Patterns of diversity varied across the 15 studies (Figure 3). Many
studies showed little diversity change, particularly studies of
long-term forest plots (Bradford et al. 2014). In these studies, total
diversity change is typically zero and the contribution of each
mechanism is negligible. Other studies are far more dynamic such as
communities experiencing disturbance regimes (Webb and Scanga 2001,
Thorn et al. 2016) or regeneration (Venturoli et al. 2011) experienced
strong rarity shifts. In these studies, all mechanisms could make a
substantial contribution to diversity change. Ernest et al. (2009)
studies seasonal flowering xeric shrubland communities in the deserts of
Arizona each summer and winter. Immigration had an unusually strong
strongly influenced diversity. Immigration increased diversity strongly
in some plots and decreased diversity strongly in others. The high
immigration rate reflected the high turnover in species, with the
dominant species frequently being replaced, largely due to the dramatic
fluctuations in precipitation (Baldwin et al. 2002). A similar
experiment examines flowering plant communities in the desert of Arizona
within plots where small mammal have been excluded (Ernest et al. 2009).
Immigration tends to increase diversity, rarity shifts tend to decrease
diversity and selection appeared to take a wide range of positive and
negative values. In this community, the most abundant species is the
Creosote bush, Larrea tridentata . This species is perennial and
its abundance changes little over the course of the study. Many of the
rarer species are annuals whose abundances fluctuate dramatically from
one season to another.
Figure
3: Partitions of Shannon diversity change across all plots and time
periods for each of the 15 studies. Diversity change (purple) is
partitioned into selection (red), immigration (green) and rarity shifts
(blue) as is described in Equation 4. The dotted line represents zero
diversity change. To improve readability, we set the x-axis from -.5 to
.5, excluding (3.7%) of values. See supporting information for more
details and Gini-Simpsons diversity comparison.
Generalized Additive Models indicate that there is a non-linear
relationship between the strength of rarity shifts related and the
strength of other mechanisms. Rarity shifts were close to zero when
selection was zero, but tended to become negative when selection was
non-zero. Rarity shifts were close to zero when immigration was close to
zero and was slightly positive when immigration was non-zero (Figure 4).
Each term in each model required a relatively complex spline with many
effective degrees of freedom (selection edf = 7.8, immigration edf =
8.8). Only a modest amount of variation was explained by the model
(R2adjusted = 0.345). Supporting
information shows similar trends in Gini-Simpsons diversity.