| Most Neotropical species are rare, narrowly distributed, and endemic to particular regions or biomes (see Albert et al., 2011 for fishes; Ter Steege et al., 2016 for plants). Species with low abundances and narrow geographic ranges, as well as those confined to special habitats or areas, represent a sizable portion of Neotropical diversity. Indeed, a recent study extrapolating population size for Amazonian trees suggests that most species in the region are represented by comparatively few individuals (Ter Steege et al., 2013). Another study suggests that a considerable fraction of the rare species in the region may actually have relatively large distribution ranges (Zizka et al., 2017). However, many apparently widespread species in most taxonomic groups have been shown to contain multiple phylogenetic species, a possibility that remains poorly explored in the Neotropics but has important consequences for our understanding of diversity patterns and conservation priorities (Bickford et al., 2007; Colli et al., 2016). The contributions of rare species to diversity patterns are difficult to quantify and remain largely obscure (Coddington et al., 2009), partly because most truly rare species will be completely unknown, and partly because rareness in the ecological sense is hard to define, depending on a variety of aspects, including the species concept adopted and the taxonomic preferences. |
| In both fish and plant taxa, areas of endemism separated by prominent biogeographic barriers, such as Amazonian and Mesoamerican rainforests currently separated by the Andes, arise from dispersal limitation, and differential environmental tolerances (Bemmels et al., 2018). In contrast, for some tree genera of rainforest trees, dispersal does not seem to be a constraining factor, meaning that community assemblages either represent random draws from the possible species pools available (Dexter et al., 2017), or functional differences arising from different habitat tolerances. The geographic distributions of many riverine and floodplain taxa are limited by river basin watersheds, and opportunities for dispersal include river capture events (Albert et al., 2017). Finally, it is not enough to know where particular species occur; we also need to know where these species do not occur (Soria-Auza & Kessler, 2008). It is, therefore, difficult to reliably say if the biodiversity patterns known to date really reflect true patterns or biases in collection effort. Further, patterns of species richness are usually discerned relatively early in the documentation of a newly explored biota, whereas patterns of species endemism are more difficult to discern as they require positive knowledge of both where species are present and absent (Soria-Auza & Kessler, 2008). |
