An unusually dense population of Sphodros rufipes (Mygalomorphae: Atypidae) at the edge of its range on Tuckernuck Island, Massachusetts

2011·1.9 MB·English

by  Andrew Mckenna-Foster

2011.TheJournalofArachnology39:171-173
SHORT COMMUNICATION
An unusually dense population ofSphodws riifipes (Mygalomorphae: Atypidae) at the edge ofits range
on Tuckernuck Island, Massachusetts
AndrewMckenna-Foster'-^ MichaelL. Draney-and CherylBeaton': 'Maria MitchellAssociation,4 Vestal Street,
Nantucket, Massachusetts02554 USA; ^UniversityofWisconsin-Green Bay, DepartmentofNatural andApplied
Sciences, 2420 Nicolet Drive, Green Bay, Wisconsin 54311 USA. E-mail: [email protected]
Abstract. We counted and measured Sphodrosrufipes(Latreille 1829) pursewebs in two survey plots on Tuckernuck
Island,Massachusetts.Ourobjectivesweretoquantifywebdensity,recordphysicalwebcharacteristicsanddeterminethe
maincomponentsof5.rufipes’diet.Wecounted479websinthetwoplotsandreportwebdensitiesbetween0.058and0.18
webs/m^,denserthanpreviouslyreportedpopulations.Webswerenotdistributedevenly,anddensitiesrangedfrom0to
0.38webs/m".Aggregationindicessuggestthatwebsareaggregatedonalandscapelevel,butaremoreevenlydistributedat
alocallevel.ContrarytomostpreviouslypublishedliteratureonS. rufipes, wenotedthepredominanceofthegrass-like
sedge,Carexpensylvanica, ratherthantrees,asawebsupport.Coleopteransandisopodsmadeup79percentoftheprey
partscollectedfrom56pursewebs.
Keywords: Pursewebspiders,webdensity,diet,spatialdistribution
Most spiders in the genus Sphodros (family Atypidae) build summer of2008 we returned to Tuckernuck with the objectives to
vertical, tube-like webs that extend from below the soil surface to estimate S. rufipes colony density, record web characteristics, and
attach to the trunk ofa tree or other solid surface (Gertsch and collectpreypartsfordietanalysis.
Platnick 1980). The aerial portion of the tube is usually well We made two trips tothe island on 5-8 Juneand 17-20August
camouflaged bythespiderwith soil particlesand debris. Thereare 2008.Wecountedwebsina50X50mplotonthewesternsideofthe
two Sphodros species in New England, USA. Sphodros rufipes island(southwestcorner41.304558°N,70.26798°W)anda37X50m
(Latreille 1829)buildsavertical tubeofsilkand usuallyattachesit plotontheeasternside(smallerduetotimeconstraints)(southwest
tothebaseofadeciduoustree(Hardy2003). Malesofthisspecies corner41.29922N,70.24516°W).Eachplotencompassedapreviously
have completely red legs, whereas females are all black. Sphodros identifiedaggregationofpursewebs.Thewesternsitewaslocatedona
niger(Hentz1842)isamorecrypticspeciesthatusuallyconstructsthe western-facinghillcoveredingrassesandscatteredheathshrubs.The
‘aerial’ portion ofits web horizontally and, at least on Cape Cod, eastern site was a Hat area with a pitch pine stand (Pinus rigida)
Massachusetts, underneath pine duff and leaf litter (Edwards & surrounded by extensive black huckleberry clones (Gciyhissacia
Edwards 1990). Males and females of this species are all black. baccata)andpatchesofgrassland.Neithersitewasnearopenwater.
