Patterns of Endangerment at the Global Level

The most objective and authoritative information on global species rarity can be found in the IUCN Red List of Threatened Species, which was first conceived in 1963. The IUCN, also known as the World Conservation Union, is an international conservation network including over 10 000 scientists and 1000 organizations from around the world whose mission is to influence societies to conserve their natural diversity. Every 4 years, the IUCN hosts a World Conservation Congress, where the current status of global species rarity is scientifically evaluated in a Global Species Assessment that is based on the IUCN Red List of Threatened Species. The Red List, which currently lists 7181 endangered or critically endangered species, includes a regularly updated online database and represents an ongoing effort to track the status of all species. The Red List evaluation of species in terms of their risk of extinction falls into three steps. First a species is taken up for evaluation - to date only 38 047 species have been considered for evaluation. Of those species considered, some have to be dropped because the data are too scarce (roughly 10% of the species IUCN has attempted to evaluate have run into problems because of data scarcity). Obviously, the initial selection of which species to evaluate is also biased towards species that might be at risk - in no way is the selection of species random. If the data are adequate, then the third step is taken where each species is assigned to one of seven categories: extinct, extinct in the wild, critically endangered, endangered, vulnerable, near threatened, and least concern. Species at risk of extinction (defined by IUCN as threatened) include the three categories of critically endangered, endangered, and vulnerable. The criteria used to assign species to different risk categories are intended to be quantitative and objective, and to focus on data that could be realistically obtained if sufficient effort were made. Although the criteria in Table 1 describe specific numerical thresholds between categories and appear to provide a data-driven way to assign rarity among species, caution is advised. One weakness of the approach is that the criteria do not reflect life-history variation or idiosyncratic natural history, and instead represent a 'one size fits all' approach. In other words, the intent is that these criteria are to be used to evaluate all species, excluding microbes. However a population of 100 fruit flies has a much greater potential for recovery than a population of 100 sea turtles that do not reproduce until they are 20 years old. Also species differ greatly in how flexible or adaptable they are in their habitat - a small population size represents a much greater risk for an inflexible species than a generalist species with ample physiological and behavioral plasticity. The implications of life-history differences can become a focal point of debate about whether species should be listed or not. For instance, some marine experts have argued that certain decline criteria in Table 1 do not apply to so-called resilient fish stocks which can be sustained at a tiny fraction of their precatch levels. Secondly, the numerical data likely embody huge uncertainties. Endangered species by their very nature are few in number and are often difficult to count and assess because they frequently occupy fragmented and/or impacted habitats and can be scattered over large areas. Moreover, analyses that are used to determine the expected population declines or probabilities of extinction for species are potentially misleading for species because data quality is often poor or population distributions are not well understood. There is a constant tension in developing the Red List between ensuring sufficient data are available to assign appropriate

Table 1 IUCN criteria to evaluate species for threatened status

Critically endangered

Endangered

Vulnerable

A. Trend of loss in total numbers of mature individuals in a taxona

B. Limited geographic range, based on either extent of occurrence*3

area of occupancyb

C. Limited remaining individuals, based on either

(i) numbers of mature individuals with decline

(ii) numbers of mature individuals

D. Expected extinction rate in the wild based on quantitative studye

<250 and 25% decline

Expected in 3 years or one generation0 <50

<5000 km2 <500 km2

<2500 and 20% decline

Expected in 5 years or two generations0 <250

<20000 km2 <2000 km2

<10000 and 10% decline

Expected in 10 years or three generations0 <1000d

aTrend over a period of 10 years or three generations, whichever is longer (up to a maximum of 100 years). A greater percentage loss is required to be critically endangered, endangered, or vulnerable (>90%, >70%, and >50%, respectively) where threats to loss are clearly reversible and understood and have ceased.

bAlso require at least two of the following: severe fragmentation/few locations, continuing decline, or extreme fluctuations in numbers. cCriteriaare still satisfied for C (i) with any continuing decline if each subpopulation has few individuals, if there is a high percentage of the total number of individuals in any one subpopulation, or if there is an extreme fluctuation in numbers of mature individuals. ^Vulnerable status also requires the area of occupancy to be less than 20 km2 or number of locations <5.

eTrend over a period of 10 years or three generations for critically endangered status, 20 years or 5 generations for endangered status, and for all categories within 100 years maximum.

A species is critically endangered, endangered or vulnerable by meeting at least one of A-D in the appropriate column.

Table 2 Definitions of threats used in IUCN Red List in order of importance®

Habitat loss/degradation Intrinsic factors

Harvest (hunting and gathering)

Alien species Pollution

Human disturbance Changes in native species Natural disasters Other (accident, persecution)

Human-induced land and/or water impacts associated with aqua/agriculture, logging, mining, land management, and infrastructure development

Species specific factors that increase vulnerability such as limited dispersal, poor recruitment/ reproduction/regeneration, high juvenile mortality, inbreeding, low densities, skewed sex ratios, population fluctuations, and restricted ranges

Reduction of species numbers for human use such as food, medicine, fuel, cultural, or scientific applications and materials for direct use or trade

Invasive species directly affecting an endangered species through competition, predation, hybridization, and pathogens/parasites

Land pollution, water pollution, and atmospheric pollution (including climate change). Pollution can be heat/cold, noise, light or chemical

Impacts related to recreation/tourism, research, war/civil unrest, transport, and fire

Changes in competitors, predators, prey/food base, hybridizers, pathogens/parasites, and mutualisms

Includes storms/flooding, temperature extremes, wildfire, volcanoes, and avalanches/landslides

Includes fisheries or terrestrial bycatch, collision with vehicles/buildings/pylons, and pest control aDefinitions are illustrative and may not include all conceivable examples for each category.

