threatStatus (http://rs.gbif.org/terms/1.0/threatStatus) @return [String] Threat status of a species as defined by IUCN: http://www.iucnredlist.org/static/categories_criteria_3_1 - categories Vocabulary: http://rs.gbif.org/vocabulary/iucn/threat_status.xml Example: "EX" "EW" "CR"