Differential survival among Tahitian tree snails during a mass extinction event: persistence of the rare and fecund C.S. BICK,DIARMAID ÓFOIGHIL and T REVOR C OOTE Abstract The deliberate introduction of the rosy wolf snail Introduction Euglandina rosea to the Society Islands in the s led to the mass extirpation of its rich Partulidae (Pilsbry, ) ceanic islands represent some of the most isolated habi- fauna, comprising approximately half of all species in this Otats on earth and their endemic biotas are character- Pacific island tree snail family. On Tahiti ongoing field sur- ized by small ranges and the absence of highly co-evolved veys have documented the survival of two of seven endemic defensive capabilities, such as anti-predator behaviours species of Partula (P. hyalina and/or P. clara)in valleys. and morphologies (Paulay, ; Vermeij, ; Roff & E. rosea is now a potent extinction agent across Oceania and Roff, ; Fullard et al., ). Consequently, island biotas determining the factors enabling these two taxa to endure are exceptionally vulnerable to introduced continental pre- ’ may have wide conservation import. We hypothesized dators (Paulay, ;DAntonio & Dudley, ). This is ’ ‘ ’ that P. hyalina and P. clara have survived because they perhaps best exemplified by Guam s empty forest were the most abundant and/or widespread species and (Redford, ) phenomenon, where a single introduced that they will eventually become extinct. We lack demo- predator, the brown tree snake Boiga irregularis, has severely graphic data contemporaneous with predator introduction, affected the endemic forest fauna (Savidge, ; Wiles et al., but an early th century study by H.E. Crampton provides ; Mortensen et al., ), prompting extraordinary con- historical demographic data for intact Tahitian partulid servation interventions (Clark & Savarie, ). populations. Crampton found that P. clara and P. hyalina, Another alien predator, the carnivorous rosy wolf snail although widespread, were consistently rarer than their Euglandina rosea, has also had an outsized impact on ocean- now-extirpated congeners, including in the valleys he sur- ic island endemic faunas, being implicated in the extinction veyed that retain surviving populations. Given this result, of at least terrestrial snail species (Régnier et al., ). A and the recent finding that P. clara and P. hyalina comprise North American native, E. rosea stalks its gastropod prey by a discrete founding lineage in Tahiti, it is plausible that some following their mucous trails, consuming small prey whole shared biological attribute(s) may have contributed to their and larger individuals piecemeal (Gerlach, ; Shaheen survival. Crampton recorded the clutch sizes of thousands et al., ; Davis-Berg, ). It is highly mobile and can of gravid Tahitian partulids and found that these two taxa climb trees, a detrimental characteristic for many arboreal had higher instantaneous mean clutch sizes than did co-oc- Pacific island terrestrial snails (Kinzie, ; Meyer & curring congeners. Higher fecundities may have contributed Cowie, ). Euglandina rosea was introduced to multiple to the survival of P. hyalina and P. clara in the valleys of Pacific island archipelagos in a misguided strategy to control Tahiti. outbreaks of the introduced giant African snail Lissachatina fulica, most notably to the Hawaiian Islands in (Cowie, Keywords Differential survival, mass extinction, oceanic ) and to the Society Islands in (Coote, ). It islands, Partulidae, Tahiti, tree snail rapidly extirpated large numbers of endemic land snail This paper contains supplementary material that can be species on both archipelagos, including members of the found online at http://journals.cambridge.org Achatinellidae in Hawaii (Hadfield et al., ) and Partulidae in the Society Islands (Clarke et al., ). The loss of the Partulidae of the Society Islands has been of particular concern because this archipelago is home to c. % of partulid species diversity (Cowie, ). It has also been the main setting for th century partulid studies, starting with the classic work of H.E. Crampton (, ) C.S. BICK (Corresponding author) and DIARMAID ÓFOIGHIL, Museum of Zoology and continuing with decades of research by B. Clarke, and Department of Ecology and Evolutionary Biology, University of Michigan, 1109 Geddes Ave, Ann Arbor, MI 48109-1079, USA. E-mail [email protected] J. Murray, M. Johnson and associates (Clarke & Murray, ; Murray & Clarke, ; Johnson et al., ). The TREVOR COOTE, Partulid Global Species Management Programme, Papeete, Tahiti, Polynésie Française collapse of Society Islands partulid populations following Received July . Revision requested November . the introduction of E. rosea