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8.3: When is a Species Extinct?

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  • While the term “extinction” is relatively easy to define (Section 8.1), determining whether a species is indeed extinct is a more difficult task. One of the most important questions conservation biologists grapple with is deciding how long to wait after the last observation before declaring a species extinct. Answering this question is particularly complicated when considering cryptic and shy species that are difficult to survey, sparsely distributed animals that are hard to find, or plants that are difficult to identify when not in flower.

    To complicate matters, over the last few decades, biologists and their colleagues have rediscovered several species that were once thought to be extinct. These rediscovered species are often called Lazarus species, in reference to their apparent return to life. Recent examples include Burundi’s Bururi long-fingered frog (Cardioglossa cyaneospila. NT) rediscovered after a 60-year absence (Blackburn et al., 2016), a Tanzanian coral tree, (Erythrina schliebenii, CR) originally known from only one specimen collected from a deforested region in the 1930s (Clarke et al., 2011), and the coelacanth (Latimeria chalumnae, CR), a fish that was once thought to be extinct for millions of years (Balon et al., 1988). To avoid declaring more extant species as extinct, there is currently a practice of only declaring a species extinct after several decades of intensive searching and “there is no reasonable doubt that the last individual has died” (IUCN, 2012). Consequently, species, such as the black-spotted false shieldback katydid (Aroegas nigroornatus, CR) and the Ethiopian sedge, Cyperus chionocephalus, Critically Endangered, last seen in 1916 (Bazelet and Naskrecki, 2014) and 1836 (Contu, 2013) respectively, have not yet been declared extinct, even though the last individual may have died a long time ago. Similarly, as many as 15 African orchid species—a group that includes some of the most beautiful and specialised plants on Earth, some of which have not seen since 1890—are currently considered Critically Endangered but may actually be extinct (IUCN, 2019).

    The rediscovery of species once thought to be extinct should not necessarily be considered a sign of conservation progress. In many cases, Lazarus species were simply overlooked because they were extremely rare and restricted to isolated locations. Such is the case for two forest birds from the island nation of São Tomé and Príncipe, namely the São Tomé grosbeak (Crithagra concolor, CR) and Newton’s fiscal (Lanius newtoni, CR). The grosbeak, the world’s largest canary (50% larger than the second largest canary), was for a long time known only from three specimens collected in 1888–1890; it was thus considered extinct, until its rediscovery over 100 years later, in 1991 (BirdLife International, 2018a). The fiscal shares a remarkably similar history: it was previously known only from records in 1888 and 1928, until its rediscovery in 1990 (BirdLife International, 2018b). Despite these rediscoveries, both species persist as very small (< 250 individuals) populations that are at risk from extinction due to ongoing habitat loss and the impact of invasive predators.

    Because extinctions may not always happen immediately after a disturbance, conservation biologists must also consider the lag time between destructive human activities and eventual extinctions. This is illustrated in a study from Kenya’s Kakamega Forest, which found that only half of the species that will eventually go extinct due to habitat loss do so in the first 50 years following habitat fragmentation (Figure 8.3). Long-lived plants can have particularly long extinction lag times, sometimes of several centuries. For example, populations of the Saint Helena olive (Nesiota elliptica, EX) fell below viable levels in the mid-1800s, but the last individual died only in 2003, when the species was officially declared extinct (Cronk, 2016). Species that are doomed to eventual extinction are considered committed to extinction (also called functionally extinct), while the total number of species committed to extinction is referred to as an area’s extinction debt. In one study, researchers used the island biogeography theory to estimate that the average extinction debt for African forest primates was over 30%—that is, more than 30% of forest primates are predicted to go extinct because of habitat destruction and other human activities that have already happened (Cowlishaw, 1999).

    Figure 8.3 Percentage of bird species expected to persist over time in isolated forest patches in western Kenya. Because of extinction debt, not all species are expected to be extirpated immediately after fragmentation; instead there is a time lag between habitat loss and species losses. The image also illustrates how forest size and degree of isolation influences the speed of losses: Kakamega (the largest forest) loses species much slower than Malava, the smallest and most isolated forest. After Brooks et al., 1999, CC BY 4.0.

    On a more positive note, extinction debts may also provide hope for conservation biologists, as the lingering presence of seriously imperilled species affords opportunities to prevent impending extinctions. Conservation biologists are currently illustrating how this can be done by preventing the extinction of three species of pale-coated, desert-adapted ungulates that were formerly common and widespread across the Sahel-Sahara region, namely the scimitar-horned oryx (Oryx dammah, EW), dama gazelle (Nanger dama, CR), and addax (Addax nasomaculatus, CR) (Durant et al., 2014; Brito et al., 2018; IUCN, 2019). The oryx once numbered around one million individuals, with herds of 10,000 seen as recently as 1936. But a population collapse soon followed: by 1985 only 500 oryx survived, and by 2000 it was declared Extinct in the Wild. The addax, relatively common as recently as in the 1970s, also experienced precipitous declines; today fewer than 30 individuals remain in the wild. Similarly, the once-common dama gazelle’s current global population numbers fewer than 250 individuals, fragmented among five subpopulations in Chad, Mali, and Niger. Conservationists noted initial declines already in the 1960s and 1970s, when wild individuals of all three species were caught to initiate captive breeding programs. Luckily, all three species responded well to these programs, and captive populations have grown so strong that reintroduction programs (Section 11.2) have been initiated for the addax (in 1985, in Tunisia), dama gazelle (in 2015, in Morocco), and oryx (in 2016, in Chad). With several reintroductions seemingly successful, there is hope that viable populations of these iconic species may one day again roam free in their previous strongholds. This will only happen if we can reverse or mitigate the threats that causes their population collapses in the first place, namely uncontrolled and illegal hunting, as well as disturbances associated with agriculture, oil exploration, and inconsiderate drilling of wells for groundwater extraction.

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