Houston ChandlerViews:
Open any field guide or book with species accounts and you are likely to see some type of range map that broadly delineates the geographic distribution of a species. While a seemingly basic concept, identifying species ranges is often challenging, and most maps are based on incomplete data that contribute to uncertainty in the map, especially around the borders. Furthermore, a species geographic distribution can be dynamic as populations come and go over time or individuals travel outside of the recognized range. I often see significant discussion about range maps being ‘wrong’ on social media (particularly common for venomous snake posts), but it is important to remember that range maps are only an approximation of the complex ecological processes that actually determine where a species can be found. Inaccuracies are to be expected for many species, particularly at fine scales where the dynamic nature of species occurrence can manifest over relatively short time periods.
So how does someone go about creating a range map? In many cases, this is actually a challenging endeavor that relies on a fair amount of expert opinion and guesswork. At a basic level range maps are based on documented species occurrences, which can be found either in museum collections, published accounts, wildlife agency databases, or, more recently, in app databases like Herpmapper and iNaturalist. Once all of this occurrence data has been collected some type of buffer is typically drawn around points, which will hopefully form a large blob that is characteristic of a range map. If existing point data is insufficient (i.e., there are large gaps were the species is presumably present), range maps may be filled in based on a species habitat requirements or expert opinion. This inexact science can often lead to discrepancies among maps, depending on who did the drawing and what dataset was used. Overall, the goal is usually to create the best approximation of a species distribution, giving the reader a general idea of where the species may be encountered.
One of the benefits of higher resolution data and better range maps becoming widely available in recent years is that it is easier to identify gaps in the current data. Most state agencies keep some type of wildlife database that documents where species have been observed, usually at the county level. New county records can thus be documented and even published in the Geographic Distribution section of Herpetological Review. Along with the Natural History Notes section, this allows the herpetological community to publish single observations that would otherwise be difficult to publish, making these data available to the broader community. For example, we recently participated in two geographic distribution notes. One was a combination of 151 new county records from Georgia that were compiled over several years of fieldwork (Stevenson et al. 2021a), and the other documented Eastern Indigo Snake observations from Decatur and Seminole Counties (Stevenson et al. 2021b), both of which are in the area of uncertainty on the above map. For rare and imperiled species, these records will benefit their conservation by increasing our knowledge of where extant populations exist, potentially guiding future land conservation or management actions.
Overall, range maps should be considered an approximation based on the best available data at the time the map was created. It is not especially surprising when species turn up outside of their known range (within some reasonable distance), but it should be documented so that better maps can be made in the future. Finally, documenting occurrence is just one of the many reasons that I encourage people to record the species that they encounter (the above mentioned apps are the easiest method). Over time these data are likely to have many applications that could benefit both conservation and our understanding of large-scale ecological processes.
Literature Cited
Stevenson, D.J., B.S. Stegenga, B. Rice, J. Barrett, J. Bolton, Z. Cava, H.C. Chandler, K.M. Stohlgren, T. Brock, C.D. Camp, J.B. Jensen, M.D. Moore, S. McGuire, C. Howe, F. Snow, J. Oguni, C. Coppola, and L.L. Smith. 2021a. New County Records for amphibians and reptiles in Georgia, USA. Herpetological Review 52:350–359.
Stevenson, D.J., B.S. Stegenga, H.C. Chandler, and R.D. Birkhead. 2021b. Drymarchon couperi (Eastern Indigo Snake) – Distribution records. Herpetological Review 52:347.