Undescribed species (estimated) 82%

Described species 17.3%

Described and evaluated 0.4%

Described, evaluated, and found to have adequate data to determine status 0.3%

Figure 2 The relationship between (hypothetical) total species and species that have been evaluated for globally threatened status. Only the bullseye area has been evaluated (0.3%) of a conservative total of 10 million species. In some rare cases (143), undescribed species were evaluated for threatened status where museum or herbarium collections were made and immediate listing would make some tangible benefit to those species' conservation.

Reptiles Cycads Amphibians Mammals Conifers Birds

Reptiles Cycads Amphibians Mammals Conifers Birds

Figure 3 The percentage of species that are classified as endangered or critically endangered by IUCN within taxa that have been comprehensively evaluated. Reptiles in this figure only include order Testudines, order Crocodylia, and family Iguanidae (~4% of all reptile species) and therefore may not represent the percentage of reptile endangerment as a whole.

Figure 3 The percentage of species that are classified as endangered or critically endangered by IUCN within taxa that have been comprehensively evaluated. Reptiles in this figure only include order Testudines, order Crocodylia, and family Iguanidae (~4% of all reptile species) and therefore may not represent the percentage of reptile endangerment as a whole.

status and erring on the side of caution to list poorly understood globally endangered species so that they receive appropriate conservation attention (Table 2).

One of the biggest challenges in addressing endanger-ment is our limited ability to prioritize conservation actions because we have incomplete and skewed geographic and taxonomic knowledge. Assuming a conservative 10 million total species in the world, only a fraction have been described, a much smaller fraction described and evaluated for risk, and an even tinier fraction upon evaluation is judged to have enough data to make a risk determination (Figure 2). In addition to not knowing much, what we do know is highly biased so that some taxa are much better studied than others; for instance, whereas nearly 40% of the known vertebrate species have been evaluated for rarity, less than 5% of the plants species have, and less than 1% of the invertebrate species have, and essentially no fungi have. Marine and freshwater species are also much less understood than terrestrial species. For example, though marine and freshwater areas cover more than 70% of the Earth's surface, only 6% of described fish species have been evaluated for rarity. Among the well-studied taxa where species are known and have been evaluated for risk of extinction, endangerment varies five to tenfold (Figure 3). There is also a wide range of endangerment within taxa, with some

Habitat loss/degradation Intrinsic factors Harvest (hunting and gathering) Alien species Pollution Human disturbance Changes in native species Natural disasters Other (accident, persecution)

Habitat loss/degradation Intrinsic factors Harvest (hunting and gathering) Alien species Pollution Human disturbance Changes in native species Natural disasters Other (accident, persecution)

Figure 4 The percentage of species that are classified as endangered or critically endangered for each IUCN threat category. Endangered species often suffer from multiple threats. Results are heavily biased toward amphibians, birds, and mammals, since threat data have only been comprehensively compiled for these groups. Definitions for threats are in Table 2.

Figure 4 The percentage of species that are classified as endangered or critically endangered for each IUCN threat category. Endangered species often suffer from multiple threats. Results are heavily biased toward amphibians, birds, and mammals, since threat data have only been comprehensively compiled for these groups. Definitions for threats are in Table 2.

classes being much more threatened than others. For example, amphibians stand out for having a rate of endangerment that is roughly twice that of mammals and four times that of birds. One reason for the vulnerability of amphibians might be their permeable skin which exposes them to pollutants and other environmental stressors, plus the fact that they typically require multiple habitats (aquatic and terrestrial) as opposed to just one habitat. If either their aquatic or terrestrial habitat has been lost or degraded they will not be able to complete their life cycle. A handful of evolutionary lineages at the family and order level can be identified as especially endangered among mammals, amphibians, and birds. These higher-order evolutionary lineages with uniquely severe levels of endangerment include albatrosses, kiwis, carnivores, manatees, salamanders, and true toads.

Globally, the greatest source of endangerment is primarily habitat loss and degradation (Figure 4); in fact, the next nearest risk factor after habitat loss is only one-fifth as frequent. The predominance of habitat loss as a source of endangerment provides the rationale for the widespread use of species-area curves in conjunction with deforestation rates to estimate global extinction in the terrestrial environment. Most terrestrial endangered species are in tropical rainforests, which parallels the fact most terrestrial species are in tropical rainforests. For well-studied taxa, one can ask how many endangered species are truly on the brink of disaster, which can be defined as those globally endangered species that are found at only one site. It turns out there are 795 such species spread among 595 sites in the world. If we were committed to zero extinction, these are the endangered species under imminent risk that would command our greatest attention. Whereas most extinction in the historical record has occurred on islands with their isolation and small population sizes, these contemporary imminent extinctions are increasingly in mainland areas where human activities have created isolated islands of habitat.

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