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Journal of Archaeological Science 40 (2013) 4024e4038

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Journal of Archaeological Science

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Intensifying collection and size increase of the tessellated nerite snail(Nerita tessellata) at the Coconut Walk site, Nevis, northern LesserAntilles, AD 890e1440

Christina M. Giovas a,*, Meagan Clark b, Scott M. Fitzpatrick b, Jessica Stone b

aDepartment of Anthropology, University of Washington, Box 353100, Denny Hall M32, Seattle, WA 98195-3100, USAbDepartment of Anthropology, University of Oregon, Eugene, OR 97403, USA

a r t i c l e i n f o

Article history:Received 13 February 2013Received in revised form9 May 2013Accepted 9 May 2013

Keywords:CaribbeanNevisZooarchaeologyResource depressionNerita tessellataShellfishSustainable resource use

* Corresponding author. Tel.: þ1 905 975 4870, þ1 23285.

E-mail address: cmgiovas@uw.edu (C.M. Giovas).

0305-4403/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jas.2013.05.008

a b s t r a c t

Recent archaeological investigation at the Coconut Walk site on the Caribbean island of Nevis revealeddense 40 cm deep midden deposits that accumulated between cal AD 890e1440. Analysis of invertebratefaunal remains reveals an assemblage dominated by nerite snails. We measured the length and width ofmore than 2700 tessellated nerite (Nerita tessellata) shells to investigate evidence for changing mean sizethat might be indicative of intensifying human predation pressure or other cultural and natural pro-cesses. Contrary to similar archaeomalacological studies in which size decline is detected, we observed astatistically significant size increase for N. tessellata over time. This size increase is coupled withincreasing levels of tessellated nerite exploitation at the Coconut Walk site. Results suggest thattessellated nerite use was sustainable over several centuries of site occupation. Our findings haveimportant implications for investigations of anthropogenic impacts on prehistoric mollusc populations.In addition, the findings reported here provide important insight into human subsistence patterns duringthe Late Ceramic Age in the Caribbean and a framework for comparison with observations from otherPre-Columbian sites in the Caribbean.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The systematic exploitation of marine resources for subsistenceextends deep into antiquity and is now widely recognized as asignificant adaptation that facilitated the expansion of human so-cieties into new environments (Erlandson, 1988, 2001; Erlandsonand Fitzpatrick, 2006; Erlandson et al., 2007; Parkington, 2004;Waselkov, 1987). Molluscs are known to have been exploited by theearliest anatomically modern humans around 165 kyr ago in SouthAfrica (Jerardino and Marean, 2010; Marean et al., 2007) withtentative evidence for human impacts on shellfish resources in thisregion dating to 72e85 kyr ago in the Middle Stone Age (Langejanset al., 2012). For some time, researchers have employed a decreasein the mean size of mollusc shells from archaeological contexts asevidence for intensifying human predation on shellfish populations(Allen, 2012; Braje et al., 2012; Erlandson et al., 2008, 2011;Erlandson and Rick, 2010; Faulkner, 2009; Jerardino, 1997; Lasiak,

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1991; Mannino and Thomas, 2001, 2002; Milner et al., 2007;Morrison and Hunt, 2007; Morrison and Cochrane, 2008; Prummel,2005; Stager and Chen, 1996). At the Pre-Columbian site of CoconutWalk on the Caribbean island of Nevis, abundant mollusc remainstestify to the importance of shellfish resources to prehistoric in-habitants there. Here we investigate the potential for humanimpact on tessellated nerites (Nerita tessellata), the most heavilyexploited molluscan species at the site, by assessing N. tessellatashell size over time at Coconut Walk.

1.1. Environment and Pre-Columbian history of Nevis

Known as Oualie (‘land of beautiful water’) to its original in-habitants, the island of Nevis (area c. 93 km2) is the smaller partnerof the two island St. Kitts and Nevis Federation which lies in thenorthern part of the Leeward Islands in the Eastern Caribbean(Fig. 1). Part of an arc of volcanic islands on the edge of a tectonicplate boundary between the Atlantic and Caribbean plates, thecentre of Nevis is dominated by a dormant volcano, the cloud-shrouded Nevis Peak, which rises to 985 m in height.

While cursory examination of the island’s prehistory byscholars had recorded a number of different sites with scattered

Fig. 1. Map of the Caribbean showing the location of Nevis.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4025

surface collections, it was not until Samuel Wilson (1989, 2006)began conducting more intensive archaeological surveys in the1980s that Pre-Columbian sites were systematically recorded andidentified. During his investigations, Wilson (2006) found evi-dence for an earlier Archaic occupation at the Hichman’s ShellHeap site, as well as an abundance of Ceramic Age sites spanningbetween ca. 500 BCeAD 1400.

1.2. Archaeological research background

While Wilson’s (1989: 427e450, 2006: 62e63) surveys identi-fied more than 20 sites on Nevis, very few of these were actuallyexcavated, with those that were mostly limited to smaller units.Apart fromwork by archaeologists at Southampton University, whohave focused primarily at the Hichman’s site and some historicruins (Morris et al., 2003), there has been a paucity of archaeo-logical investigation into Amerindian occupation on Nevis. As such,it was an opportune time to begin a new campaign to document theisland’s archaeologically rich record of prehistoric occupation.Given a host of current issues related to accessibility, logistics, andsite visibility, it was determined that the site of Coconut Walk,located on the southeast part of the island, would be an ideallocation to examine Pre-Columbian lifeways during the later stagesof occupation known as the Late Ceramic Age, i.e., post-AD 500 toEuropean contact. In addition, a previous attempt in 1998 by theU.K. television program Time Team to examine Coconut Walk hadproduced some interesting results (Bellamy, 2001; Nokkert, 2001)and suggested that the site still had much more to offer.

Field investigation began in 2010 with the establishment of anarbitrary grid system across the site. Three 5 � 5 m trenches (3073,2973, and 2273) were excavated, the two more northerly of which

(3073 and 2973) extended eastwards from one of the Time Team’slarger excavation units from 1998 (Fig. 2). Trenches 3073 and 2973were opened using mattocks and trowels, revealing a shallow (c.30 cm) humic layer with few artifacts and faunal remains overlyingmostly sterile, sandy subsoil. Based on this finding, and informationgleaned from the Time Team excavation reports, it was clear thatthis part of the site was used primarily for habitation and not refusedisposal. Therefore, after the removal of topsoil by hand, a me-chanical backhoe was employed for controlled removal of theremaining humic layer to reveal the lighter colored subsoil andidentify possible features (e.g., postholes) and the boundaries of theold 1998 Time Team trench.

Investigations in Trench 2273 focused on an extensive middendeposit (Fig. 3). The trench was subdivided into 25 1 � 1 m units,each of which was labeled with a unique computer-generatedbarcode number. After clearing the surface by hand, an initial5 cm layer of disturbed topsoil (Planum 0) was removed andexcavation proceeded by hand troweling in 10 cm arbitrary levels,termed plana, since midden boundaries were diffuse. The trenchwas excavated down to subsoil, approximately 45 cm below thesurface, resulting in four plana in addition to the uppermostdisturbed layer.

