Population demography maintains biogeographic boundaries

Abstract

Global biodiversity is organised into biogeographic regions that comprise distinct biotas. The contemporary factors maintaining differences in species composition between regions are poorly understood. Given evidence that populations with sufficient genetic variation can adapt to fill new habitats, it is surprising that more homogenisation of species assemblages across regions has not occurred. Theory suggests that expansion across biogeographic regions could be limited by reduced adaptive capacity due to demographic variation along environmental gradients, but this possibility has not been empirically explored. Using three independently curated data sets describing continental patterns of mammalian demography and population genetics, we show that populations near biogeographic boundaries have lower effective population sizes and genetic diversity, and are more genetically differentiated. These patterns are consistent with reduced adaptive capacity in areas where one biogeographic region transitions into the next. That these patterns are replicated across mammals suggests they are stable and generalisable in their contribution to long‐term limits on biodiversity homogenisation. Understanding the contemporary processes that maintain compositional differences among regional biotas is crucial for our understanding of the current and future organisation of global biodiversity.

High-resolution density assessment assisted by deep learning of Dendrophyllia cornigera (Lamarck, 1816) and Phakellia ventilabrum (Linnaeus, 1767) in rocky circalittoral shelf of Bay of Biscay

“…Finally, QGIS software (version 3.22; QGIS, London, UK) was used to create maps based on the species density data obtained from each transect image point. We applied a symbology scheme based on graduated symbol sizes to represent density categories ( Schmidt et al, 2022 ). The density data series was classified into intervals according to their values, and each interval was assigned a corresponding symbol size, with larger sizes indicating higher densities.…”

Determinants of genetic diversity and species richness of North American amphibians

“…These hypotheses are not mutually exclusive; total energy availability and heterogeneity are thought to interactively produce biodiversity patterns (Kerr & Packer, 1997; Schmidt, Dray, & Garroway, 2022). Although the more individuals and heterogeneity hypotheses both invoke population processes, they are rarely extended to or tested on the genetic level (but see Lawrence & Fraser, 2020; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022).…”

“…Such macroecological patterns ultimately arise from processes operating at the population level (e.g. demographic factors; Charlesworth et al, 1982; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022); thus, we might achieve a more cohesive understanding of the general factors that underlie species richness gradients across taxonomic groups by linking them to microevolutionary processes. Biogeographic patterns of population genetic diversity can suggest links between population processes and macroecological patterns (Lawrence & Fraser, 2020; Leigh et al, 2021; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022).…”

“…demographic factors; Charlesworth et al, 1982; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022); thus, we might achieve a more cohesive understanding of the general factors that underlie species richness gradients across taxonomic groups by linking them to microevolutionary processes. Biogeographic patterns of population genetic diversity can suggest links between population processes and macroecological patterns (Lawrence & Fraser, 2020; Leigh et al, 2021; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022). Broad‐scale patterns of population genetic biodiversity across species have only recently begun to be mapped thanks to the accumulation of open data in public repositories.…”

“…The causes of population genetic diversity are rarely studied at the same time or scale as patterns of species richness (but see Marshall & Camp, 2006; Schmidt, Dray, & Garroway, 2022; Schmidt, Muñoz, et al, 2022), yet the presumed mechanisms related to the more individuals hypothesis and heterogeneity hypotheses are closely related to population‐level processes (Schmidt, Dray, & Garroway, 2022). The more individuals hypothesis predicts a positive relationship between species richness and population genetic diversity because bigger populations and communities tend to have higher levels of genetic and species diversity, respectively (Hubbell, 2001; Kimura, 1983; Schmidt, Dray, & Garroway, 2022).…”