A substantial proportion (more than a third) of mammal species that have been classified as extinct or possibly extinct, or flagged as missing, have been rediscovered. Searching for missing species takes substantial effort and funding, and many missing species have a high scientific or public profile and high potential conservation importance if found [3
]. It is therefore important that investigators prioritize their effort to missing species that are most likely to be detected [7
The missing species most likely to be rediscovered are those with large ranges that declined from habitat loss. Mammal extinctions have been attributed to habitat loss only in the last two centuries, and our analysis of this time period showed that larger range size predicted higher probability of rediscovery only in species affected by habitat loss. This is consistent with most evidence in birds, showing that habitat loss causes disproportionate global and local extinction of restricted range endemics in comparison with other threats [13
], and with models showing that endemics–area relationships predict extinction rates from habitat loss better than species–area relationships [34
Our finding that mammals affected by habitat loss are most likely to be rediscovered suggests that the current number of species considered extinct owing to habitat loss is likely to be overestimated. Because small range is a variable used to both ascertain extinction risk and assign the cause (habitat loss), circularity might lead to overestimation of the proportion of extinctions that are due to habitat loss [11
]. Severely declined mammals are likely to be considered as specialists on their last detected habitat. Now being restricted to a small range, they will be categorized as threatened by habitat loss, even if the cause of previous decline was different. This will inflate perceived extinction risk owing to habitat loss if some of these species actually persist undetected in other habitats or distant sites. Fisher [35
] found that species affected by habitat loss are more likely to be rediscovered at the periphery than the centre of their former range, suggesting that spreading habitat change has pushed them to the range edge, and that high human population pressure was associated with rediscovered species changing habitat from previous records in primary forest, to rediscovery in marginal habitat such as regrowth, cropland and plantations. Both of these effects are likely to make mammals that have declined from habitat loss particularly hard to detect. We found no significant effect of human population density on the probability of rediscovery, although increased frequency of extinction from habitat loss and overkill might be expected in more populated regions. It is possible that this effect was cancelled out because there were also more opportunities for rediscovery in populated areas, because of increased encounter rates and number of people with identification skills.
Across all centuries, range size was strongly correlated with the probability of rediscovery of missing mammals, and species with very small ranges were unlikely to be rediscovered. This might simply be due to the elevated extinction risk associated with small ranges. All recent analyses have concluded that small range size and the closely correlated trait of small population size are the most important indicators of extinction risk and declines of threatened mammals [10
]. The rediscovery rate of species with large ranges might also be higher because scattered remnant populations are more likely to escape detection, an explanation reinforced by the finding that species originally occurring at lower population density were rediscovered at higher rates, despite the fact that low population density predicts extinction risk in mammals generally [10
]. This interpretation also seems inconsistent with previous assertions that large geographical range is the best predictor of early species description, because it increases the encounter rate with collectors [37
]. However, the high detectability of initially widespread species before decline, and their low detectability after decline, makes sense if they contracted to a very small range that was not at the site where they were last seen but one anywhere within the former wide distribution, or at a remote edge of it [35
Our finding that, throughout historical time, species with small ranges are unlikely to be rediscovered is not an effect of island endemics being extirpated by introduced predators. Unlike birds, which are disproportionately exterminated by predators introduced to islands [39
], invasive predators have had continental-scale impacts on mammal extinction rates [2
]. Being restricted to islands was not correlated with the probability of rediscovery in our analyses (60% of rediscoveries were on continents). We found that, overall, mammals were unlikely to be rediscovered if the cause of extinction was an introduced predator or disease, but they were likely to be rediscovered if the cause was habitat loss. This conclusion parallels recent findings in birds. Although more birds are classified as threatened to some degree by habitat loss than by biological invasion, bird families threatened mainly by invasive species are more extinction-prone, and families containing species primarily threatened by habitat loss are less extinction-prone [39
Moderate search effort was associated with increased rediscoveries, in comparison with low search effort. We could not separate this from the effect of century, because all species missing in the 19th century and before were subject to low search effort (two or fewer expeditions), except for the Talaud flying fox (Acerodon humilis
), which was missing in 1897 and found alive in 1999 after three searches. Most missing mammals have not been adequately searched for, but a few flagship species (charismatic large mammals) received disproportionately high numbers of searches. The highest search effort in our dataset was confined to a handful of species that remain missing, namely the thylacine (Thylacinus cynocephalus
), wild horse (Equus ferus
, extinct in the wild), kouprey (Bos sauveli
) and Baiji (Lipotes vexillifer
). We suggest that this is because it is possible to keep searching indefinitely without success if the species is actually extinct. These large-bodied mammals all declined mainly from overkill. However, body size did not independently predict rediscovery rate in any of our models, although persecuted and harvested species are predominantly large and conspicuous [11
]. A species must be identifiable and detectable to be persecuted, exploited or harvested, so publicity about its supposed extinction is also more likely, which might result in more search effort. Our data suggest that mammals purportedly exterminated by overkill receive more attention, because they were targeted by more than twice as many reported searches on average as the more enigmatic species that declined from habitat loss or introduced predator impacts. Mammals that declined from human persecution were more likely to be rediscovered than those presumed to have been driven extinct by introduced species, particularly in the 20th century. Increased public attention and searching probably explain why species that declined in the 20th century tended to be rediscovered more frequently, especially if they declined from overkill.
Our major findings are robust to varying definitions and time scales, because the same conclusions were important whether we used the overall dataset with a broad definition of missing species, or subsections (19th and 20th centuries only, species with one reported threat only, or the restricted dataset of species with a narrower definition of ‘missing’).