Radiocarbon dates for Planum 1 (1350 þ 40 BP or cal AD 970e1170 at 2s), Planum 3 (570 � 30 BP or cal AD 1320e1440 at 2s) andPlanum 4 (1410 þ 40 BP or cal AD 890e1080 at 2s; 720 � 30 BP orcal AD 1170e1300 at 2s), suggest that deposits accumulated over aperiod of up to five and half centuries, between cal AD 890e1440(Table 1). All radiocarbon dates are on shell, calibrated using Calib6.0 with a global marine reservoir correction (Stuiver et al., 2009). Itshould be noted that while the earliest date for the site (cal AD890e1080) is from the lowest Planum (4), the other radiocarbon

Fig. 2. Map of Coconut Walk grid showing location of the larger 1998 Time Team trench (white block) with trenches (hatched) excavated in 2010.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e40384026

assay from this context dates at least a century later (cal AD 1170e1300). In addition, the date for the uppermost Planum (1) fallsbetween the two ages from Planum 4 (ca AD 970e1170). This is notentirely unexpected given that midden deposition is often notevenly distributed horizontally or vertically. The fact that thesethree samples come from the same vicinity (squares 6, 8, and 9)testifies to this occurring within a restricted area (Fig. 3). Theyoungest date, which comes from Planum 3 (cal AD 1320e1440),was from the farthest corner square (25) in the 5� 5m trench in anarea of dense cobble accumulation and mixing. This square appearsto have been disturbed more than any other one in the trench,suggesting that the dated specimen could have been displaced.Additional dating may help to resolve the discrepancy between theradiocarbon ages and midden depositional processes. Because theradiocarbon dates from square 6, 8, and 9 all overlap at two stan-dard deviations, we treat the deposits they represent and thoseimmediately surrounding them as stratigraphically intact and

consistent with rapid deposition over a few centuries (Table 1). Asdiscussed below, the nerite samples analyzed in this study comefrom this more secure context.

Artifacts and faunal remains from Trench 2273were collected byhand during excavation. However, four 1 m2 units (square numbers7, 9, 17, and 19) within the trench were designated as environ-mental squares for systematic collection of zooarchaeological andpaleobotanical data (Fig. 3). The fill from these units was carried inbuckets down to the sea to bewet sieved through 6.4mmmesh andscreened archaeological materials were cleaned, bagged, andcataloged for further analysis. Molluscan remains were transportedback to North Carolina State University Archaeology Laboratorywhere they were identified by three of the authors (MC, SMF, andJS) using published guides and comparative collections. Remainswere quantified based on number of identified specimens (NISP)and minimum number of individuals (MNI) (Grayson, 1984; Reitzand Wing, 2008). MNI values are based on counts of a non-

Fig. 3. View of Trench 2273 at Coconut Walk. Raised units are “environmental squares” 7, 9, 17, 19 designated for wet sieving.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4027

repeating element (part) of the shell. For gastropods this is the shellapex.

2. The tessellated nerite

2.1. Nerita tessellata exploitation and the potential foranthropogenic impacts

More than 37,000 tessellated nerite individuals were recordedfor Trench 2273, making it the most abundant mollusc speciesoverall at Coconut Walk (63.8% of analyzed mollusc MNI). Inaddition, it was clear that N. tessellata exploitation intensified overtime at the site. Tessellated nerites increase from 58.7% ofmolluscan assemblage MNI in the lowermost planum to 66.4% ofthe assemblage by the end of site occupation. In light of this sig-nificant abundance, we undertook a size analysis of N. tessellata toinvestigate whether harvesting by Coconut Walk residentsimpacted prehistoric populations of this small intertidalgastropod. In particular, we were interested in the potential forresource depression in this species due to intensifying predationpressure.

Resource depression refers to the phenomenon of decliningencounter rates with a prey type and is signaled by a decrease in itsrelative abundance on the landscape (Broughton, 1994a, 1994b;Lupo, 2007). Informed by optimal foraging theory models derivedfrom behavioral ecology, archaeologists have identified prehistoricresource depression for numerous vertebrate and invertebrate

Table 1Radiocarbon dates from the Coconut Walk site.

Lab no. Sampleno.

Database no. Type Species Unit Square

Beta-290340 NEV-01 10NCW003SHE Shell Eustrombus gigas(juvenile)

2273 6

Beta-324951 NEV-03 10NCW0201SHE Shell Cittarium pica 2273 25Beta-290341 NEV-02 10NCW0166SHE Shell Cittarium pica 2273 8Beta-324952 NEV-04 10NCW0212SHE Shell Cassis tuberosa 2273 9

fauna in a variety of environmental settings (e.g., Broughton, 1997,1999, 2002; Butler, 2001; Byers and Broughton, 2004; Cannon,2003; Erlandson and Rick, 2010; Grayson and Delpech, 1998;Morrison and Hunt, 2007; Nagaoka, 2000, 2002a, 2002b, 2005;Wolverton et al., 2012, 2008; Whitaker, 2010). In this study, we arespecifically concerned with evidence for anthropogenic exploita-tion depression where prey numbers decrease because humanpredation exceeds the level at which a targeted species can sustainits population.

Identification of exploitation depression in the archaeologicalrecord relies on multiple lines of evidence. For molluscs, these havebeen comprehensively discussed by Claassen (1986, 1998) andMannino and Thomas (2002), so we only summarize them asrequired here. Indicators for molluscan exploitation depressioninclude: 1) a decline in the relative abundance of the preyspeciesdClaassen (1998) adds that absolute abundances shouldalso track changing relative frequencies; 2) evidence for decliningforaging efficiency in the form of diet breadth expansion, (asmeasured by species richness, diversity, or evenness) along with anincreasing reliance on species that involve more effort to procureand process (Allen, 2012; Grayson et al., 2001; Nagaoka, 2001;Jones, 2004; Mannino and Thomas, 2002); 3) a change in the ageprofile of the prey species, typically seen as a decrease in mean agegiven that fewer individuals are likely to survive to older ages withincreasing predation (Claassen, 1986; Faulkner, 2009; Milner et al.,2007;Mannino and Thomas, 2001, 2002); and finally, 4) a change inthe mean size of the prey species (Braje et al., 2012; Campbell,

Planum Feature cm Belowsurface

Wt. (g) 13C/12Cratio

Measured14C age

Cal (2 sigma)

1 TOP 0e10 67.0 1.8 1350 � 40 AD 970e1170

3 L001 20e30 55.4 0.3 570 � 30 AD 1320e14404 L003 30e40 59.8 2.6 1410 � 40 AD 890e10804 L003 30e40 58.2 2.7 720 � 30 AD 1170e1300

Fig. 4. Tessellated nerite measurements points: SL ¼ shell length; SW ¼ shell width.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e40384028

2008; Claassen, 1986, 1998; Erlandson et al., 2011; Fenberg and Roy,2007, 2012; Jerardino, 1997; Lupo, 2007; Mannino and Thomas,2001, 2002; Stager and Chen, 1996). We focus on the relationshipbetween the latter two indicators as these are especially relevant tothis investigation.

