What is a population (a first, simple answer)
To use the indicators, populations must be defined, making the definition of “population” a critical step. A ‘population’ is a group of individual organisms that can mate with each other and have low or no connectivity with other populations. Populations are usually genetically distinct due to no or very low gene flow (movement of individuals) and/or significant adaptive differences (e.g., different environments [high/ low altitude], different soil types, differences in precipitation or temperature).
Not every occurrence, site or locality is a population; if there is significant exchange of individuals or breeding among sites or localities, occurrences should be considered one population. The spatial size of a population does differ depending on the species being considered, often due to differences in dispersal capacity (e.g. the spatial extent for a tree population and a salamander may not be the same)
The word ‘subpopulation’ may describe clusters of organisms across a landscape (including family units) that 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.
See section How To Guides for advice on How to define populations?
In IUCN Red List reports the term ‘population’ and ‘subpopulation’ are used differently than intended in here. 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’ more closely resembles our definition of a ‘population’! 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.