Each of the checklists is based on a single taxonomic source that covers all species in a group on a global basis. For the sake of consistency, each of these taxonomic sources is used as the final authority on the validity of species regardless of subsequent taxonomic changes or new discoveries. Although most of these sources also included some information on distribution, all required checking additional sources to confirm ranges. Islands or island groups divided into more than one political unit (such as Borneo, Hispaniola, the Solomons, and the Comoros) almost invariably required such checking.

An endemic species for the purposes of the checklists is considered to be one that currently occurs as a breeding species in a single political unit (for example New Caledonia or South Sudan). Formerly more widespread species that currently survive in only a single country are also considered endemic (for example the Hirola in Kenya). Species that have been introduced to other countries are also considered endemic to their original country of origin (for example South Africa's Bird of Paradise Flower). The situation is more complex for the handful of species in which reintroduction attempts are being made into their former range. For example, the Crested Ibis was included in the checklist as endemic to China until reintroduced birds in Japan successfully bred in 2012.

Species known from border regions and frequently assumed to occur also in similar habitats in a neighboring nation are treated as endemic unless a definite record exists for the second country (an exception is freshwater fishes collected on a stretch of river that serves as a border which are assumed to occur in both countries even if specimens are known only from one bank).

In cases of doubt species are usually included as an endemic. Recently extinct species (such as the Golden Toad in Costa Rica) are included in the lists if they were included in the taxonomic source, both because they may be indicators of areas that may include other endemic species and because some "extinct" species have turned out to be not quite as extinct as had been previously assumed (for example: the Forest Owlet in India).


How accurate are the checklists?

It is safest to assume that most of the longer checklists (and many of the shorter ones) can be improved upon. Although for some island nations the checklists may accurately reflect current knowledge, for many mainland nations the situation is quite a bit messier. In a number of cases a species will be included as an endemic because an inadequate search of the literature by the compiler failed to discover records from other countries.

One complicating factor is a lack of consensus on species boundaries that can result in misidentifications in the literature or differing usages of the same scientific name to mean different things. For example the name Antidorcas marsupialis is most commonly used to refer to an antelope found in several countries (IUCN), but has been used at times in a more narrow sense to refer to a population occuring only in South Africa (Google Books).

A number of species may have been inadvertantly omitted from the list by the compiler not realizing that a name was being used in a different sense. Misidentifications in databases such as GBIF are unfortunately all too frequent and one study (Current Biology) concludes that perhaps as many as half of all museum tropical plant specimens have been misidentified creating additional problems in determining accurate species ranges. Specimen data are subject to a variety of possible location errors (BHL). Museum databases also sometimes misinterpret collection localities to an incorrect modern country (and sometimes continent).

It is also important to remember how much is still unknown to science. Thousands of vertebrate species, tens of thousands of vascular plant species, and millions of invertebrate species still await discovery and scientific description. For example, according to the State of Observed Species report 19,232 new species were described in 2009 (ESF WM pdf). Over 8000 new species of fish have been described since 2004 (Catalog of Fishes). There is often also a long lag time between the first collection of a species and its scientific description. One recent study found that this averages over 21 years (ResearchGate).

Distributions for many species, especially in the tropics, are also incompletely known with much of the world very poorly sampled (NSO pdf). For example, one in seven lizard species have been found solely in the location where it was originally discovered (ResearchGate).

Some parts of the world are also much more poorly known than others - for example only two of the 105 freshwater fish species known solely from Laos had even been described before 1995. A sizeable majority of the species newly described are known from only a single nation. 205 out of 220 (93.2 %) reptile species described in 2019 (according to Reptile Database accessed 25 May 2020) and 138 out of 149 (92.6 %) amphibian species described in 2019 (according to AmphibiaWeb accessed 11 April 2020) are so far known exclusively from a single country.


Political Units

Countries and dependent areas are those recognized by the CIA World Factbook (CIA). Additional accounts appear for species of Unknown Origin, those found solely in International Waters, those unique to the Korean Peninsula, and those restricted to the island of Timor. The Cyprus account includes the nation of Cyprus plus the United Kingdom administered areas of Akrotiri and Dhekelia. Israel, the West Bank, and the Gaza Strip are discussed jointly for practical reasons. Threatened endemic species can be found using the Advanced Search function at the IUCN Red List which includes filters for countries and endemics (near the bottom of the page).


Country Overview Pages

These pages provide introductions to the wealth of resources on the web dealing with the world's animal and plant species. It also allows for the mention of endemic invertebrate and vascular plant species plus additional vertebrate species that are recognized by some authorities or were described since the sources used for the checklists. Countries and dependent areas lacking known endemic species are not given a country overview page.


Mammals

American Society of Mammalogists (2020) Mammal Diversity Database, Version 1.2 https://www.mammaldiversity.org, accessed 24 September 2020.

An online version of the above work can be found here.

Lists of additional species recognized by the Illustrated Checklist of the Mammals of the World (Lynx) are also included. Mammal species unique to a single nation added to the Mammal Diversity Database after this date are periodically included as addenda in the appropriate national checklists.

The MDD recognized 6485 recent mammal species of which 2896 species (44.7 %) are single nation endemics. 307 of 1330 (23.1 %) mammal genera are also restricted to a single country.


Birds

del Hoyo, J. & N.J. Collar. HBW and BirdLife International Illustrated Checklist of the Birds of the World. 2014 Vol. 1: Non-passerines & 2016 Vol. 2: Passerines. Lynx Edicions, Barcelona. The lists are based on the December 2017 update available here.

11,122 bird species are recognized in the December 2017 update of which 3450 (31.0 %) are single nation endemics. Bird species unique to a single nation recognized after the December 2017 update are periodically included as addenda in the appropriate national checklists. Another addenda indicates additional species recognized as valid by one of the following checklists: IOC 10.1 (IOC), Howard & Moore 4.1 (TAS), and eBird/Clements v2019 (Cornell).