Predictions for archaeological mollusc size and prehistoricexploitation follow from the assumption that people preferentiallytarget larger individuals (Fenberg and Roy, 2007, 2012; Langejanset al., 2012), except in cases where shellfish beds may have beenpurposefully managed for long-term productivity (Cannon andBurchell, 2009; Whitaker, 2008). Within the context of intensi-fying collection, the average size of a mollusc species is expected todecrease because, as larger individuals are selectively removed,only smaller individuals remain within the population. Sustained,high levels of exploitation limit the opportunity for molluscs togrow larger, contributing to this effect. As larger individualsbecoming rarer, foragers are expected to increasingly incorporatesmaller individuals into the diet. As a consequence, the archaeo-logical prey populationwill reflect the decreasingmean shell size ofthe natural population being exploited.

Since body size and age are generally correlated, particularly forspecies of indeterminate growth, a decline in the frequency oflarger individuals in a population also results in fewer older,sexually mature individuals. This effect may have additional lifehistory repercussions for a prey species, including a reduction inthe reproductive potential of the population, changes in growthrate, and a lowering of the age of onset for sexual maturity (Fenbergand Roy, 2007). In some cases, the latter effect may further depressmean size since many molluscan taxa cease or greatly reduce therate of growth once sexual maturity is achieved (Axelsen, 1968;Fenberg and Roy, 2007; McCarthy, 2007). Life history variablesinteract in complex ways with predation effects and environmentalfactors such that the relationship between changes in prey age andsize is not always straightforward (Pernet, 2007; Ray and Stoner,1995). In light of this, many investigators have cautioned thatarchaeological assessments of changing prey size must considerspecies biology and ecology when evaluating the potential foranthropogenic exploitation depression (Broughton, 2002; Claassen,1998; Giovas et al., 2010; Mannino and Thomas, 2002; Thakar,2011). We, therefore, provide a brief overview of tessellated ner-ite ecology and life history below.

2.2. Nerita tessellata ecology and life history

The tessellated, or checkered, nerite, is an intertidal gastropodbelonging to the family Neritidae. It is one of the smaller Neritaspecies in the Western Atlantic and is found along subtropical andtropical sea coasts from Florida to Texas, through the Caribbean toBrazil (Abbott and Morris, 1995; Rehder, 1981; Rosenberg, 2009).Tessellated nerites inhabit rocky shorelines, often congregating inlarge numbers, just at or above the tide line, but may be immersedin up to 0.5 m of water (Axelsen, 1968; Bovbjerg, 1984; Chislett,1969; Potts, 1980). The species is readily recognized by the blackand white checkered markings that give it its name. The shell issturdy, thick and globose, with spiral cords, a low apex, andcalcareous operculum (Fig. 4). The D-shaped aperture exhibits awell-defined parietal wall marked with small irregular beads andtwo small, nub-like teeth on the columellar margin. Snails reach areported maximum length of 25 mm (Rosenberg, 2009), but aretypically under 20 mm (Axelsen, 1968; Lewis, 1971). Like othernerite species, as the tessellated nerite grows, it resorbs posteriorportions of its shell, enlarging the interior chamber from within toaccommodate its relatively ample visceral mass (Vermeij, 1987:224). This process also allows the snail to maintain a reservoir ofwater within the shell, a key adaptation that facilitates evaporative

cooling and prevents desiccation with exposure above the tide line(Vermeij, 1973).

The tessellated nerite exhibits a marked habitat zonation withits congeners, Nerita peloronta and Nerita versiclor (Axelsen, 1968;Bovbjerg, 1984; Cairns and Wagner, 2000; Chislett, 1969;Kolipinski, 1964). N. peloronta, N. versicolor, and N. tessellata ar-ranged themselves, respectively, within the high, medium, and lowintertidal zone, maintaining their relative position within thesesectors by migrating with the tide as it moves in and out. Thezonation exhibited by Nerita spp. results from species-specificphysiological adaptations that make certain microhabitats moresuitable for each taxon, rather than competitive exclusion, where ataxon successfully pushes aside others competing for the sameresources. N. tessellata’s relatively large gill capacity, for instance,allows it to tolerate the lowered oxygen levels associated withfrequent submersion in the splash zone (Cairns andWagner, 2000).

Like other nerites, N. tessellata is nocturnal, emerging from itsshell at night to feed on the biofilm layer covering rocky substrates(Bovbjerg, 1984). Sensitive to sunlight and strong wave action, itwill seek out crevices and the underside of rocks and may be easilycollected from these locations during low tide (Bovbjerg, 1984;Rehder, 1981). Information regarding natural mortality in tessel-lated nerites is limited, although octopuses have been recorded as apredator (Bovbjerg, 1984).

Snails reach sexual maturity at between 14 and 17 mm in length(Chislett, 1969; Kolipinski, 1964), and show a marked decline ingrowth rate once this stage is achieved (Axelsen, 1968). Sexes areseparate and fertilization is internal. Females lay egg capsules inwater filled depressions on rocks and will annually depositapproximately 160 capsules, each of which contains more than 100eggs (Hughes, 1971a; Kolipinski, 1964). Once the planktonic veligerlarvae hatch, they are dispersed by the currents and remain in thewater column for several months until they reach the terminaldevelopmental stage, whereupon they settle onto a suitable sub-strate and metamorphose into juvenile snails (Kolipinski, 1964).Tessellated nerites have a lifespan of about three and half years(Hughes, 1971b) and continue to grow throughout life.

3. Methods

Nearly 7000 N. tessellata shells from the Trench 2273 samplewere measured, but only a subset of these originated from unitswhose deposits were wet sieved through 6.4 mm (1/400) meshscreens. This study focuses exclusively on screened samples(n ¼ 2710) from squares 9 and 17 to eliminate potential method-ological bias from tests of nerite size change. Future research willemploy the full sample to explore the impact of screening protocolson such tests, however.

Nerite specimens in this study were each labeled numericallyabove the inner lip of the shell for individual identification related

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4029

to data quality control. Measurements on shell length and widthwere taken using an Absolute Digimatic caliper andwere rounded tothe nearest hundredth of a millimeter. Shell length was taken fromthe apex of the spire to the most inferior portion of the body whorl.Shell width was measured as the distance between the two mostlateral portions of body whorl and the outer lip (Fig. 4). Data werecompiled into an Excel database and statistically evaluated using R2.15.1 and SPSS 13.0 software.

