Final answer:
The main criterion for identifying species based on the PSC is the regular occurrence at a site of a significant proportion of the global population of a species, with a focus on evolutionary history and primary biodiversity data.
Step-by-step explanation:
The main criterion for identifying species based on the Phylogenetic Species Concept (PSC) is the regular occurrence at a site of a significant proportion of the global population of a species. This concept is primarily concerned with the evolutionary relationships and common ancestry of species, rather than just reproductive isolation. The implementation of this concept often relies on primary biodiversity data that may include various factors such as range size, habitat requirements, and threat levels as defined by the IUCN Red List. The importance of using primary biodiversity data lies in its ability to offer a more accurate representation of species distribution and conservation needs, as opposed to basing models solely on bioregional classification or subjective opinions. To improve conservation strategies, identifying species groups that are good indicators of broader ecological health is essential, which is why the study of traits affecting a species' extinction risk is crucial.