The International Union for Conservation of Nature (IUCN) Red List of Threatened Species is a valuable database containing information on species status, population trends, and threats. While it has its flaws—including some out-of-date species assessments and overall coverage of only a fraction of known species—it provides important status indicators for more than 150,000 animal and plant species.
Based on a number of factors, the Red List places assessed species in one of several categories, including critically endangered (CR), endangered (E), vulnerable (V), near threatened (NT) and least concern (LC). In 2022, Red List data revealed that 18 percent of all vertebrate and insect species assessed are considered at the very least threatened (i.e., categorized as CR, E, or V). Broken down by taxonomic groups, the percentages are as follows: amphibians (35% of species), mammals (22%), insects (19%), reptiles (18%), fish (14%), and birds (12.5%).
While these numbers are troubling, a new analysis published in Biological Reviews—by Catherine Finn and Dr. Daniel Pincheira-Donoso of Queens University Belfast and Dr. Florencia Grattarola of Czech University of Life Sciences Prague—presents an even more alarming picture of species decline. Rather than rely on placement within Red List categories, the authors looked at Red List population trend data (e.g., decreasing, stable, increasing).
Their analysis revealed that 48 percent of CR, E, and V species were in decline. Among 33,305 species of vertebrates and insects in these categories for which population data was available, significant declines were found in each taxonomic group: amphibians (63% of 4,991 species); mammals (56% of 3,383); birds (53% of 10,305); insects (54% of 3,006); fish (41% of 6,445); reptiles (28% of 5,175). These figures may even be an underestimate, as population trend data are not up to date for a number
Even for “non-threatened” species—those categorized as NT or LC on the Red List—the authors found a 33 percent decline among the 24,185 species for which population trend data were available (81% of 2,610 NT species; 27% of 21,575 LC species), suggesting eventual reclassification into categories of greater concern if threats to these species remain unabated.