prompted the emergency estab- Accepted April . First published online October . lishment of off-archipelago captive populations for Oryx, 2016, 50(1), 169–175 © 2014 Fauna & Flora International doi:10.1017/S0030605314000325 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.8, on 30 Sep 2021 at 05:33:15, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605314000325 170 C. S. Bick et al. Society Island species (Murray et al., ; Tonge & Bloxam, ; Pearce-Kelly et al., ). Until recently, only of en- demic Society Islands partulid species were thought to per- sist in the wild (Coote & Loève, ) but subsequent field surveys have found scattered extant populations on Raiatea, Moorea and Tahiti representing four additional species (Lee et al., , ). Seven of these surviving taxa (Partula otaheitana, Partula hyalina, Partula clara, Partula affinis, Samoana attenuata, Samoana burchi, Samoana diaphana) occur on Tahiti, the largest and highest island in the archi- pelago (Coote, ; Lee et al., a, ). The predom- inant Tahitian pattern is one of low elevation extirpation and montane persistence: partulid survivors are most com- mon in cloud forest refuges of ., m altitude (Coote, ; Lee et al., a, ), where predator activity is probably impaired by cooler temperatures (Gerlach, , FIG. 1 Tahiti, showing the distribution of surviving low elevation ). However, extensive field surveys beginning in partulid populations detected by Coote (; unpubl. data) in have located small clusters of low elevation survivors in a surveys during –. The survivors in of valleys number of Tahitian valleys (Coote, ), currently totalling exclusively comprised Partula clara and/or Partula hyalina.A (Fig. ; Table ). Of Tahiti’s eight species of Partula, two third species, Partula affinis, was found in one valley (, now dominate low elevation extant populations; i.e. of the Faaroa), together with P. clara and P. hyalina. See Table for the names of the numbered valleys and the number of survivors. valleys with known survivors were exclusively populated by P. clara and/or P. hyalina (Fig. ; Table ). Partula hyalina and P. clara are closely-related species, populations with their pre E. Rosea introduction equivalents distinguished by shell coloration, which together represent across the island as a whole, as well as for the valleys a discrete Tahitian founder lineage of Moorean origin Crampton surveyed that retain surviving populations (Lee et al., ). Their ability to endure almost years (Table ). of predation by E. rosea in the valleys of Tahiti is surprising because predation models predict partulid extirpation with- Methods in years of initial predator contact (Gerlach, ). We are interested in understanding what aspect(s) of their biology Crampton () systematically surveyed the partulid popu- underlies this survival, not only for their individual conser- lations of Tahiti. He divided the island of Tahiti into five vation but because of the possible implications for the sur- subunits: Tahiti-Nui, comprising four quadrants (north, vival of the many endemic land snails across Oceania now south, east and west) and the peninsula Tahiti-Iti threatened by E. rosea (Régnier et al., ). (Taiarapu). Over four annual surveys during – he The inverse relationships of population size (Pimm et al., surveyed a large fraction of the coastal valleys present in ; Schoener & Spiller, ) and geographical range each geographical subunit: western, southern, (Payne & Finnegan, ; Cardillo et al., ) to extinction northern, eastern and in Tahiti-Iti. Crampton () risk have been well documented. Our initial hypothesis is did not detail his sampling methodology except to state therefore that P. hyalina and P. clara have survived because that he walked into each valley along the primary trails dur- they were the most abundant and/or widespread species in ing daylight hours, collecting snails from the adjacent trees Tahitian valleys and that they will eventually be driven to and vegetation. He was particularly interested in extinction by the predator. To test this, ideally we need a de- population-level variation and typically obtained large sam- tailed census of Tahitian partulid populations contempor- ple sizes (hundreds) from each valley. aneous with the introduction of E. rosea. Such a Modern-day surveys of Tahitian valleys for surviving resource is not available but we do have access to a partulids largely follow Crampton’s() methodology, ex- century-old dataset of Tahiti’s intact partulid populations. cept that the snails are much rarer and that more valleys During – Crampton () surveyed and collected have been surveyed (Coote, ). Each survey of a valley Tahitian valley tree snail populations, publishing a detailed