Species distribution maps are an integral part of Red List assessments, because they inform some of the criteria for the assessments by allowing calculations of extent of occurrence (EOO) and area of occupancy (AOO). They provide a visual representation of where a species occurs, where threats have to occur for a species to be affected by these processes and allow spatial analysis of distribution patterns of species, and other macroecological analyses. Spatial information can also be used to identify conservation priorities, for example by identifying priority areas for conservation and informing conservation policy, identifying gaps in scientific knowledge, and helping to inform business decisions (e.g. where not to expand development).
There are a number of ways in which species distributions can be mapped, for example through gridded maps or localities denoting species’ presence (see Figure 1 for an example from Switzerland’s National Red List of Molluscs) for the approach used to derive species distributions). For IUCN Red List assessments, specific guidance is provided on how to map species distributions to make sure these maps are consistent across taxa. This guidance can be accessed on the IUCN’s species mapping wiki.
Figure 1. Species distribution map for Unio crassus in Switzerland
In summary, for IUCN Red List assessments, and therefore for national Red List assessments which are passed on to the IUCN for inclusion in the global Red List, species distribution maps represent the known or inferred limits of a species’ range as a polygon shape – thus, any area outside the polygon is unlikely to contain the species (Figure 2). This does not mean that the species is distributed equally within the polygon or occurs everywhere in the polygon. In order to establish inferred range of a species, techniques such as habitat suitability modelling may be applied.
Figure 2. Terrestrial species distribution map created for the IUCN Red List (source: IUCN)
Mapping protocols employed by the IUCN differ slightly depending on whether a species is terrestrial, freshwater or marine. For example, distribution maps of freshwater species are generally produced at the catchment level (Figure 3). At present, the IUCN employs the WWF Hydrosheds layer, with an average basin size of 100 km2. This and other resources (including guidance and best practice on species mapping) can be accessed through the IUCN’s species mapping wiki and the spatial resources page on the IUCN Red List website.
Figure 3. Freshwater species distribution map created for the IUCN Red List (source: IUCN)
Species distribution maps are often produced in Geographical Information System software. QGIS is open source GIS software which can be downloaded from the QGIS homepage.
Species distribution maps can also be generated in Google Maps and Google Earth, and the IUCN provides instructions on how to do so here.
In addition, we are happy to help with any queries on distribution mapping, or the subsequent analysis of spatial data.