The following data limitations should be kept in mind when using these data:
The rate of mammal descriptions remains high, with 61 species described in the years 2006 and 2007 alone. Although we have endeavored to include all recently described species, it is possible a few may have eluded our attention, especially if published in obscure media. In general, the cut-off date for including species in the 2008 IUCN Red List was Dec 31st, 2007; however, a few species described in 2008 (Rhynchocyon udzungwensis, Miniopterus petersoni, Cacajao hosomi and C. ayresi) are included. A single species currently in press (Mico sp. nov.) has also been included.
Some parts of the world remain poorly known in terms of their mammal faunas, including, for example, the Andes, most of Central Africa and parts of West Africa, Angola, parts of South and Southeast Asia, and Melanesia. In addition, many species' names, especially in the tropics, actually represent complexes of several species that have not yet been resolved. For our purposes, and pending the availability of published information to the contrary, these are treated as single species, until resolution of their taxonomic status is published.
Domestic species (for example Dromedary Camelus dromedarius) are not included in the assessment.
Because of the conservative approach taken in mapping species, the ranges for many are likely to be minimum estimates of the limits of species' distributions. A rule was followed allowing interpolation of occurrence between known localities if the ecological conditions seemed appropriate, but not permitting extrapolation beyond known localities. In other words, to the best of our knowledge, maps represent current known limits of distribution within historical native range (any introductions are coded accordingly, and are excluded for the purposes of analysis), with the obvious caveat that species occurrence is not homogeneous within the polygon. Some species are therefore almost certain to occur more widely than mapped. Because of this, some regions are recorded as having lower mammal diversity than may eventually prove to be the case. On the other hand, species' ranges were mapped as generalized polygons which often include areas of unsuitable habitat, and therefore species may not occur in all of the areas where they are mapped.
The information on the relative importance of different threatening processes to mammal species is incomplete. We coded all threats that appear to have an important impact, but not their relative importance for each species.
The percentage of mammals assessed as Data Deficient (15%) in 2008 is higher than previously found for mammals on the IUCN Red List in 2004 (7.8%). There are three possible explanations for this. The first has to do with the large number of recently described species for which it is often difficult to determine their real taxonomic and distributional limits. This is particularly the case for many of the recently described lemurs in Madagascar, and hence 42 were assessed as Data Deficient. The second explanation is due to a number of species previously incorrectly listed as Least Concern moving to the category Data Deficient, particularly in the New World. The third is simply due to lack of knowledge that permits a reliable assessment. Because many Data Deficient species are likely to have small distributions or populations, or both, they are intrinsically likely to be threatened. Consequently, in accordance with IUCN guidelines, species assessed as Data Deficient should not be considered as "not threatened". With further survey work and the availablility of improved information, it is anticipated that many of these species, if indeed proven to be valid taxa, will move out of the Data Deficient Category. This is a deliberate precautionary approach in accordance with the IUCN guidelines.
Wilson, D.E., Reeder, D.A. Mammal Species of the World: A Taxonomic and Geographic Reference, 3rd edn (Johns Hopkins Univ. Press, Baltimore, 2005).
Sechrest, W.W. 2003 Global Diversity, Endemism and Conservation of Mammals, Thesis. University of Virginia, Charlottesville, USA.