Biomes as evolutionary arenas: convergence and conservatism in the trans-continental Succulent Biome

Aim: Biomes are globally-distributed, structurally and functionally
similar vegetation units, but there is debate about whether these
similarities are superficial, and about how biomes are defined and mapped.
We propose that combined assessment of evolutionary convergence of plant
functional traits and phylogenetic biome conservatism provides a useful
approach for characterising biomes. We focus on the little-known succulent
biome, a trans-continentally distributed assemblage of succulent-rich,
drought-deciduous, fire-free forest, thicket and scrub vegetation as a
useful exemplar biome to gain insights into these questions. Location:
Global lowland (sub)tropics. Time period: Present. Major taxa studied:
Angiosperms. Methods: We use a model-ensemble approach to model the
distribution of 884 species of stem succulents, a plant functional group
representing a striking example of evolutionary convergence. Using this
model, phylogenies, and species occurrence data, we quantify phylogenetic
succulent biome conservatism for ten non-succulent trans-continental plant
clades including prominent elements of the succulent biome, representing
over 800 species. Results: The geographical and climatic distributions of
stem succulents provide an objective and quantitative proxy for mapping
the distribution of the succulent biome. High fractions of succulent biome
occupancy across continents suggest all ten non-succulent study clades are
phylogenetically conserved to the succulent biome. Main conclusions: The
trans-continental succulent and savanna biomes both show evolutionary
convergence in key biome-related plant functional traits. However, in
contrast to the savanna biome, which was apparently assembled via repeated
local recruitment of lineages via biome shifts from adjacent biomes within
continents, the succulent biome forms a coherent trans-continental
evolutionary arena for drought-adapted tropical biome conserved lineages.
Recognizing the important functional differences between the
succulent-rich, grass-poor, fire-free succulent biome and the
grass-dominated, succulent-poor, fire-prone savanna biome, and defining
them as distinct seasonally dry tropical biomes, occupying essentially
non-overlapping distributions, provides critical insights into tropical
biodiversity and the extent of biome stasis versus biome shifting.