Fig. 6. Box plots of tessellated nerite width. Circles represent outliers circles representoutliers; see text for explanation.

4. Results

4.1. Evaluation of mean length and width

Statistical analysis was performed on length and width mea-surements from a total sample of 2710 tessellated nerite shells fromPlana 1, 2, 3 and 4. Table 1 and Figs. 5 and 6 summarize associateddescriptive statistics for mean length and width of shells by periodand graphically illustrate these using box plots to show the spreadof data. Mean and median shell length increase from the earliestdeposit represented by Planum 4 (x ¼ 9.96 mm) through to theuppermost layer, Planum 1 (x¼ 10.44 mm). This trend is duplicatedin shell width measurements (Pl. 4: x ¼ 12.70 mm; Pl. 1: x ¼ 13.27).There are some outliers in the samples, represented by circles in theboxplots and defined as cases with values between 1.5 and 3 timesthe interquartile range (i.e., the box length) from the upper or loweredge of the boxes (Table 2). However, skewness and kurtosis arelimited and sample means andmedians are typically close together,indicating that outliers do not positively or negatively skew samplemeans. Overall, the data indicate a 0.5e0.6 mm increase in tessel-lated nerite size over the full span of site occupation, cal AD 890e1440, which represents about a 5% size increase in these smallsnails.

Tests for homogeneity of sample variances were insignificant(Levene statistic, length: 1.501, p ¼ 0.212; Levene statistic, width:0.640, p ¼ 0.589), but QeQ plots (Fig. 7; Fig. 8) and significant re-sults for Lilliefors tests of normality indicated non-normal datadistributions in some of the samples (Pl. 1 length: D ¼ 0.031,p ¼ 0.013, df ¼ 1145; Pl. 1 width: D ¼ 0.030, p ¼ 0.020, df ¼ 1145).These could not be corrected using standard data transformations,therefore, non-parametric tests were used to assess the statisticalsignificance of the observed trends based on a significance level of

Fig. 5. Box plots of tessellated nerite length. Circles represent outliers; see text forexplanation.

0.05. A KruskaleWallis test returned significant results for bothlength (c2 ¼ 34.14, p < 0.001) and width increases (c2 ¼ 30.85,p < 0.001). To further explore these trends we employed post hocmultiple pairwise comparisons using GameseHowell tests to con-trol for the experiment wise Type 1 error rate. The results appear inTable 3 with statistically significant comparisons highlighted inbold font. As can be seen from these data, increases in length andwidth are significant between the lowermost planum (Planum 4)and all others, but not between any other plana. Nerites increaseapproximately 0.40e0.45 mm in mean length and width betweenPlanum 4 and 3. Thereafter they remain relatively unchanged insize, suggesting possible stabilization at this time of the conditionsthat promoted size increase.

4.1.1. Evidence for intensified exploitation of Nerita tessellataThe trend for size increase occurs against a backdrop of

increasing tessellated nerite exploitation as measured by relativeabundance (% NISP and % MNI of molluscs) for squares 9 and 17combined (Table 4). At the same time, nerites increase dramati-cally in absolute abundance, and are nearly ten times as abun-dant in the uppermost excavated level (MNI ¼ 3290) as thelowermost (MNI ¼ 337). The results of chi square tests of MNI-based abundance for N. tessellata as compared to all other mol-luscs support these assessments (Table 5). Statistically significantincreases in nerite abundance (indicated in bold font, Table 5) areregistered for all paired planum comparisons except those ofPlana 2 and 3, and Plana 3 and 4 (insignificant test results arelikely due to the smallness of incremental changes from oneplanum to the next, coupled with the effect of smaller, unevensample sizes). Notably, longer term comparisons between Plana 1and 4, and Plana 2 and 4 are significant. On the whole, the evi-dence presented here confirms a gradual 8e9% increase inN. tessellata abundance at Coconut Walk over several centuries ofsite occupation.

Molluscan species richness (the number of taxa) also appears toincrease over the same period (Table 4), suggesting that dietbreadth increases. However, taxonomic counts are loosely corre-lated with sample size (NISP), which also increases over time(R ¼ þ0.937, p ¼ 0.063), making taxonomic richness an imperfectindicator of diet breadth and changing foraging efficiency in thisparticular case.

Fig. 7. QeQ plots for tess

Table 2Descriptive statistics for tessellated nerite length and width at the Coconut Walksite.

Descriptive statistics

Planum 1 Planum 2 Planum 3 Planum 4

LengthMean 10.44 10.30 10.37 9.9695% Conf. Interval for mean Lower 10.36 10.21 10.26 9.81

Upper 10.51 10.38 10.48 10.10Median 10.50 10.29 10.33 9.97Variance 1.628 1.556 1.397 1.605Std. deviation 1.276 1.247 1.182 1.267Minimum 6.160 5.660 6.350 5.810Maximum 14.110 15.510 13.810 13.160Range 7.950 9.850 7.460 7.350Interquartile range 1.725 1.590 1.570 1.790Skewness �0.209 0.029 �0.104 �0.283Kurtosis �0.012 0.599 0.123 �0.019

WidthMean 13.27 13.41 13.17 12.7095% Conf. interval for mean Lower 13.16 13.29 13.01 12.50

Upper 13.38 13.53 13.33 12.91Median 13.31 13.44 13.10 12.79Variance 3.339 3.156 3.190 3.217Std. deviation 1.827 1.777 1.786 1.794Minimum 7.260 6.200 7.290 7.580Maximum 18.580 18.810 18.110 17.380Range 11.320 12.610 10.820 9.800Interquartile range 2.375 2.265 2.240 2.510Skewness �0.182 �0.162 �0.023 �0.272Kurtosis �0.038 0.481 �0.031 �0.129

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e40384030

5. Discussion

The abundance of N. tessellata at Coconut Walk is consistentwith high levels of nerite exploitation observed for other LesserAntillean Pre-Columbian archaeological sites, including those onSt. Martin (Serrand and Bonnissent, 2005), Carriacou (Fitzpatricket al., 2009; Giovas, 2009, 2013), Grenada (Newsom and Wing,2004: 87), and the Hichman’s (GE-5) site also on Nevis (Nokkert,2002). Tessellated nerites were a principle component ofmolluscan foraging from the earliest occupation of the site,beginning around cal AD 890. Early in site history these small snailsincreased in size and then remained relatively large through to theend of occupation. This phenomenon occurs in conjunction withincreasing levels of nerite exploitation over time. Evidence for anincrease in mean size that is linked with intensified predation is aninfrequent finding among studies such as this one. Of greater sig-nificance is the fact that this phenomenon is inconsistent withstandard predictions derived from foraging theory and anthropo-genic exploitation depression models. In such models increasingpredation pressure is typically expected to yield a decrease in meanprey size (Broughton, 1994a, 1994b, 1997, Butler, 2001; Lupo, 2007;Mannino and Thomas, 2002; see Broughton 2002 for an exceptioninvolving age (and by extension, size) and behavioral depression).

