Biogeographic affiliation and centers of richness as predictors of elevational range-size patterns for Malesian flora

Our goal was to interrogate the idea that “mountain passes are higher in
the tropics” by investigating ecological and biogeographic drivers of
elevational range-sizes patterns among equatorial flora. We used herbarium
records for sixty species-rich plant families, representing 18535 species
total, to estimate distributions over a 4500 m elevational gradient. For
each family, we estimated the change in average range-sizes with
increasing elevation (i.e. Rapoport’s rule, abbreviated as ERR) and
quantified 15 metrics of familial richness distribution, evolutionary age,
and biogeographic affiliation. We visualized covariation across families
using phylogenetic principal components analysis (pPCA). We then evaluated
how family-level ERR slopes correlated with each metric individually, as
well as when using multivariate techniques to reduce dimensionality. We
hypothesized that if long term climate stability over millions of years
promotes habitat specialization, then among taxa with longer-term tropical
affiliations, we would expect smaller range-sizes within lowland forests,
with greater range-size expansion towards higher elevations, expressed as
a positive ERR slope. Conversely, variation in growing conditions should
promote larger, relatively consistent, range-sizes at all sections of an
elevational gradient, expressed as a neutral ERR slope. Our results
support this corollary because of the dichotomy of ERR slopes observed in
relation to the elevational distribution of richness and historical
biogeographic positioning. We found that families with greater Sundaland
endemism, or richness that was restricted to tropical lowland forests, had
positive ERR slopes. Families with stronger Sahul affiliation, or montane
centered richness, had shallower, neutral, or negative ERR slopes, as did
clades with temperate origins. Families with Wallacea affiliation, broader
latitudinal or elevational distributions, cosmopolitanism, greater
richness, or older evolutionary age had mixed results. We conclude that
the relative steepness of an ERR slope is an indicator of a taxonomic
group’s tolerance of habitat variation and vulnerability to contemporary
climate change.