Extinction in a hyperdiverse endemic Hawaiian land snail family and implications for the underestimation of invertebrate extinction

Abstract

The International Union for Conservation of Nature (IUCN) Red List includes 832 species listed as extinct since 1600, a minuscule fraction of total biodiversity. This extinction rate is of the same order of magnitude as the background rate and has been used to downplay the biodiversity crisis. Invertebrates comprise 99% of biodiversity, yet the status of a negligible number has been assessed. We assessed extinction in the Hawaiian land snail family Amastridae (325 species, IUCN lists 33 as extinct). We did not use the stringent IUCN criteria, by which most invertebrates would be considered data deficient, but a more realistic approach comparing historical collections with modern surveys and expert knowledge. Of the 325 Amastridae species, 43 were originally described as fossil or subfossil and were assumed to be extinct. Of the remaining 282, we evaluated 88 as extinct and 15 as extant and determined that 179 species had insufficient evidence of extinction (though most are probably extinct). Results of statistical assessment of extinction probabilities were consistent with our expert evaluations of levels of extinction. Modeling various extinction scenarios yielded extinction rates of 0.4-14.0% of the amastrid fauna per decade. The true rate of amastrid extinction has not been constant; generally, it has increased over time. We estimated a realistic average extinction rate as approximately 5%/decade since the first half of the nineteenth century. In general, oceanic island biotas are especially susceptible to extinction and global rate generalizations do not reflect this. Our approach could be used for other invertebrates, especially those with restricted ranges (e.g., islands), and such an approach may be the only way to evaluate invertebrates rapidly enough to keep up with ongoing extinction.

Discordance between morphological and taxonomic diversity: land snails of oceanic archipelagos

“…Oceanic island snail faunas have suffered widespread extinction because of human activities both recently (e.g. Solem, ; Cowie, ; Régnier et al ., , b) and during prehistory (e.g. Preece, ).…”

Impact of land‐use change on flower‐visiting insect communities on an oceanic island

“…Over the last century, island ecosystems have been disproportionately affected by anthropogenic alterations and a large proportion of recorded extinctions have taken place on islands (e.g. Cardoso et al, 2010b;Rando et al, 2013;Alcover et al, 2015;R egnier et al, 2015;Terzopoulou et al, 2015). Land conversion of native forest to agricultural and exotic forest is now recognised as one of the major causes of island biodiversity decline, with many extant species predicted to be committed to future extinction as a result of land-use change ('the extinction debt'; Triantis et al, 2010).…”

Type Specimens of Hawaiian Land Snails in the Smithsonian Institution, National Museum of Natural History, with Lectotype Designations

“…The spectacularly diverse assemblages of land snails on these islands have been particularly heavily affected, with many species already extinct and the remaining fauna disappearing rapidly (Lydeard et al, 2004;Régnier et al, 2009Régnier et al, , 2015Richling and Bouchet, 2013;Sartori et al, 2014). Among the Pacific islands, the most species-rich land snail fauna is that of the Hawaiian Islands, with more than 750 described species, over 99% of them endemic to the archipelago and many to single islands (Cowie et al, 1995).…”