One possible explanation for the observed patterning is that itresults from analytic bias associated with collection methods ortaphonomic processes. There are good reasons to believe neither ofthese factors is at play in this study, however. First, the samecollection procedures, including systematic screening of excavateddeposits, were employed across all plana throughout the excava-tion of the samples used in this analysis. It could be argued that

ellated nerite length.

Fig. 8. QeQ plots for tessellated nerite width.

Table 3Post hoc pairwise comparisons for mean length and width between plana. Statis-tically significant results in bold type.

GameseHowell post hoc comparisons

Planum Planum Mean difference Std. error p

Length1 2 0.140 0.058 0.075

3 0.069 0.066 0.7204 0.482 0.083 <0.001

2 1 �0.140 0.058 0.0753 �0.071 0.070 0.7404 0.341 0.086 <0.001

3 1 �0.069 0.066 0.7202 0.071 0.070 0.7404 0.412 0.092 <0.001

4 1 �0.482 0.083 <0.0012 �0.341 0.086 <0.0013 �0.412 0.092 <0.001

Width1 2 �0.140 0.083 0.330

3 0.105 0.098 0.7054 0.569 0.118 <0.001

2 1 0.140 0.083 0.3303 0.245 0.103 0.0824 0.709 0.122 <0.001

3 1 �0.105 0.098 0.7052 �0.245 0.103 0.0824 0.464 0.133 0.003

4 1 �0.569 0.118 <0.0012 �0.709 0.122 <0.0013 �0.464 0.133 0.003

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4031

there is a survivorship bias against smaller shells in older depositsdue to an increased vulnerability of these to shell dissolution and/or fragmentation. Since the deposits accumulated over a relativelyshort period, however, it seems unlikely that the chemical break-down of smaller, less robust shells could account for significantdifferences between plana that are at most, a few centuries apart. Inaddition, tessellated nerite specimens from Coconut Walk wereobserved to be well preserved (i.e., coloration was still largelyvisible on many specimens, regardless of the planum in which theywere found) and relatively intact (Poteate and Fitzpatrick, 2013).This is consistent with quantitative studies demonstrating minimalfragmentation in this taxon compared to other gastropod species inLesser Antillean zooarchaeological assemblages (Giovas, 2009).

In light of these considerations, the observed size increase intessellated nerites appears genuine and must, therefore, arise fromeither human behavioral (cultural) or ecological causes. Behavioralfactors include not only the impact of human predation pressure,but also the effects, associated with specific foraging methods (e.g.,location of patches exploited), or forager attributes (e.g., gender andage) (Bird, 1997; Bird and Bliege Bird, 1997; Campbell, 2008). Theseare discussed in greater detail below.

Teasing environmental and cultural variables apart is potentiallymade easier by establishing whether size changes arise from achange inmean snail growth rate or a change in themean age of thepopulation. This is because for gastropod species of indeterminategrowth, shell size at any given time is a function of both the age ofthe snail and its growth rate. A number of researchers have notedthe importance of assessing snail age in addition to length andwidth metrics for the purpose of distinguishing between causalvariables (Campbell, 2008; Claassen, 1998; Faulkner, 2009; Giovas

Table 4Tessellated nerite NISP, MNI, and assemblage relative abundance.

Class Species Trench 2273: Squares 9 and 17 mollusc remains

Planum 4 Planum 3 Planum 2 Planum 1

NISP % NISP MNI % MNI Wt. (g) NISP % NISP MNI % MNI Wt. (g) NISP % NISP MNI % MNI Wt. (g) NISP % NISP MNI % MNI Wt. (g)

Bivalvia Anadara floridana e e e e e e e e e e e e e e e 1 0.0 1 0.0 20.5Arca zebra e e e e e 1 0.0 1 0.1 10.0 e e e e e e e e e e

Chama sp. e e e e e e e e e e 1 0.0 1 0.0 17.7 5 0.1 3 0.1 125.6Donax denticulatus 89 8.9 44 7.5 16.7 221 9.4 94 5.6 242.0 365 7.8 172 5.4 95.6 285 3.9 111 2.2 65.2Codakia orbicularis e e e e e 30 1.3 2 0.1 25.9 38 0.8 6 0.2 40.2 29 0.4 13 0.3 21.5Lucinoma sp. e e e e e 5 0.2 1 0.1 12.5 e e e e e e e e e e

Raeta plicatella e e e e e e e e e e 1 0.0 1 0.0 8.0 e e e e e

Indeterminate Bivalvia e e e e 6.7 e e e e 26.2 e e e e 24.5 e e e e 92.9Gastropoda Hesperisternia multangulus e e e e e e e e e e e e e e e 1 0.0 1 0.0 0.2

Engina turbinella e e e e e 1 0.0 1 0.1 0.4 e e e e e 2 0.0 2 0.0 3.1Cassis tuberosa 1 0.1 1 0.2 134.8 e e e e e e e e e e e e e e e

Cypraecassis testiculus e e e e e e e e e e 1 0.0 1 0.0 7.0 5 0.1 4 0.1 40.0Cerithium eburneum e e e e e e e e e e e e e e e 1 0.0 1 0.0 0.3Cerithium litteratum e e e e e e e e e e e e e e e 1 0.0 1 0.0 0.8Columbella mercatoria e e e e e 3 0.1 3 0.2 1.1 e e e e e 6 0.1 6 0.1 2.5Columbella rusticoides e e e e e 2 0.1 2 0.1 0.7 e e e e e 2 0.0 2 0.0 1.0Conus flavescens e e e e e e e e e e e e e e e e e e e e

Conus regius e e e e e e e e e e 1 0.0 1 0.0 3.1 1 0.0 1 0.0 3.4Conus spurius e e e e e 1 0.0 1 0.1 7.0 12 0.3 7 0.2 20.9 e e e e e

Conus sp. 7 0.7 7 1.2 3.3 e e e e e e e e e e 12 0.2 7 0.1 19.1Macrocypraea zebra e e e e e e e e e e 1 0.0 1 0.0 3.3 12 0.2 2 0.0 40.7Epitonium lamellosum e e e e e 1 0.0 1 0.1 0.6 e e e e e e e e e e

Leucozonia nassa e e e e e e e e e e 1 0.0 1 0.0 3.8 e e e e e

Leucozonia ocellata e e e e e e e e e e e e e e e 6 0.1 6 0.1 5.5Diodora listeri e e e e e e e e e e e e e e e 1 0.0 1 0.0 1.2Fissurella nimbosa e e e e e e e e e e 2 0.0 2 0.1 1.7 3 0.0 1 0.0 2.4Fissurella nodosa e e e e e e e e e e 4 0.1 4 0.1 3.5 e e e e e

