Habitat preferences of the Critically Endangered greater Bermuda land snail Poecilozonites bermudensis in the wild
A LISON C OPELAND and T HOMAS H ESSELBERG
Abstract The endemic, Critically Endangered greater Poecilozonites bermudensis (Plate ) was believed extinct Bermuda land snail Poecilozonites bermudensis is known by the s as a result of the introduction of invertebrate from only two relict subpopulations. Little is known of its predators, primarily the rosy wolf snail Euglandina rosea habitat preferences in the wild. Observations of released and the flatworm Bipalium vagum (Ovaska & Outerbridge, zoo-reared P. bermudensis suggested an affinity for lime- a). In a subpopulation was rediscovered in an stone, which we investigated on Port’s Island. Previous urban alley and ex situ breeding was started (Outerbridge qualitative observations on Port’s Island suggested an aver- et al., ). A second subpopulation was found in on sion to the litter of the invasive tree Casuarina equisetifolia, Port’sIsland,a. ha island unconnected to the rest of the which we examined. Additionally, we hypothesized that archipelago (Fig. ). An unmanaged, invasive-dominated, sec- snail abundance would increase with elevation, distance ondary growth forest covers %ofPort’sIsland,andthere- from the sea, and with increased plant species diversity. mainder is a camping ground. The rediscovery of this wild During May– June , we found live P. bermuden- subpopulation offered the opportunity to study the habitat sis at sites across Port’s Island. We found no correlation preferences of the species, to aid identification of suitable re- between the number of live snails at a site and either the lease sites for captive-bred snails and to guide searches for number of plant species, elevation or distance from the additional wild subpopulations. shoreline, but snails were significantly less abundant at Observations of zoo-reared P. bermudensis released in sites dominated by C. equisetifolia. Significantly more snails indicated an affinity for limestone (Outerbridge et al., were found around limestone features, indicating future ). Chiba () suggested that invasion by Casuarina reintroductions and searches for any undiscovered sub- species can reduce densities of native snails on islands under populations should focus on limestone features where certain conditions. Casuarina equisetifolia, native to South- C. equisetifolia is absent. east Asia, northern Australia and Pacific islands, is invasive on Bermuda’s coasts, and previous anecdotal observations Keywords Bermuda, habitat preference, land snail, suggest P. bermudensis has an aversion to its leaf litter. In Poecilozonites bermudensis, protected species recovery, this study on Port’s Island, we investigated whether P. bermu- reintroduction, terrestrial snail survey densis was more abundant around limestone features and less abundant where C. equisetifolia waspresent.Additionally,we slands around the world support assemblages of endemic expected P. bermudensis abundance to be higher at sites with Iland snails (Chiba & Cowie, ), many of which have greater plant species diversity, and for snail counts to increase been driven to near extinction and rely on intensive man- with distance from the coast and altitude. agement and captive breeding for their survival (Coote & To test our hypotheses we used two approaches to iden- Loève, ; Outerbridge et al., ). Poecilozonites is a tify sites: ( ) random sites were chosen to assess differences genus of land snails endemic to Bermuda, with two extant in snail abundance and plant diversity at various altitudes species (Outerbridge & Sarkis, ). Both are categorized and distances from the sea, and ( ) targeted surveys to assess on the IUCN Red List as Critically Endangered (Ovaska if the snails preferred limestone features and avoided C. & Outerbridge, a,b). The greater Bermuda land snail equisetifolia trees. The contours of Port’s Island were used to stratify ran- dom sites by randomly selecting locations along the contour lines using ArcGIS .. (Esri, Redlands, USA), ensuring ALISON COPELAND* (Corresponding author, orcid.org/0000-0001-8549-5054) Department of Environment and Natural Resources, Government of Bermuda, sites were chosen from sea level to the peak of the island 17 North Shore Road, Hamilton Parish, FL04, Bermuda at m altitude (Fig. ). Targeted surveys at C. equiseti- E-mail [email protected] folia trees and limestone features were conducted op- THOMAS HESSELBERG*( orcid.org/0000-0002-9735-2488) Jersey International Centre of Advanced Studies, St. Helier, Jersey portunistically when we encountered appropriate features. Limestone features included natural outcrops, ledges, over- *Also at: Department for Continuing Education, University of Oxford, Oxford, UK hangs and rubble piles. An effort was made to select features ’ Received May . Revision requested May . from different parts of Port s Island (Fig. ). A paired control Accepted August . site for each feature was selected by moving m away from
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microhabitats is not understood, only live snails were used in data analysis. As it was the dry season, aestivating snails were only dislodged if obscuring others. An SM soil moisture sensor with HH moisture meter (Delta-T Devices, Cambridge, UK) was used at the centre of the . m quadrat once it was cleared. The altitude and dis- tance to the nearest coast were later determined with ArcGIS. Targeted surveys at C. equisetifolia trees, limestone features and paired control sites used the same sized quad- rat, soil sensor and methods as for the random sites, but no vegetation survey was performed. The live snail counts in the random quadrats were analysed with a generalized linear model using the quasi Poisson error distribution, with plant species richness, dis- tance to coast, altitude and soil moisture as predictor vari- ables. Non-significant terms were dropped from the model using the backward elimination method, with probability values estimated from the type II Wald χ test. The experi- PLATE 1 Two adult-sized greater Bermuda land snails mental paired design was used to analyse the response of Poecilozonites bermudensis on Port’s Island, Bermuda (Fig. ). the number of live snails to the presence of C. equisetifolia, limestone or neither habitat feature (control site). The non- the feature in a random direction. Control sites were a parametric Mann–Whitney test, the paired t test or the similar distance from the sea and within the same plant paired non-parametric Wilcoxon signed-rank test were community, but without rocks, C. equisetifolia trees or used depending on whether the data was normally distrib- large amounts of C. equisetifolia litter. uted or not. All tests were conducted in R .. (R Core Sites were surveyed during May– June . Nested Team, ). quadrat surveys were conducted at the random sites, On Port’s Island, live and , dead P. bermudensis using a . × . m(. m ) quadrat for snail counts within were found at sites. Our linear model found no signifi- a × m( m ) quadrat for vegetation. In each m quad- cant effect of plant diversity on the number of live snails rat, plants were identified to species and counted, except for (χ = .,df=,P=.), suggesting that plant diversity species of Calophyllum, which were only identified to genus. is not an important predictor of snail abundance on Port’s Each . m quadrat was searched down to the soil surface, Island. Similarly, we found no effect of soil moisture with leaves, rocks and twigs checked for snails. Live and (χ = .,df=,P=.), elevation (χ = .,df=, dead P. bermudensis were counted and photographed. P=.) or distance from the coast (χ = .,df=, Since the invasive-dominated plant assemblage is relative- P=.). Poecilozonites bermudensis were found in both ly recent, and the differential preservation of shells in coastal habitat and interior woodlands. The hypothesis
FIG. 1 Port’s Island in the Bermuda archipelago, indicating locations surveyed for Poecilozonites bermudensis.
