Abstract
One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground-sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness.
Highlights
One of the most fundamental questions in ecology is how many species inhabit the Earth
If hyperdominance of a small fraction of species in the tropics [17] is a general phenomenon, it would mean that these regions generally harbor a very large number of rare species, many of which are endemic
These results highlight the vulnerability of global tree species diversity to anthropogenic changes
Summary
Because of limited available data, estimates of tree diversity at large geographic domains still rely heavily on published lists of species descriptions that are geographically uneven in coverage These limitations have precluded efforts to generate a global perspective. We estimated a global tree richness (≈73,300) that is ≈14% higher than numbers known today, with most undiscovered species being rare, continentally endemic, and tropical or subtropical. These results highlight the vulnerability of global tree species diversity to anthropogenic changes. A lack of saturation (driven by the existence of high numbers of species uncommon in the landscape, incomplete sampling, or both), in the South American accumulation curve (Fig. 2B), suggests that our estimates may still be incomplete accounts of continental and global tree species richness. Our findings are in general agreement with recent studies of Amazonian plant diversity, which suggested that there are many undiscovered species; different approaches to the problem arrived at different estimates of total numbers of known and unknown
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