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docs/2_Theoretical_background/What-is-a-population.md

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 For example, there are two IUCN subpopulations that make up the IUCN population (entire species distribution) for Capensibufo rosei, a critically endangered toadlet endemic to Table Mountain National Park in Cape Town, South Africa. These subpopulations have been found to be genetically distinct, with no current gene flow possible between them likely owing to the distances between subpopulations exceeding the dispersal distances for the species (see Cressey et al. 2015; da Silva et al. 2016). We therefore define the two genetically distinct IUCN subpopulations as separate populations.
 
 
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-This is one of the most important concepts to agree on before searching for data.  The definition of a population does differ depending on the species (e.g. the spatial extent for a tree population and a salamander may not be the same) but here follows some general guidance.
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-Briefly, ‘population’ refers to a genetically distinct group of individuals.  Within populations the individuals are capable of interbreeding.  Genetic distinction of populations is usually due to no or very low gene flow (movement of genes e.g. by movement of offspring) and/or significant adaptive differences, such as due to occurring in different environments (high/ low altitude, different soil types, differences in precipitation or temperature).  Not every occurrence is a population (see Appendix)!  The word ‘subpopulation’ may describe clusters of organisms across a landscape (including family units) which are near enough to exchange gene flow. Usually several ‘subpopulations’ are considered together as a population (also called a ‘metapopulation’). When the potential for gene flow is large, e.g., viable tree pollen that can travel tens of kilometers, ‘populations’ can range across large distances, sometimes hundreds of km. 
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-In IUCN Red List reports the term ‘population’ and ‘subpopulation’ are used differently than intended in this study. An ‘IUCN population’ refers to the entire species (i.e., the total number of individuals of the taxon). ‘IUCN subpopulations’ are geographically or otherwise distinct groups of the species between which there is little exchange (IUCN 1995);  thus an ‘IUCN subpopulation’ is what we refer to as ‘populations’! 
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 Some published reports will clearly define what a population is based on the knowledge of biodiversity and taxonomic experts.  Some judgment may be required in interpreting population designation in a report, but in general the designation of a population by experts should be considered strongly.  In other cases, the reports or database may not clearly designate population boundaries and will require interpretation.  Visual examination of maps may result in ‘merging’ occurrences that are likely to experience extensive gene flow- ‘likely to experience extensive gene flow’ may be defined on spatial proximity e.g. close geographic distance and lack of clear barrier (mountain, fence, etc.). 
 
 The distance between subpopulations (or discrete habitat patches where the species occurs) can be used as a proxy to evaluate to what extent subpopulations are likely functionally connected. When the edge of a subpopulation is within a reasonable dispersal distance (for that particular species, a distance within which some 75-90% of realized dispersal distances occur- expert discretion is fine here) of another subpopulation edge, and there are no known physical barriers impeding dispersal, the subpopulations can be considered as part of the same metapopulation. Common trees may disperse pollen tens to hundreds of kilometers, while salamanders or frogs may be dispersing kilometers or less, while wolves may disperse hundreds of kilometers.  Designation of populations should consider human induced gene flow e.g. genetic rescue, translocations etc. Experts in the species type (e.g. amphibians) and/or text from the data source can help identify populations.