Final answer:
Species that face the highest extinction risk are usually rare, either because they have a small geographic range or a low population density. The highest extinction rates are currently estimated to be 500-1000 times the background rate of extinction seen in the Earth's history, and it is projected to cause a decline in biodiversity in the next century.
Step-by-step explanation:
Species that face the highest extinction risk are usually rare, either because they have a small geographic range or a low population density. This applies to terrestrial vertebrate species, plants, and many marine groups. The highest extinction rates are currently estimated to be 500-1000 times the background rate of extinction seen in the Earth's history, and it is projected to cause a decline in biodiversity in the next century.