Table 1.

Simplified overview of thresholds for the IUCN Red List criteria.

criterion	critically endangered	endangered	vulnerable	qualifiers and notes
A1: reduction in population size	≥90%	≥70%	≥50%	over 10 years/3 generations in the past, where causes are reversible, understood and have ceased
A2–4: reduction in population size	≥80%	≥50%	≥30%	over 10 years/3 generations in past, future or combination
B1: small range (extent of occurrence)	<100 km2	<5000 km2	<20 000 km2	plus two of (a) severe fragmentation/few localities (1, ≤5, ≤10), (b) continuing decline, (c) extreme fluctuation
B2: small range (area of occupancy)	<10 km2	<500 km2	<2000 km2	plus two of (a) severe fragmentation/few localities (1, ≤5, ≤10), (b) continuing decline, (c) extreme fluctuation
C: small and declining population	<250	<2500	<10 000	mature individuals. Continuing decline either (1) over specified rates and time periods or (2) with (a) specified population structure or (b) extreme fluctuation
D1: very small population	<50	<250	<1000	mature individuals
D2: very small range	N/A	N/A	<20 km2 or ≤5 locations	capable of becoming critically endangered or extinct within a very short time
E: quantitative analysis	≥50% in 10 years/3 generations	≥20% in 20 years/5 generations	≥10% in 100 years	estimated extinction-risk using quantitative models, e.g. population viability analyses