US Species at Risk

Source: Precious Heritage: The Status of Biodiversity in the United States © The Nature Conservancy and NatureServe
Proportion of U.S. Species at Risk - 1997 Species Report Card - freshwater mussels are most at risk, followed by crayfishes, amphibians, freshwater fishes, flowering plants, conifers, ferns, tiger beetles, dagronflies/damselflies, reptiles, butterflies, mammals and birds.

Conservation Status Ranks
GX	Presumed Extinct (not located despite intensive searches)
GH	Possibly Extinct (of historical occurrence; still some hope of rediscovery)
G1	Critically Imperiled(typically 5 or fewer occurrences or 1,000 or fewer individuals)
G2	Imperiled (typically 6 to 20 occurrences or 1,000 to 3,000 individuals)
G3	Vulnerable (rare; typically 21 to 100 occurrences or 3,000 to 10,000 individuals)
G4	Apparantly Secure (uncommon but not rare; some cause for long-term concern; usually more than 100 occurrences and 10,000 individuals)
G5	Secure (common; widespread and abundant)