Mapping species distributions: living with uncertainty

Abstract

news and update ISSN 1948‐6596 commentary Mapping species distributions: living with uncertainty decreases with time because of range changes associated with factors such as climate and land‐ use change, habitat degradation, biological inva‐ sions, etc. Moreover, changes in taxonomic status resulting from reclassifications (Hey et al. 2003) may mean that even if the assemblage remains unchanged, the quality of the information de‐ grades. Thus, the utility and accuracy of informa‐ tion about species occurrences to describe current distributions depends upon when the database is consulted. This process of information decay also applies to data based on expert assessments, such as range maps, which are ultimately based on knowledge coming from surveys and observa‐ tions. Finally, the decay in community similarity with increasing geographic distances is a well known pattern in biogeography (Nekola & White 1999). The main drivers of this pattern are envi‐ ronmental gradients (e.g. climate, soil chemistry, etc.), spatial configuration of habitats and popula‐ tion aggregation (Morlon et al. 2008). Species dis‐ tribution maps are typically based on the spatial extrapolation of survey data through expert as‐ sessment or species distribution modeling (Rocchini et al. 2011). It follows that their accu‐ racy for unsurveyed regions will decrease with distance from the original samples, and (in the absence of comprehensive coverage of the study area) our knowledge about the actual distribution of the species may be poor. The proposed Map of Life project aspires to create a type of omniscient GIS for all the Earth’s living species – such as the one imagined by Colwell & Coddington (1994). This is a wonderful target, with the potential to dramatically improve the spatial and temporal resolution of biodiversity data. Nevertheless, the biodiversity data that un‐ derpin any such enterprise have intrinsic and un‐ avoidable limitations that result in loss of knowl‐ edge over both time and space. This is well known A recent paper by Jetz et al. (2012) presents the ‘Map of Life’, a web‐based tool that aims to pro‐ vide a representation of the distribution of every species on Earth 1 . This database, if it is fully real‐ ized, would provide an invaluable resource for the advancement of biogeographical research, in addi‐ tion to its potential utility for conservation plan‐ ning. As most biogeographers, we would give al‐ most unconditional support to any initiatives that may improve the quantity, quality and availability of species distribution data – such as the Map of Life, GBIF 2 , Encyclopedia of Life 3 , etc. (see review in Riddle et al. 2010). We are certain that they will provide a deeper and wider knowledge about life on Earth through the uprising field of biodiversity informatics (see Soberon & Peterson 2004). Even so, it is important to bear in mind that there are intrinsic limitations to the quality, longevity and coverage of biodiversity data. There are at least three factors that com‐ promise the quality of our knowledge of species distributions at any given spatial scale: survey completeness, and the decay of information with time and space. First, as any field ecologist knows, many taxa are difficult to detect because of their phenotype (cryptic coloration and behavior, high mobility, phenology, etc.) and the characteristics of the habitat. Thus, most biological surveys on which species distribution maps are based are un‐ avoidably incomplete, even when a range of sam‐ pling methodologies is utilized (Colwell & Cod‐ dington 1994). In other words, it is normally im‐ possible to record the entire assemblage for a de‐ fined area at a given moment of time. Moreover, the completeness of a sample will typically de‐ crease with increasing extent of the survey area (see Chiarucci et al. 2011). Second, knowledge about species distribu‐ tions and the composition of local assemblages is in a constant state of information decay. The reli‐ ability of the information provided by any survey 1 http://www.mappinglife.org/ 2 http://www.gbif.org/ 3 http://eol.org/ frontiers of biogeography 5.1, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society