Unequal Contribution of Widespread and Narrow-Ranged Species to Botanical Diversity Patterns

Abstract

In conservation studies, solely widespread species are often used as indicators of diversity patterns, but narrow-ranged species can show different patterns. Here, we assess how well subsets of narrow-ranged, widespread or randomly selected plant species represent patterns of species richness and weighted endemism in Gabon, tropical Africa. Specifically, we assess the effect of using different definitions of widespread and narrow-ranged and of the information content of the subsets. Finally, we test if narrow-ranged species are overrepresented in species-rich areas. Based on distribution models of Gabonese plant species, we defined sequential subsets from narrow-ranged-to-widespread, widespread-to-narrow-ranged, and 100 randomly arranged species sequences using the range sizes of species in tropical Africa and within Gabon. Along these sequences, correlations between subsets and the total species richness and total weighted endemism patterns were computed. Random species subsets best represent the total species richness pattern, whereas subsets of narrow-ranged species best represent the total weighted endemism pattern. For species ordered according to their range sizes in tropical Africa, subsets of narrow-ranged species represented the total species richness pattern better than widespread species subsets did. However, the opposite was true when range sizes were truncated by the Gabonese national country borders. Correcting for the information content of the subset results in a skew of the sequential correlations, its direction depending on the range-size frequency distribution. Finally, we find a strong, positive, non-linear relation between weighted endemism and total species richness. Observed differences in the contribution of narrow-ranged, widespread and randomly selected species to species richness and weighted endemism patterns can be explained by the range-size frequency distribution and the use of different definitions of widespread or narrow-ranged. We call for a reconsideration of the use of widespread species as an indicator of diversity patterns, and advocate using the full ranges of species when assessing diversity patterns.

Contribution of rare and common species to subterranean species richness patterns

“…Range size of each species was calculated as maximum linear extent (MLE) and defined as linear distance between the two most distant point localities (Gaston, , ), and thereby, difficulties were avoided related to the use of two‐dimensional metrics (Gaston & Fuller, ). In determining MLE of the species, we considered all available species' occurrence records, including those occurring outside our study area (as in van Proosdij et al, ). The proportion of such species in our dataset was 13% in amphipods (19 species) and 0.9% in beetles (four species).…”

“…Species richness, quantified as number of species per geographic unit, is not distributed evenly around the globe (Gaston & Blackburn, ; Zagmajster, Malard, Eme, & Culver, ). Studies of factors that shape species richness patterns (SRPs) explored either various environmental variables (for reviews see Beck et al, ; Field et al, ; Gaston & Blackburn, ; Stein, Gerstner, & Kreft, ) or species traits, in particular sizes of species' distribution ranges (Heegaard, Gjerde, & Saetersdal, ; Lennon, Beale, Reid, Kent, & Pakeman, ; Lennon, Koleff, Greenwood, & Gaston, ; Reddin, Bothwell, & Lennon, ; van Proosdij, Raes, Wieringa, & Sosef, ). In most taxonomic groups, species with small ranges (hereafter rare species) are more numerous than species with large ranges (hereafter common species) (Gaston, , , ).…”

“…Indeed, after correction for the amount of information each species adds into the analyses, some studies yielded opposite conclusions that rare species are more important in shaping overall SRPs (Heegaard et al, ; Lennon et al, , ; Reddin et al, ). The relative importance of rare and common species may change with the spatial scale of the analyses, but depend also on the studied taxon (Heegaard et al, ; van Proosdij et al, ). Overall, the contradictory results coupled with relative paucity of explicit studies, call for further testing of the hypothesis stating that common species shape SRPs.…”

“…A striking characteristic of subterranean fauna is an exceptionally high proportion of extreme endemics, that is, species limited to few square kilometers or even one site Niemiller & Zigler, 2013;Trontelj et al, 2009;Zagmajster et al, 2014). For example, in different parts of United States, 20%-45% of subterranean species are single-site endemics (Christman, Culver, Madden, & White, 2005;Niemiller & Zigler, 2013), while in France, 38% of aquatic subterranean species have latitudinal linear range extents <3 km (Ferreira, Malard, Dole-Olivier, & Gibert, 2007).…”

The global biogeography of lizard functional groups

“…Much effort has been directed toward studying the spatial richness pattern of particular taxa (e.g., Ceballos, Ehrlich, Soberon, Salazar, & Fay, 2005;Grenyer et al, 2006;Jetz, Thomas, Joy, Hartmann, & Mooers, 2012;Roll et al, 2017). Analysing an assemblage that contains wide variation in traits may mask the processes underlying richness patterns (Proosdij, Raes, Wieringa, & Sosef, 2016). Lumping all species in an assemblage implicitly assumes that species richness of all sub-groups responds to similar underlying drivers.…”