Patterns and processes behind the range size distributions of the New World plants

Background/Question/Methods

Geographic range size varies by several orders of magnitude among species and is used as one of the key criteria to determine the conservation status of species. Small- and broad-ranged species are not evenly distributed across geographic space, and, although many hypotheses exist on the drivers of range size, the mechanisms determining why some species are rare and where they occur remain unresolved. Using the largest botanical dataset for the New World to date, we map the characteristics of range size frequency distributions across space for ~90,000 plant species. Furthermore, we assess hypotheses on the importance of ecological, geometric and historical factors in explaining the geographic patterns found using spatially explicit models in an information-theoretic framework.

Results/Conclusions

The distribution of range sizes in the New World shows striking geographic patterns. While northern North America is characterized by mean ranges larger than would be expected by chance, the rest of the two continents show the opposite pattern. We also find that ranges in most of North America and the Amazon basin are skewed towards relatively large sizes, while small-ranged species dominate along the Andes, the Brazilian Mata Atlantica, Central America and the North American Great Plains. Three factors were most important in explaining the patterns of mean range size: 1) temperature seasonality, implying support for Rapoport’s rule, 2) Late-Quaternary climate-change velocity, stressing the importance of low-velocity areas for the conservation of endemics, and 3) the total area of land surrounding each site, indicating the role of geometric constraints on range sizes in addition to environmental factors.