Variation in ERR slopes and metrics of richness and distribution among families
When grouping all species together, richness was highest within lowland tropical forests and average range-sizes increased with elevation (i.e. in support of Rapoport’s rule), with differences emerging when comparing individual families. Average elevational range-sizes significantly corresponded with elevation (ERR) for 56 out of the 60 plant families examined (see Supporting information). Observed ERR slope values (without scaling) for individual families were mostly positive, averaging 0.28 across families, but ranged from negative (- 0.13 Asteraceae) to positive (0.62 Celastraceae). Among families, there was considerable variation in the predictive metrics explored (see Supporting information). For Malesian richness , the number of species ranged from 66 (Podocarpaceae) to 3015 (Orchidaceae), with differences in the skew and shape of richness distribution curves across the elevational gradient.
Families with a high localized richness ratio were mostly affiliated with lowland tropical forests, with the exception of Fagaceae and Podocarpaceae. Low localized richness ratio families included Begoniaceae and Gesneriaceae. Values for the metric elevational extent were comparable to those for the localized richness ratioin that families with restricted distributions tended to have concentrated richness, but differed in that elevational extentidentified families which span the majority of the elevation gradient, such as Asteraceae with an elevational extent of ~ 4500 m. Families with peaks in richness near sea-level included trees or shrubs (Anacardiaceae, Burseraceae, Calophyllaceae, Dipterocarpaceae, Ebenaceae, Myristicaceae, Myrtaceae, Sapindaceae), palms (Arecaceae), and mixed growth forms (Acanthaceae, Apocynaceae, Celastraceae, Euphorbiaceae, Fabaceae, Malvaceae, Pandanaceae, Polygalaceae, Sapotaceae). Families with peaks in richness towards the middle of the elevational gradient, transitioning from premontane to lower montane cloud forests, included Fagaceae, Cyatheaceae, and Orchidaceae. Families with the highest elevationpeaks in richness , often in habitats ranging from upper montane cloud forests to subalpine thickets, included woody growth forms (Ericaceae, Podocarpaceae, Rosaceae), ferns (Polypodiaceae, Aspleniaceae) and mosses, all of which also had the highest (lower or upper ) average elevational limits .
Each family had at least one endemic species per biogeographical region, but endemism proportions differed among families and locations (Figs. 1; see Supporting information). Families with the highest Sundaland endemism ( > 50% of species) included Dipterocarpaceae, Fagaceae, Polygalaceae, Calophyllaceae, Annonaceae, Ebenaceae, Anacardiaceae, Gesneriaceae, Zingiberaceae, and Arecaceae. Using the same threshold forSahul endemism , notable families included Ericaceae and Solanaceae. Wallacea had fewer endemics, with maximum endemism values of 26.5% for Acanthaceae, Higher cosmopolitanism values were noted for ferns (Hymenophyllaceae), Podocarpaceae, and taxonomic groups not included in cross-family analyses, such as Bryophytes, grasses (Poaceae) and sedges (Cyperaceae).
For the pPCA analysis using a dozen factors (we excluded metric of overlap), we found that the first two loadings explained 62% of the data variance (Table 1, Fig. 2, see Supporting information), with pPC1 accounting for ~ 43% and PC2 ~ 19%. Lower pPC1 scores were associated with greater Sundaland endemism(represented by magenta), or high localized richness ratiovalues. At the other end of the spectrum included montane affiliated families, based on locations of richness peaks, the lower or upper limits of occurrence, and Sahul endemism (represented by cyan). When the components were plotted (Fig. 2), this created a spectrum of colors across the horizontal axis, with Dipterocarpaceae and Anacardiaceae having the lowest pPC1 scores and Ericaceae and Asteraceae having the highest pPC1 values. Positive pPC2 values were ambiguous (associated with age, richness, or Wallacea endemism ) with less distinction between families. However, negative pPC2 values included metrics of cosmopolitanism and broad latitudinal extent . Families with strongly negative pPC2 values included Fabaceae, Convolvulaceae, Lamiaceae, Vitaceae, and Moraceae, which are families noted for global agriculture use, including sweet potato, legumes, grapes, breadfruit, and aromatics.
As a post-hoc investigation of the interplay between factors we examined correlations between Sundaland endemism and latitudinal extent (r = - 0.44, P < 0.0017), with the negative relationship reinforcing species’ tropical affiliation within this biogeographic region. In contrast, we found positive correlations between latitudinal extent and Wallacea overlap (r= 0.54, P < 0.001), as well as cosmopolitanism(r = 0.57, P < 0.001), with the latter two factors also correlated to each other (r = 0.83 , P< 0.001) suggesting a broad tropical-subtropical species interchange within this centrally positioned biogeographical region.Latitudinal extent and familial elevational extent also had a positive relationship (r = 0.49, P < 0.001; see Supporting information), reinforcing our prediction that larger range-sizes are indicative of tolerance to variations habitat conditions.