Abstract
Global patterns of species and evolutionary diversity in plants are primarily determined by a temperature gradient, but precipitation gradients may be more important within the tropics, where plant species richness is positively associated with the amount of rainfall. The impact of precipitation on the distribution of evolutionary diversity, however, is largely unexplored. Here we detail how evolutionary diversity varies along precipitation gradients by bringing together a comprehensive database on the composition of angiosperm tree communities across lowland tropical South America (2,025 inventories from wet to arid biomes), and a new, large-scale phylogenetic hypothesis for the genera that occur in these ecosystems. We find a marked reduction in the evolutionary diversity of communities at low precipitation. However, unlike species richness, evolutionary diversity does not continually increase with rainfall. Rather, our results show that the greatest evolutionary diversity is found in intermediate precipitation regimes, and that there is a decline in evolutionary diversity above 1,490 mm of mean annual rainfall. If conservation is to prioritise evolutionary diversity, areas of intermediate precipitation that are found in the South American ‘arc of deforestation’, but which have been neglected in the design of protected area networks in the tropics, merit increased conservation attention.
Highlights
Our understanding of the influence of evolutionary history on major gradients in the global distribution of biodiversity is framed by two hypotheses
Contrary to predictions from the OTH or TCH, whereby highest lineage diversity would be found in wetter regions, we find that communities in areas with intermediate MAP have the highest lineage diversity (Fig. 2)
The mismatch that we uncovered between higher species richness in wetter environments[4,5,6,7] and higher lineage diversity in areas under intermediate MAP (Fig. 2a,b) indicates that the distribution of evolutionary diversity in neotropical tree communities might not be as simplistic as previously thought, based on studies with limited coverage of environmental gradients[19,20,32]
Summary
Our understanding of the influence of evolutionary history on major gradients in the global distribution of biodiversity is framed by two hypotheses. Phylogenetic conservatism for dry biomes has been demonstrated for multiple plant clades, and these can be tens of millions of years old[15] If these clades spill out of dry extremes into areas with intermediate precipitation to coexist with members of the majority of angiosperm clades that prefer high rainfall environments, we may expect areas with intermediate precipitation to have higher amounts of evolutionary diversity because they can contain specialised lineages from both extremes. We use a database of 2,025 tree communities from moist forests to savannas and dry woodlands, covering the full breadth of environmental space of lowland tropical South America (http://neotroptree.info) We combine it with a new time-scaled molecular phylogeny for 852 angiosperm genera
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