Q.What criteria should one use in categorizing a species as threatened?

To decide whether a species is threatened, you must look at the evidence that its population is declining or at risk. The IUCN Red List provides the most widely accepted framework for this — the same body the NCERT textbook cites for species counts and extinction records (e.g. the IUCN Red List (2004) documenting 784 extinctions in the last 500 years, and more than 15,500 species worldwide currently facing the threat of extinction).

The key criteria used by this framework are:

• A sharp reduction in population size, observed or estimated over a short period (usually 10 years or three generations).
• The geographic range of the species is small, fragmented, or shrinking — for example, its area of occupancy or extent of occurrence is limited.
• The total number of mature individuals is very low.
• A continuing decline is projected or observed in the number of mature individuals, even if the total population seems stable. …

The question of when a species becomes threatened is not just a matter of counting heads in the wild. The International Union for Conservation of Nature (IUCN) has developed a set of standardised criteria used worldwide — the same organisation whose Red List the NCERT textbook cites when it notes that the IUCN Red List (2004) documents the extinction of 784 species (including 338 vertebrates, 359 invertebrates and 87 plants) in the last 500 years, and that more than 15,500 species worldwide currently face the threat of extinction.

The first criterion is population size. A species with a very small number of mature individuals is clearly at risk. But size alone is not enough — the rate of decline matters just as much: a sharp population reduction over a short period is one of the standard criteria for classifying a species as Endangered or Critically Endangered.

The second major criterion is geographic range. A species that lives only in a tiny area is far more vulnerable to a local disaster than one spread across a continent. Two measures are used: the extent of occurrence and the area of occupancy.

Third, population fragmentation and number of locations — even a moderate total population broken into many small, isolated groups that cannot interbreed puts each group at risk of local extinction.

Fourth, quantitative analysis — a formal population viability analysis can be used to model extinction probability over a given time period.

Group the scattered IUCN criteria into a short checklist (population size, rate of decline, range size/fragmentation, modelled extinction probability), no …