
diversity change examined. For Shannon entropy, selection had a
substantial effect on diversity change (Figure 2 A). Selection could
either increase or decrease diversity (median: 0, inter quartile range:
-0.025 - 0.061). Immigration tended to increase diversity (median:
0.024, inter quartile range: 0 - 0.085) (Figure 2 B), Intuitively,
immigration increases diversity because the number of species increases
as new species enter a community. Immigration occasionally decreased
diversity when highly diverse communities were replaced by a new
community of immigrants with lower diversity, or when the new immigrant
species became the most dominant species. Rarity shifts commonly
decreased diversity (median: -0.011, inter quartile range: 0.072 - 0)
(Figure 2 C). Total diversity change was centered on zero (median: 0,
inter quartile range: -0.065 - 0.124) (Figure 2 D). Gini-Simpsons
diversity showed similar trends (Supporting information).