Hipponix antiquatus e e e e e e e e e e 1 0.0 1 0.0 1.3 e e e e e

Echinolittorina ziczac 14 1.4 14 2.4 1.2 97 4.1 57 3.4 8.1 97 2.1 66 2.1 14.4 40 0.5 30 0.6 6.9Cenchritis muricata 18 1.8 12 2.1 10.2 65 2.8 46 2.7 25.9 113 2.4 78 2.5 60.4 160 2.2 116 2.3 85.6Tectura (Lottia) antillarum 6 0.6 6 1.0 4.3 14 0.6 14 0.8 8.6 34 0.7 28 0.9 18.7 28 0.4 28 0.6 15.3Marginella sp. e e e e e e e e e e 2 0.0 2 0.1 0.3 e e e e e

Mitra barbadensis e e e e e e e e e e e e e e e 1 0.0 1 0.0 0.52Plicopurpura platula e e e e e 4 0.2 4 0.2 10.5 1 0.0 1 0.0 3.06 5 0.1 5 0.1 6.3Stramonita haemastoma e e e e e 2 0.1 2 0.1 1.4 6 0.1 6 0.2 8.1 1 0.0 1 0.0 0.88Stramonita rustica 1 0.1 1 0.2 0.63 12 0.5 11 0.7 46.4 11 0.2 11 0.3 42.2 5 0.1 5 0.1 3.88Thais deltoidea 3 0.3 3 0.5 3.45 e e e e e 17 0.4 17 0.5 50.9 12 0.2 12 0.2 85.12Muricidae sp. e e e e e e e e e e e e e e e 11 0.2 11 0.2 12.8Polinices lacteus e e e e e e e e e e 1 0.0 1 0.0 1.0 e e e e e

Nerita tessellata 400 39.9 337 57.7 230.5 1030 43.6 1030 61.3 884.0 2056 43.8 1987 62.6 1564.6 3403 46.4 3290 66.1 2445.1Nerita versicolor 48 4.8 46 7.9 20.9 141 6.0 141 8.4 260.2 273 5.8 269 8.5 170.0 499 6.8 499 10.0 323.3Nerita virginea 1 0.1 1 0.2 0.8 e e e e e 2 0.0 2 0.1 2.8 e e e e e

Nerita sp. e e e e e 108 4.6 e e 18.8 1 0.0 1 0.0 0.2 3 0.0 3 0.1 2.4Oliva sp. e e e e e e e e e e e e e e e 1 0.0 1 0.0 2.8Supplanaxis nucleus 19 1.9 12 2.1 2.3 27 1.1 27 1.6 6.6 47 1.0 47 1.5 11.4 115 1.6 114 2.3 22.3Planaxidae sp. e e e e e e e e e e e e e e e e e e e e

Charonia sp. e e e e e e e e e e e e e e e 1 0.0 1 0.0 36.23Monoplex nicobaricus 2 0.2 2 0.3 1.27 1 0.0 1 0.1 3.6 6 0.1 6 0.2 5.8 7 0.1 7 0.1 19.2Cymatium sp. e e e e e e e e e e e e e e e 1 0.0 1 0.0 2.1Eustrombus (Lobatus) gigas e e e e e 9 0.4 3 0.2 34.9 25 0.5 2 0.1 143.6 6 0.1 3 0.1 341.1Strombidae sp. 22 2.2 3 0.5 7.33 e e e e e e e e e e 1 0.0 1 0.0 120.6Tegula excavata 55 5.5 42 7.2 22.8 194 8.2 146 8.7 279.2 282 6.0 223 7.0 197.5 367 5.0 302 6.1 219.7Lithopoma caelatum 2 0.2 1 0.2 2.19 16 0.7 5 0.3 65.7 16 0.3 6 0.2 72.9 16 0.2 10 0.2 103.4

C.M.G

iovaset

al./Journal

ofArchaeological

Science40

(2013)4024

e4038

4032

Lithop

omatube

r23

2.3

61.0

64.7

793.3

201.2

418.4

129

2.7

270.9

517.6

271

3.7

741.5

699.9

Cittarium

pica

484.8

91.5

73.6

903.8

241.4

452.4

169

3.6

371.2

1197

.255

47.6

491.0

3436

.0Cy

clostrem

iscu

sbe

auii

ee

ee

ee

ee

ee

10.0

10.0

0.1

ee

ee

e

Turbocailletii

ee

ee

ee

ee

ee

10.0

10.0

0.5

ee

ee

e

Turbocastan

eae

ee

ee

ee

ee

e1

0.0

10.0

4.47

ee

ee

e

Indeterminateop

ercu

la6

0.6

61.0

4.4

190.8

191.1

18.1

370.8

371.2

33.5

781.1

771.5

62.9

IndeterminateGastrop

oda

ee

ee

10.41

ee

ee

4.3

ee

ee

99.9

ee

ee

252.7

Polyplaco

phora

Chiton

tube

rculatus

237

23.7

315.3

49.5

187

7.9

241.4

145.2

942

20.1

119

3.7

277.8

1374

18.7

173

3.5

401.4

IndeterminateMollusca

ee

ee

17.99

ee

ee

238.0

ee

ee

128.4

ee

ee

663.9

Total

1002

100.0

584

100.0

689.9

2360

100.0

1680

100.0

3256

.746

9810

0.0

3174

100.0

4857

.973

3310

0.0

4977

100.0

9818

.0Ntaxa

(mutually

exclusive

taxa

)19

2535

36

Table 5Chi square test results for tessellated nerite MNI-based abundance between plana.Statistically significant results in bold type.

Chi square tests

Comparison % MNI change c2 p

Planum 1 e Planum\ 2 D3.5 10.26 0.001Planum 2 e Planum 3 þ1.3 0.73 0.394Planum 3 e Planum 4 þ3.6 2.20 0.138Planum 2 e Planum 4 D4.9 4.81 0.028Planum 1 e Planum 4 D8.4 15.89 <0.001Planum 1 e Planum 3 D4.8 12.46 <0.001

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4033

et al., 2010; Mannino and Thomas, 2001, 2002; Milner et al., 2007;Roy et al., 2003). Unfortunately, tessellated nerite shells do notpossess any overt age indicators, such as pronounced varices, usefulfor this purpose. In addition, the QeQ plots and histograms fromthis study exhibit limited evidence for multimodality whichmay beused, although not unproblematically, as a proxy for age classes(Fig. 7; Fig. 8; Fig. 9; Fig. 10) (see discussions in Campbell, 2008;Claassen, 1998; Giovas et al., 2010). The apparent lack of multi-modality may be due to the effects of archaeological time averaging(Lyman, 2003), since living N. tessellata populations typically showa bimodal size distribution (Axelsen, 1968: Fig. 24 and 25).

As discussed earlier, anthropogenic decreases in mean snaillength and/or width are expected in cases of intense human pre-dation pressure based on the assumption that people will prefer-entially select larger (older) individuals when collecting molluscs(Fenberg and Roy, 2007). Removing these individuals lowers theaverage age of the population and depresses mean size. Alterationin natural or culturally influenced ecological conditions can affectsnail growth rate, also resulting in a change in mean size. In suchcases, however, the age structure of the population remainsunchanged.

