RESULTS
When we analyzed patterns of alpha diversity including only relatively
uninvaded plots and native species, we found a positive relationship
between precipitation and all three facets of diversity on O’ahu, the
oldest island in our study, but no relationship with precipitation on
the two younger islands, Maui Nui and Hawai’i (Figure 2 a-f and Figure
S4 a and c). This pattern was true regardless of the weighting of common
versus rare species (q=0, 1, 2). When we included plots that were highly
invaded, and measured diversity with both native and alien species, the
pattern differed (Figure 2 g-l). Specifically, we found a positive
relationship between precipitation and taxonomic, functional and
phylogenetic diversity for O’ahu and Hawai’i, but a negative
relationship between precipitation and all three types of diversity on
Maui Nui (Figure 2 g-l; see model estimates and both 85% and 95%
confidence intervals being negative for Maui Nui in Figure S4b). On
O’ahu, the positive relationship between phylogenetic diversity and
precipitation was stronger for Hill number q=0 than q=2 (see model
estimates for phylogenetic q=0 being greater than those of q=2 for O’ahu
in Figure S4 b and d). In all cases, the interaction term between island
age and mean annual precipitation was significant (Table S1).
When we only considered native species in relatively less invaded plots,
we found that species compositional turnover increases with
precipitation differences among plots on O’ahu and Maui Nui (Figure 3a
and Figure S5 a and c). However, on Hawai’i, the compositional turnover
of native species was somewhat negatively related to precipitation
differences among plots (Fig 3a Figure S5 a and c). When we included
plots that were highly invaded, we found that the relationship between
species compositional turnover and precipitation differences among plots
was positive for Hawai’i and O’ahu, but negative for Maui Nui (Figure 3b
and Figure S5 b and d). In all cases, the interaction term between
island age and mean annual precipitation was significant (Table S1)
When we truncated mean annual precipitation to a common range among the
three islands, we found qualitatively similar results (Figure S6).
Furthermore, the significant interaction between precipitation and
island age on diversity remained (and even strengthened) (Figure S7).
Finally, when we redid analyses using a lower level of sample
completeness (30%), we found the relationship of species compositional
turnover with precipitation across islands was different, but
qualitatively showed a similar pattern that turnover among different
precipitation levels was higher on O’ahu) (Figure
S8).