Sphodrosrufipesisasouthernspeciesreportedintheliteratureasfar Thesubstrateatbothsiteswassandyloam.Weassignedacoordinate
northasBlockIsland,RhodeIsland,whileS.nigerisamorenorthern systemtoeachplotandbegancountingwebsstartingatthesouthwest
speciesthatoccursasfarsouthasNorthCarolinaandextendsinto cornerdesignatedas(0m,0m)(Fig. 1).Walkingupanddownthe
Canada(Gertsch&Platnick 1980). north axiswecounted webswithin a Im-wide path, startinga new
Mostlikelyduetotheircrypticlifestyle,previousresearchershave pathtotheeast.Inthisway,wezig-zaggedthroughtheplotparallel
onlydescribedattributesandbehaviorsthatcanbestudiedwithsmall totheeastaxis.Wehelda Im"quadratframetomeasurethemeter-
numbers of Sphodros spiders such as mating, prey capture, web wide path as we walked, and we used survey Hags to mark our
placement,andweb-buildingbehaviors(e.g.,McCook1888;Muma& previouspathandlineusupforthenextpassthroughtheplot.We
Muma 1945;Coyle&Shear 1981;Coyle 1983;Edwards&Edwards recordedthelocationofeachwebbymeasuringitsdistancealongthe
1990;Hardy2003).Theonlypopulation-levelstudyweareawareof northandeastaxes.
was conducted over a two-year period in eastern Kansas on Inadditiontotheplots, weusedarandomsearchingprotocolto
populations of S. niger and S. rufipes. Results were inconclusive, assesshowlikelyoneistofind morethanonewebinagivenarea.
because the populations appeared to suddenly decline. To our Afterlocatingaweb,wewalkedinthreerandomdirections,eachfora
knowledge,nootherdemographicdataexistforSphodrosspecies. randomdistancebetweenzeroand50m,andcountedthenumberof
Tuckernuck Island, 50 km south ofCape Cod, Massachusetts, webs weencountered. In all webencounters, weassumed that any
consistsof3.3 km^ ofprivate property located 2.9 km west ofthe webthatwascylindricalratherthanfiattenedwasoccupied.
largerislandofNantucketand14kmeastoftheevenlargerislandof Withinandaroundtheeasternandwesternplotswecollectedthe
Martha’s Vineyard. The largest mammal is white-tailed deer, and remains ofprey items from 56 webs for diet analysis. These prey
therearenolargescavengersorpredators,suchasraccoons,skunks, remnants consisted of disarticulated sclerotized arthropod parts,
orfoxes. usuallyhangingfromsilkthreadsatthetopofaweb.
In 2006, duringan ongoing spiderspecies survey ofTuckernuck OurresultssuggestthattheS.rufipespopulationonTuckernuckis
Islandthatincludedfivehoursofgroundsearching,weconfirmedthe verylarge.Wecountedatotalof479webs,146inthewestand333in
presenceofS. rufipesintheformofnumerouspursewebsingrassy theeast(Fig. 1).Dividingbythesurveyedarea(2,500m"inthewest
areasoftheisland.Sphodrosrufipeshasbeenknownlocallyformany and 1,850m^intheeast)givesadensityof0.058webs/m^inthewest
yearstooccuronTuckernuck(D.Brownpers.comm.).Weexcavated and 0.18 webs/m^ in the east. We used APACK 2.23 to calculate
specimens(allfemale)intheirwebstoconfirmspeciesidentityasS. aggregationindicesforeachsite(Mladenoff&DeZonia2004).This
rufipesrather than S. niger (Gertsch & Platnick 1980). During the softwareprovidesbothaclass-specificaggregation index(AI)anda
171
172 THEJOURNALOFARACHNOLOGY
pers.comm.)studiedonewithadensityof0.01webs/m^inLouisiana.
Morrow(1986)inKansasandTomChase(pers.comm.)onMartha’s
Vineyard, Massachusetts,studiedpopulationsthatappearedtohave
morethanonehundredwebsinunmeasuredareas.OnTuckernuck,
thedensityattheeasternsite(0.18)ismorethanfourtimesPoteat’s
(1889) population density and 14 times the density Hardy (2003)
describes.OurrandomsearchprotocolshowedthatS.nifipesarenot
usually found alone or in isolated groups. We came across 12
additionalwebsoutsidethestudyplots,locatedinsevengroups(each
groupcontainedbetweenoneandthreewebswithinonem^)spread
across theisland. We usedourrandomsearchingprotocolatthese
sevensitesandlocatedanadditional 17webs.Welocatedadditional
websatfiveofthesevengroups(71%).Oursuccessatfindingmore
websafterlocatingoneweborasmallgroupofwebs,suggeststhatS.
nifipes on Tuckernuck occur in groups ranging from small
aggregationstolargecolonies.
