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).