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.