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

Abstract

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 60 species‐rich plant families, representing 18 535 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.