367 of 2365 (15.5 %) bird genera are also restricted to a single country. An independent assessment of single nation endemics using the above reference can be found at Avibase (Avibase) which also provides assessments based on several other authorities as well. See also (Lynx pdf). An especially detailed source of distributional information for all the world's birds can be found at BirdLife International (BirdLife Int'l). For recently described bird species see (listaptakow.eko.uj.edu.pl).


Reptiles

Uetz, P., Freed, P. & Hošek, J. (eds.) (2018), The Reptile Database, https://www.reptile-database.org, accessed 14 November 2018. The current online version of the Reptile Database can be found here.

The 14 November 2018 version of the Reptile Database recognized 10,885 reptiles species of which 7060 (64.9 %) are single nation endemics. Reptile species unique to a single nation added to the Reptile Database after this date are periodically included as addenda in the appropriate national checklists. 373 of 1193 (31.3 %) reptile genera are also restricted to a single country. For lists of very recently described reptile species see (Reptile Database) (Reptile Database) or search by year and location at (Reptile Database).


Amphibians

AmphibiaWeb (based on species recognized as of 28 October 2018) The current online version of this database can be found here.

The 28 October 2018 version of AmphibiaWeb recognized 7941 species of which 5816 (73.2 %) are single nation endemics. Amphibian species unique to a single nation added to AmphibiaWeb after this date are periodically included as addenda in the appropriate national checklists. 179 out of 547 (32.7 %) amphibian genera are also restricted to a single country. For lists of recently described amphibian species search by year of publication and country at (Amphibian Species of the World) or see the new species page at AmphibiaWeb (AmphibiaWeb). Lists of endemic species by country can be generated at (Amphibian Species of the World).

Fishes

Fricke, R., Eschmeyer, W. N. & Van der Laan, R. (eds) 2023 Catalog of Fishes: Genera, Species, References. Version downloaded 9 February 2023. The current version can be accessed here

18,535 species of fish are found in freshwater or brackish habitats (including marine species extending into freshwaters, species only known from brackish waters, but not incuding marine species that extend into brackish waters but not freshwater) according to Fricke, et al. 2023. 10,914 (58.9 %) of these species are single nation endemics.

The marine fish checklists include only species that occur mainly in habitats between 0 and 200 m deep and therefore usually do not include bathypelagic and bathydemersal species. Additional lists of deep-sea species (below 200 m) known from a single country are listed at the bottom of each page. Maritime boundaries follow those mapped at (marineregions.org).

About 932 out of 5229 (17.8 %) of fish genera are restricted to a single country. About 15,922 fishes (including deep-sea fishes) are known from a single country out of 36,415 total fish species known (43.7 %) according to Fricke, et al. 2023..

For recently described fish species see (worldfish.de) and search by year and location at (Catalog of Fishes) or by year at (FishBase).


Vertebrate Genera

The sources for the vertebrate genera are the same as those for species. However some genera have been excluded as being endemic based on the ranges of subsequently described or undescribed species. Approximately 2158 out of 10,664 (20.2 %) vertebrate genera are single nation endemics.


Swallowtail & Nymphalid Butterflies

Jonko, C. (2019), Lepidoptera Mundi, https://lepidoptera.eu, accessed 12 January 2020. The current version can be accessed here

These two butterfly families have been selected as examples of invertebrate endemism. 2030 out of 6691 (30.3 %) of these two butterfly families are single nation endemics. National checklists at the species level are available for a number of other invertebrate groups through the 2019 Annual Checklist of the Catalogue of Life using the search by distribution feature (Catalogue of Life).


Vascular Plant Genera

Mabberley, D.J. (2017) Mabberley's Plant-book. Fourth Edition. Cambridge University Press, Cambridge.

Mabberley recognized 13,763 vascular plant genera of which 3300 (24.0 %) are single nation endemics. According to the GlobalTreeSearch database (BGCI pdf), 33,760 out of the world's 58,497 (57.7 %) tree species are single country endemics. Trees are believed to represent about 20 % of all angiosperm and gymnosperm species. Endemic plant species are indicated for most countries in checklists that can be accessed at (worldplants.de). Note that it is also possible to create endemic species checklists for many plant families and countries using the "Build a Checklist" feature at the World Checklist of Selected Plant Families website (Kew). Recently described species can be found using the advanced search function at the International Plant Names Index (IPNI).


Families & Orders

The sources for the vertebrates and vascular plants are the same as given above.

For invertebrates the primary sources are Zhang (2013) and Zhang (2011). Roskov et al. (2014) is used as the main source for groups not covered at the family level by Zhang (2013) and Zhang (2011), but this was supplemented by other sources at times as it also has some gaps (for example with terrestrial gastropods). A number of more recently described families have also been included (usually indicated by year of description).

Out of 7177 families (1182 vertebrates, 5521 invertebrates, and 474 vascular plants), 512 (7.1 %) are single nation endemics.

Zhang, Z.-Q. (Ed.) (2013) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013). Zootaxa 3703: 1–82. (here)

Zhang, Z.-Q. (Ed.) (2011) Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness. Zootaxa 3148: 1–237. (here)

Roskov Y., Kunze T., Orrell T., Abucay L., Paglinawan L., Culham A., Bailly N., Kirk P., Bourgoin T., Baillargeon G., Decock W., De Wever A., Didžiulis V., eds. (2014). Species 2000 & ITIS Catalogue of Life, 2014 Annual Checklist. Digital resource at www.catalogueoflife.org/annual-checklist/2014. Species 2000: Naturalis, Leiden, the Netherlands.