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large P. bermudensis have been observed eating limestone (R. Marirea, pers. comm., ). The breeding season of P. bermudensis in the wild has not been documented, but we found eggs on Port’s Island on May, shallowly buried in the soil. Availability of suitable, protected habitat is critical to the success of snail reintroductions (Coote & Loève, ). Our finding that P. bermudensis is common at random sites throughout the invasive-dominated woodland of Port’s Island is encouraging, given that all of Bermuda’s protected areas are now dominated by invasive plant species. If P. ber- mudensis does not associate strongly with native plants and therefore requires a managed habitat; the number of potential reintroduction locations is greater. The use of specific indigen- ous and invasive plant species by P. bermudensis for food and shelter at Port’s Island requires further study. Coastal sites above the storm surge zone can be considered as potential re- introduction sites if they are not dominated by C. equisetifolia. At least one shaded limestone feature should be present, preferably a stacked stone wall, rubble pile, or similar rough surfaced feature, to provide refuge in hot, dry weather. Searches for other wild subpopulations of P. bermudensis FIG. 2 Mean ± SE number of live P. bermudensis found in are called for in the species recovery plan (Outerbridge & targeted quadrats and paired control quadrats in areas with Casuarina equisetifolia and with limestone. Sarkis, ). As P. bermudensis uses habitats close to the sea, low elevation islands and coastal sites should be in- cluded in future searches. that vegetation diversity contributes to higher abundance of snails inland (as fewer plant species tolerate the salt expos- Acknowledgements This research received no specific funding ’ from any agency. In-kind support and salary for AC were provided ure at the coast) can be rejected. Furthermore, a Spearman s by the Bermuda Department of Environment and Natural Resources. rank correlation of the number of plant species in the We thank the Department of Youth and Sport for granting access to quadrats against their distance from the coast showed no Port’s Island; Sarah Manuel and Mark Outerbridge for loaning equip- significant correlation (r = .,df=,P=.). As most ment; Peter Drew, Mandy Shailer, Mark Outerbridge and Stephen of the island is covered by invasive plants, this is unsurpris- Copeland for assistance with fieldwork; Robbie Smith and Kristiina Ovaska for help with study design; and two anonymous reviewers ing. The number of individual plants in the m quadrats – for their comments. This is contribution number 283, Bermuda was (mean ), with a total of species. Biodiversity Project. If P. bermudensis is differentially drawn to or repelled by limestone and C. equisetifolia, differences in live snail counts Author contributions Study design, data analysis, writing: both would be expected between the two sets of quadrats; a authors; fieldwork: AC. Mann–Whitney test demonstrated a significant difference in Conflicts of interest None. medians between them (U = .,N=,P=.; Fig. ). A Wilcoxon matched pairs test showed the number of live Ethical standards This protected species was handled with permis- P. bermudensis found on limestone was significantly greater sion of the Bermuda Government, and no specimens were collected. than at the control sites (V = .,N=,P=.), and The research otherwise abided by the Oryx guidelines on ethical the number was significantly less beneath C. equisetifolia standards. trees than at the control sites (t = −.,df=,P=.). Our findings suggest that P. bermudensis is not more References abundant in places with higher plant diversity and at sites
away from the coast, but is more abundant on limestone CHIBA,S.() Invasive non-native species provision of refugia for and less abundant where the invasive tree C. equisetifolia endangered native species. Conservation Biology, , –. is present. The dry season timing of this survey probably CHIBA,S.&COWIE, R.H. () Evolution and extinction of land contributed to the strength of the association with lime- snails on oceanic islands. Annual Review of Ecology, Evolution and Systematics, , –. stone, as snails were using overhangs and damp crevices COOTE,T.&LOÈVE,E.() From species to five: endemic tree as refugia. Snails concentrated around limestone were pos- snails of the Society Islands fall prey to an ill-judged biological sibly seeking calcium for their egg shells. In breeding tanks, control programme. Oryx, , –.
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