Differential survival among Tahitian tree snails during a mass extinction event: persistence of the rare and fecund

Abstract

The deliberate introduction of the rosy wolf snail Euglandina rosea to the Society Islands in the 1970s led to the mass extirpation of its rich Partulidae (Pilsbry, 1900) fauna, comprising approximately half of all species in this Pacific island tree snail family. On Tahiti ongoing field surveys have documented the survival of two of seven endemic species of Partula (P. hyalina and/or P. clara) in 38 valleys. E. rosea is now a potent extinction agent across Oceania and determining the factors enabling these two taxa to endure may have wide conservation import. We hypothesized that P. hyalina and P. clara have survived because they were the most abundant and/or widespread species and that they will eventually become extinct. We lack demographic data contemporaneous with predator introduction, but an early 20th century study by H.E. Crampton provides historical demographic data for intact Tahitian partulid populations. Crampton found that P. clara and P. hyalina, although widespread, were consistently rarer than their now-extirpated congeners, including in the 23 valleys he surveyed that retain surviving populations. Given this result, and the recent finding that P. clara and P. hyalina comprise a discrete founding lineage in Tahiti, it is plausible that some shared biological attribute(s) may have contributed to their survival. Crampton recorded the clutch sizes of thousands of gravid Tahitian partulids and found that these two taxa had higher instantaneous mean clutch sizes than did co-occurring congeners. Higher fecundities may have contributed to the survival of P. hyalina and P. clara in the valleys of Tahiti.

Deconstructing an infamous extinction crisis: Survival of Partula species on Moorea and Tahiti

“…A century ago, Clade 1 taxa collectively dominated Tahitian valley populations, typically comprising >95% of the tree snails in individual valleys (Bick, Ó Foighil, & Coote, 2016;Crampton, 1916). The two Tahitian Clade 2 species (P. clara and P. hyalina) were widely distributed in Tahitian valleys but typically represented <5% of individual valley partulid populations (Bick et al, 2016;Crampton, 1916). Taxa from Clades 1 and 2 occurred in sympatry throughout Tahiti's valleys (Crampton, 1916), but we observed little to no evidence for introgression among them either in this study ( Figure 5; Supporting Information Table S5), or in earlier mt phylogenies (Lee, Burch, Jung et al, 2007;Lee et al, 2014).…”

Revision of Partulidae (Gastropoda, Stylommatophora) of Palau, with description of a new genus for an unusual ground-dwelling species

“…Low reproductive rate is likely to make the new species particularly vulnerable to population suppression from introduced predators or habitat disturbance. In fact, Partula species on Tahiti that were rare but had the largest instantaneous clutch sizes (Crampton 1916) persist, whereas their more common congeners with smaller instantaneous clutch sizes have not (Bick et al 2015). …”

Negative impacts of invasive predators used as biological control agents against the pest snail Lissachatina fulica: the snail Euglandina ‘rosea’ and the flatworm Platydemus manokwari

“…A similar pattern of decline occurred in three Trochomorpha species (relatively fresh shells of T. cressida are still found only on Mt Marau, Tahiti) (Gerlach 2016). Further evidence in support of Euglandina predation being the primary cause of partulid extinctions is that the few species that coexist with the predators have unusually high reproductive rates for the family (Bick et al 2016).…”