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

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.