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

Our goal was to interrogate Janzen’s idea that “mountain passes are higher in the tropics” by investigating putative ecological and biogeographic drivers of patterns of elevational range-sizes among equatorial plant families. We used herbarium records for sixty species-rich plant families, representing a total of 18535 species, 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 used phylogenetic principal components analysis (pPCA), with color illustrating endemism. We then evaluated how family-level ERR slopes correlated with each metric individually, and the first two pPCA components, using phylogenetic independent contrasts. 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. Families with Wallacea affiliation, broader latitudinal or elevational distributions, cosmopolitanism, or older evolutionary age had mixed results. Our findings support Janzen’s hypothesis that “mountain passes are higher in the tropics.” If long term climate stability over millions of years promotes habitat specialization, then among taxa with long-term tropical affiliations, we would expect smaller range-sizes within lowland forests, with range-size expansion towards higher elevations, expressed as a positive ERR slope. Conversely, variation in growing conditions should promote larger, relatively consistent, range-sizes expressed as a neutral ERR slope. Our results support this corollary because of the dichotomy of ERR slopes observed in relation to elevational and historical biogeographic positioning, which may be an indicator of vulnerability of these plant groups to contemporary climate change.