Nerite growth rate has been shown to be influenced by anumber of factors, including microhabitat, seasonal air tempera-tures, sea surface temperatures, wave action, access to food,intraspecific competition, latitude, and the maturation of gonadsduring the spawning season (Axelsen, 1968; Chislett, 1969;Faulkner, 2009; Kolipinski, 1964; Lewis, 1971; Lewis et al., 1969;Underwood, 1975, 1976). For N. tessellata, these factors appear tointeract in complex ways so that the mean growth rate of a

Fig. 9. Histogram of tessellated nerite length.

Fig. 10. Histogram of tessellated nerite width.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e40384034

population may or may not vary according to location, season, oryear. For example, both Kolipinski (1964) and Axelsen (1968) foundmarked seasonal variation in the growth rate of Florida andBarbados tessellated nerites, but this was driven by differentenvironmental factors in each study. Chislett (1969), on the otherhand, observed no significant monthly changes in the rate ofgrowth in his study of Barbados N. tessellata.

In this study, we have demonstrated a shell size increase withsubsequent size stabilization against a backdrop of increasingnerite exploitation. It is difficult to conceive of a scenario in whichintensifying predation would yield an increase in mean tessellatednerite size through increases in mean age, even if, for example, thenatural population was enhanced by additional juvenile recruit-ment. Underwood (1976) has demonstrated that nerite growth rateis negatively correlated with high population densities due to foodcompetition, and specifically showed through experiments on therelated Nerita atramentosa that increased population densitiesdriven by high levels of recruitment actually increased the mor-tality of adults and lowered juvenile growth rates. In the experi-ments, high densities of nerites ultimately could not bemaintained.In light of this, the only alternative explanations for the CoconutWalk pattern are those involving specific changes in humanshellfishing strategies or a naturally or culturally induced increasein the growth rate of snails. Importantly, neither of these supports asimple model of exploitation depression as previously outlined.

There are certain behavioral aspects of shellfishing strategiesthat may explain the observed patterning at Coconut Walk. Forinstance, sincemollusc populations (including those ofN. tessellata)can exhibit differing size frequency distributions depending onlocation, a change in the patch exploited may results in a change inthe mean size of archaeological shells (Axelsen, 1968; Claassen,1998; Campbell, 2008). In the case of Coconut Walk, the increasein tessellated nerite size may simply arise from residents exploitingincreasingly more distant (less disturbed) shellfish patches wherenerites were larger. Typically, for both vertebrate (Binford, 1978;Broughton, 1999; Cannon, 2003; Faith, 2007; Nagaoka, 2005) andinvertebrate taxa (Bird et al., 2002;Whitaker and Byrd, 2012; O’Dayand Keegan, 2001), an increase in the field processing of prey isexpected to result from foraging at greater distances from a centralplace, following from the expectation that foragers will attempt tomaximize energy return rates associatedwith the increased costs ofmore distant travel. Because of their small size relative to othertaxa, however, nerites are not amenable to this type of carcass

utility transport analysis. Ethnoarchaeological work has demon-strated that Nerita and other similarly small molluscs are typicallyreturnedwhole to a central residence because, in terms of energeticreturn, it is notworthwhile to process these in field (Bird,1997; Birdand Bliege Bird, 1997). Because of this, detecting changes in thesource location of exploited tessellated nerite populations at Co-conut Walk is probably not possible.

Other potential explanations involving shellfishing strategieshave to do with the behavior of specific foragers or groups of for-agers. In ethnographic studies, Bird and Bliege Bird (2000) andothers (De Boer et al., 2002;Weiant and Aswani, 2006) have shown,for instance, that children are more likely than adults to collectsmaller molluscs, including Nerita spp., so a size increase in neritescould be interpreted as a shift away from children’s foraging. Here,again, however, establishing independent archaeological supportfor this scenario is problematic as the identity of individual pre-historic foragers or groups in this context is unknown.

The possibility that tessellated nerite growth rate increased overtime should be considered. That growth rate can be affected by ahost of natural (non-human) factors has been discussed above. Sizeincrease between Planum 4 and 3 coincides roughly with the Me-dieval Warm Period, c. AD 950e1300, which multi-proxy recordsfrom the circum-Caribbean indicate was a period of warmer seasurface temperatures (SST) and perhaps increased frequency ofhurricanes and major storms (Gischler et al., 2008; Malaizé et al.,2011; Richey et al., 2007). The association between temperature,SST, andNerita growth rate is unclear, with some studies suggestinga positive correlation (Kolipinski, 1964) and others indicating norelationship or even increased mortality in instances of moreextreme temperature and aridity (Axelsen, 1968; Chislett, 1969).

An alternative explanation for increase in N. tessellata growthrate may be found within Nerita’s response to high populationdensities and competition with conspecifics. Potentially, an in-crease in N. tessellata size could result from population thinningand reduced intraspecific competition due to human predation.This has been suggested previously as one possible explanationamong several to account for the size increase of Gibberulus(formerly Strombus) gibberulus in Palauan archaeological deposits(Giovas et al., 2010). For Nerita, data available to evaluate this hy-pothesis are limited. Sharpe and Keough (1998) experimented withthe removal of N. atramentosa individuals on the Australianshoreline to assess the impact on growth rate and weight of theremaining population. The authors did not detect any statisticallysignificant effects on either variable, but the experiment was short-lived, fewer than five months, and involved a very small samplesize, �5 individuals. Additional experimental investigation withcontrolled, graduated levels of nerite collection could providefurther insight into these processes.

At this stage, it is not possible to state conclusively what causedtessellated nerite size increase at Coconut Walk between AD 890e1440, or even whether a statistically significant size increase ismeaningful from a biological perspective. We do note, however,that while archaeological evidence for prey size increase is limited,it is not entirely unknown. On Santa Cruz in the California ChannelIslands, Thakar (2011) detected a size increase in archaeologicalPismo clams, in this case, coupled with a decline in exploitation.This pattern was explained as the continued, short-duration har-vest of a single clam recruitment event during which time thedwindling population aged, resulting in older, larger shells in theuppermost site levels. McCoy (2008) recorded a size increase inlimpets (Cellana spp.) from Moloka’i, Hawai’i during the proto-historic and historic periods that he suggested may have beendue to declining human populations and a release from predationpressure. Both these cases differ from Coconut Walk in that sizeincreases are associated with decreased exploitation.

Fig. 11. Cumulative frequency distribution curves for tessellated nerite length byplanum.

