We know very little about the world’s biodiversity. A recent study suggests that, despite 250 years of taxonomic effort, a mere 14% of the world’s species are recognised by scientists.
Worryingly, anthropogenic effects, including habitat loss, climate change, and invasive species, threaten to exterminate thousands of species before they are even described. In this race against time, scientists are working to describe new species and characterise the extinction risk of known species so they can plan actions to reduce extinctions.
The International Union for the Conservation of Nature (IUCN) has been working since 1994 to identify which species are at greatest risk of immediate extinction and place them on the Red List of threatened species.
The IUCN uses quantitative and objective criteria (such as population size, rate of decline, and range size) to classify species as imperilled (Vulnerable, Endangered, or Critically Endangered), Near Threatened, or Least Concern. Through the collaboration of many scientists, and regular refinement of the categories and criteria, the IUCN Red List has emerged as the leading global threatened species list.
Many countries use national “red lists” to protect locally threatened species and evaluate species at the local level where they are managed. One of the best known national lists is the United States Endangered Species Act (ESA), which legally protects species. It is arguably the world’s most effective conservation law.
The ESA classifies a species as endangered if it is “in danger of extinction throughout all or a significant portion of its range”. It is threatened if it is “likely to become endangered in the foreseeable future”. If sufficient information is available to warrant listing but listing is “precluded by higher listing actions”, species are considered “warranted but precluded" and not listed. This means that species deemed to be at greater risk of extinction are often listed before “warranted but precluded” species.
ESA listing decisions often become political because listings have the power to stop development projects that impact listed species.
The ESA has succeeded in improving the conservation status of most listed species over time and may have prevented 227 extinctions. Nonetheless, the US government’s implementation of the ESA has been problematic, including political intervention and protracted listing times.
For example, the listing rate varies greatly depending on who is president. The mean listing time from 1974–2003 was greater than 10 years (in contrast to stated maximum of one year). Partly as a result of these shortcomings, at least 42 species or subspecies have gone extinct while awaiting ESA listing.
Given the ESA’s status as one of the world’s most prominent national lists, its track record at conserving species is of international interest. A previous study found that the ESA does not recognise at least 90% of the United States’ imperilled species listed by NatureServe. But no studies have analysed the ESA’s coverage of species listed as globally imperiled by the IUCN.
We undertook the first comparison of IUCN and ESA listings of US birds, mammals, amphibians, gastropods, crustaceans, and insects. We studied the listing histories of three bird species and Pacific salmon in more detail. We found that 40% of IUCN-listed birds, 50% of mammals, and 80–95% of species in the other groups were not recognised by the ESA as imperilled.
Our research suggests that a nearly 10-fold increase in listing would be required if the ESA were to protect the gamut of IUCN-listed species. Our data indicate that less imperilled (but at-risk) species are most likely to be overlooked. This does not bode well for the ESA’s ability to mitigate declines before species become critically imperilled.
The bird case studies exemplify how rapidly declining species can be carefully evaluated by the ESA but still not listed. By contrast, the salmon example shows an alternative situation: agencies were effective in evaluating and listing multiple (closely-related) species.
Lack of funding, vague definitions of the ESA’s threatened and endangered categories, and the existence of the “warranted but precluded" category likely contribute to the ESA’s under-recognition of imperiled species.
The ESA is a powerful environmental law, but its impact is limited because most imperilled species (measured by the IUCN Red List) are not ESA-listed. The case of the ESA illustrates a tradeoff between strong species protection and poor coverage of threatened species caused by the substantial implications of listing. The successes and failures of the ESA provide rich lessons in threatened species conservation stategies that should inform managers in other countries.