How many mammal species are there now? Updates and trends in taxonomic, nomenclatural, and geographic knowledge.

The Mammal Diversity Database (MDD) is an open-access resource providing up-to-date taxonomic, nomenclatural, and geographic data for global mammal species. Since its launch in 2018, the MDD has transformed the traditionally static process of updating mammalian taxonomy into regular online releases reflecting the latest published research. To build on this foundation, we here present version 2.0 of the MDD (MDD2), which catalogues 6,759 living and recently extinct mammal species, representing net increases of 4.1% and 24.8% over MDD version 1.0 and Mammal Species of the World, 3rd edition (MSW3), respectively. Additionally, we identify a net increase of 68.8% (+2,754; 3,149 splits + de novo, 395 lumps) species since 1980 at a rate of [~]65 species/year based on past totals from 14 mammalian compendia, leading to projections of [~]7,084 species by 2030 and [~]8,382 by 2050 if these trends continue. Key updates in MDD2 include: (i) codings of US state, country, continent, and biogeographic realm geographic categories for each species; (ii) a comprehensive nomenclatural dataset for 50,230 valid and synonymous species-rank names, curated with type locality and specimen information for the first time; and (iii) integration between the MDD and the databases Hesperomys and Batnames for greater data accuracy and completeness. These updates bridge critical gaps in the taxonomic and nomenclatural information needed for ongoing revisions and assessments of mammalian species diversity. Using these data, we evaluate temporal and geographic trends over the past 267 years, identifying four major time periods of change in mammalian taxonomy and nomenclature: (i) the initial monographic description of traditionally charismatic species (1758-1880); (ii) the peak of descriptive taxonomy, describing subspecies, and publishing in journals (1881-1939); (iii) the shift toward revisionary taxonomy and polytypic species (1940- 1999); and (iv) the current technology-driven period of integrative revisionary taxonomy (2000- present). Geographically, new species recognition since MSW3 has been concentrated in equatorial, mountainous, and island regions, highlighting areas of high mammal endemism (e.g., Madagascar, Philippines, Andes, East Africa, Himalayas, Atlantic Forests). However, gaps in 21st century taxonomic activity are identified in West and Central Africa, India, and some parts of Indonesia. Currently lagging conservation assessments are alarming, with 25% of the MDD2-recognized mammal species allocated to the understudied conservation threat categories of Data Deficient (11%) or Not Evaluated (14%), underscoring the need for greater taxonomic integration with conservation organizations. Governance advancements in MDD2 include the establishment of external taxonomic subcommittees to guide data collection and curation, a rewritten website that improves access and scalability, a cross-platform application that provides offline access, and new partnerships to continue linking MDD data to global biodiversity infrastructure. By providing up-to-date mammalian taxonomic and nomenclatural data--including links to the text of original name descriptions, type localities, and type specimen collections--the MDD provides an integrative resource for mammalogists and conservationists to more easily track the status of their study organisms. Teaser Text: The Mammal Diversity Database 2.0, listing 6,759 mammal species and 50,230 species-level synonyms, unifies 267 years of taxonomic, nomenclatural, and geographic data to track global mammal biodiversity and provide a continually updated resource for the mammalogical community. Teaser Image: Figure 3. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=117 SRC="FIGDIR/small/640393v1_fig3.gif" ALT="Figure 3"> View larger version (34K): org.highwire.dtl.DTLVardef@1012e67org.highwire.dtl.DTLVardef@8d99d7org.highwire.dtl.DTLVardef@6f21b9org.highwire.dtl.DTLVardef@f23fd_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 3.C_FLOATNO Type locality locations for species-rank mammal taxa described from 1 January 2000 to 15 August 2024. Map gray-scale country coloration represents the total number of currently valid species type localities found per country described within this period. Colored dots represent the exact georeferenced type locality of all currently valid species (red circles) and names currently considered synonyms and subspecies (blue diamonds) in MDD2. Most coordinates were generated from the original description of each name when included directly in publication, while others were georeferenced using GeoLocate or WikiMedia GeoHack place coordinates when not included in the original description publication. The georeferenced localities were mapped and both programmatically and visually vetted for accuracy. Latitude and longitude in decimal degrees is included for each of the names mapped here in the MDD2 synonym list, Supplementary Data SD2. C_FIG