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e4038 4035

There are a few instances, however, where size increase invertebrate taxa has been detected in conjunction with relativelyhigh exploitation levels. Leach and Davidson (2001) documentedincreases in the mean live fork length of several fish species from anumber of archaeological sites around New Zealand. These wereattributed to changes in fishing technology, targeted fishinggrounds, and environmental factors, including altered sea surfacetemperatures. Lastly, for California’s Emeryville Shellmound,Broughton (2002) recorded a mean age increasedand, therefore, ade facto size increasedin several taxa associated with behavioraldepression in female and juvenile prey, which abandoned the areain response to human hunting pressure.

A final observation is warranted. Assuming that N. tessellata sizeat the onset of sexual maturity did not alter with a change ingrowth rate, the mean shell lengths in this study all fall below the14e17 mm benchmark documented for maturity in living pop-ulations. (Chislett, 1969). This suggests that the majority of theexploited population at Coconut Walk was reproductively imma-ture, a conclusion which is supported by size frequency distribu-tions (Fig. 9). As Lasiak (1991) and others (Fenberg and Roy, 2007,2012; Mannino and Thomas, 2002) have noted, if the size atmaturity is larger than the size at which foragers reject a molluscindividual, then the recruitment potential of the population will beimpacted, as all reproducing individuals are potential targets forharvest. The degree to which recruitment is negatively affected willdepend on rates of veliger recruitment from elsewhere, which inturn depend on larval characteristics coupled with local andregional hydrographic conditions. In this study, we have docu-mented an increase in mean length and width in of tessellatednerites based on a robust sample. The shell size of this littoral snailthen remains stable for the remainder of site occupation even asrates of exploitation increase nearly tenfold. To us, this evidencestrongly implies that tessellated nerite exploitation by CoconutWalk inhabitants was sustainable over the duration of severalcenturies of habitation at Coconut Walk.

Whether sustainability in this context arises from intentionalrestraint in harvesting (conservation) by humans is open to ques-tion. Indigenous knowledge about intertidal ecology, Nerita habits,and the appropriate use and harvest of these animals could informthis issue, as it has for other taxa in the Caribbean and elsewhere(Aswani and Vaccaro, 2008; Drew and Henne, 2006; Grant andBerkes, 2004; Turner et al., 2000; Valdés-Pizzini and García-Quijano, 2009; Weiant and Aswani, 2006). Unfortunately, in theLesser Antilles much of such knowledge was lost with the collapseof indigenous island populations following European conquest(Allaire, 2013). In addition, because it is not a commercial species,limited scientific study has been conducted on N. tessellata, and weare not aware of any investigations of traditional ecologicalknowledge (TEK) based in contemporary artisanal fisheries for thisspecies in the insular Caribbean. Elsewhere in the South Pacific, TEKstudies (Aswani and Vaccaro, 2008; Weiant and Aswani, 2006)indicate that Nerita spp. are generally collected by women andchildren, often at night when snails are active. In some cases theyare considered to have therapeutic effects (Aswani and Vaccaro,2008).

Recent studies of prehistoric shellfish use have presentedcompelling evidence for human conservation of molluscan re-sources (Cannon and Burchell, 2009; Whitaker, 2008). On the bal-ance of the evidence presented here, however, we think it likelythat the sustainability of N. tessellata exploitation at Coconut Walkrepresents an instance of epiphenomenal conservation (Alvard,1995; Hunn, 1982), in the sense that sustainability arises fromcombined effects of prey resilience and low human populationdensities, rather than a conscious strategy of sacrificing short-termgains for the long-term benefit of enduring resource availability.

Cumulative frequency distributions for N. tessellata length at Co-conut Walk exhibit a sigmoidal curve relative to the distribution ofa natural population (approximate reconstruction from size fre-quency histogram data based on a natural population sample fromLittle Bay, Barbados taken November 1965 in Axelsen, 1968, Fig. 24;note that data represent a single point in time, not a time-averagedpopulation structure) (Fig. 11). In previous studies of Californiamussel (Mytilus californianus) exploitation, this pattern of

C.M. Giovas et al. / Journal of Archaeological Science 40 (2013) 4024e40384036

sigmoidal distribution that falls below the line described by anatural population has been linked to a “plucking” strategy thatopportunistically targets larger individuals, i.e., a short-term, rate-maximizing strategy (Jones and Richman, 1995; Whitaker, 2008).

These data, together with the size increase exhibited by tessel-lated nerites, lead us to suggest the most parsimonious explanationfor the trends exhibited at Coconut Walk involves mounting, butstill relatively moderate, human harvest pressure that results in alowered mean size relative that of a natural population. Mean snailsize increases over time in response to an increase in growth ratedue to population thinning and reduced intraspecific competition.However, the reproductive potential of the population remains low,as few snails reach sexual maturity (although increased growth ratemay have lowered somewhat the size at onset of sexual maturity).Without outside recruitment, tessellated populations likely wouldhave crashed. In other words, increasing levels of nerite exploita-tion by residents of Coconut Walk were sustainable so long aslarvae could be recruited from healthy populations outside localwaters to offset harvesting impacts.

6. Conclusions

While cases for decreasing mollusc size attributed to intensi-fying predation pressure have become almost routine, herewe havepresented an instance that contravenes standard expectations.Tessellated nerites at the Coconut Walk site increase in size andremain relatively large in the face of intensifying collection byhumans. The specific cause(s) of this patterning are not certain, butalmost assuredly result from a change in growth rate and/or spe-cific foraging practices. Tessellated nerite exploitation appears tohave been sustainable over the duration of site occupation. Thedirect effects of predation pressure (exploitation depression basedon the harvest of larger, and necessarily older, individuals) are notsupported as a causal mechanism, although it is possible that in-direct effects, such as the reduction of interspecific competition,may ultimately be responsible.

This study, in combination with those discussed above, illus-trates the equifinality surrounding causality and mean size in-creases in prey species. They also highlight an important point. Ifthe factors instigating size increase are varied, complex, and notalways clear, it is plausible that those forcing a size decrease in preyare equally complicated. Because of the established theoretical andempirically-observed relationship between human predation andsize decline in prey, it is all too easy to attribute this pattern in thearchaeological record to anthropogenic exploitation depressionwithout first vetting other possible causes. Clearly, we cannotresolve this issue in a single publication, but we hope like others(e.g., Thakar, 2011) to call attention to this particular phenomenonof increased prey size concomitant with intensified exploitation.Human environmental impacts in the past are likely far morecomplex thanwe currently appreciate, and wewill need to developmore nuanced analytic methods and interpretive frameworks tocontinue to address them.

Acknowledgments

We thank the Nevis Historical and Conservation Society,particularly Evelyn Henville, Paul Diamond, Jane Ebbit, and CynthiaHughes, who were all instrumental in arranging logistical supportfor the project. Thanks are also given to Aaron Poteate for providingadditional, valuable data on the Coconut Walk mollusc assemblage.We also appreciate the help given from students who participatedin the 2010 field project at Coconut Walk and in the laboratory toidentify and catalog the faunal remains. This paper benefited from

comments provided by two anonymous reviewers whomwe thankfor their insights.

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