The commonness and distribution of rarity: Quantifying the botanical diversity of all plant species in the Americas

Background/Question/Methods

The study of rarity at large scales: (i) can elucidate ecological and evolutionary mechanisms driving larger scale gradients in biodiversity; (ii) has implications for conservation priorities; and (iii) is a measure of risk of extinction to climate change. However, efforts to quantify the total number of species and the nature of rarity at increasingly larger scales is impeded by informatics challenges and taxonomic issues associated with harmonizing species names from disparate data sources. Here we utilize new botanical informatics tools and apply them to a suite differing data sources to quantify, for the first time the diversity and ‘global rarity’ for the plants of the Americas. We, the BIEN working group, combine disparate data sources including: herbaria; ecological plots and surveys, and plant traits. 

Results/Conclusions

The BIEN database consists of 22.5 million observations from 760 data providers. The total number of plant species within the Americas is approximately 120,000. Analysis of random subsets of names indicates that 10% of species names or 12,000 species are naturalized non-native species. Given the estimate of the total number of Embryophytes in the world we estimate that approximately 43% of the estimated global Embryophyte diversity is in the Americas. 

Analysis of the the distribution of observations across these species is highly skewed and reveals that rarity is commonplace. There is wide variation in the number of observations per species. Intriguingly, approximately one half of all of the species recorded for the Americas are rare being recorded five times or less. Further analyses reveal that these results are not driven by sampling biases. Plotting all the observations for rare species (those with 3 observations or less) reveals several patterns. First, the distriubiton of rare species are not uniform but clustered in unique areas. Second, the absolute number of rare species cluster around mountainous regions, especially in the tropics (Andes, the Sierra Madre in Mexico and Central America) indicating that mountainous regions harbor a disproportionate number of the total species in the Americas. Third, globally rare species also cluster in rare habitats includig the Mata Atlantica region along the south eastern coast of Brazil. Lastly, there is a relative dearth of rare species throughout the Amazon confirming past claims that the Amazon consists of widespread abundant species. Together, these results provide several insights into several hypotheses concerning variation in diversity across gradients and the role of topography in maintianing and creating botanical diversity.