VegetationusedforwebattachmentisunusualonTuckernuck.At
thewesternplot,83percentofthewebswereattachedtonon-woody
objects(predominantlyPennsylvaniasedge,Ccirexpensylvanica),and
16percentwereattachedtoawoodyshrub.Theaverageaerialweb
lengthwithstandarderrorwas 11 ± 0.36cm,butthedistancefrom
thegroundtothetopofanyonewebvariedgreatly(1-15cm).Inthe
eastern plot, 51% ofthewebs wereattached to non-woodyobjects
(again,predominantly C.pensylvanica),and41%wereattachedtoa
woodyshrub(predominantlyGayliissaciahaccata).Oneofthesewebs
wasattachedtoapitchpine(Piunusrigida)(25cmdiameteratbreast
EasternPlot height).Thisisunusual,forpinesarenotmentionedaswebsupports
inanyotherstudy.Another8%wereattachedtootherobjectssuchas
a dead leaf, a dead log, or dead pine needles. The webs were on
average9.9±0.88cmlongandtheheightfromthegroundtothetop,
again,variedgreatly(0.5-15cm).
ThereisonlyonepreviousreportofS. nifipesusinggrassasaweb
support(Muma&Muma1945),andmoststudiesdescribethespiders
usingtrees. Hardy(2003) reported that S. rufipesin his studyarea
used deciduous trees and avoided coniferous trees. Our findings
stronglysupportaviewthat S. nijipeswill usewhateversupportis
available,eventherareconifer. Deciduoustrees(mostlyoaks)exist
on Tuckernuck and form acentrally located forest, but incursory
surveyswedidnotfindanywebsattachedtothesetrees.Largeoaks
werenotpresentinoursurveyplots.Spidersdidusethesmallwoody
shrubGayliissaciahaccata.Coyle&Shear(1981)notedthat5.rufipes
inFloridapreferredsmallertrees(< 10cm)tolargerones.
Wefoundpreyremnantson50%ofwebs(n= 111).Coleopterans
andisopodswerethemostabundantpreyitems,foundon42%and
38%ofthesampled webs, respectively(sampledwebsrefertowebs
that contained prey parts). The most common coleopterans were
Meters Scarabaeidae(43%ofcoleopteranspecimens)andElateridae(17%).
We found several other orders represented on only a few webs,
(wiutpFhpiiegnrurtaehnedIn.e—laorwSeepsarttigdarelaepidhmises,ttrerirebsuaptteictothnievoeslfay5)m..plneifdipweesstweerbnsa(nddotes)asptleortntepdlottso aSinhncedlaurHdyi(nm1ge9n8D1oi)pptlaoendrdaaM((11u.4m8%%)a.of&OwueMrbusd)ma,taaOp(ai1rl9ei4o5sn)ie,msilw(a3hr.o6t%oa)l,tshoAorscaeonleoleafcetCeo(d7y.lp1er%e)&,y
partsfromS. rufipeswebs.
A possible explanation for the high densities we observed on
landscape aggregation index (AR) with values between zero Tuckernuckislowpredationrates.Wedidnotfindevidenceofany
(disaggregated) and one (completely aggregated) (He et al. 2000). predation, and there are no mammal scavengers on the island.
TheAll represents the level ofaggregation forbothquadrats that However,predationonS. rufipeswebshasbeenobservedonBlock
contained websand thosethatdid not. At 1-m resolution, theweb Island,R.I.inlateMarch(E.Edwards,pers.comm.).Edwardsfound
specificAlforthewesternsiteis0.265andtheAIlis0.938.Theweb webspulledupanddugoutoftheground,probablybyring-necked
specificAlfortheeasternsiteis0.294andtheAIlis0.827,alsoat 1- pheasants.Toourknowledge,therearenopheasantsonTuckernuck.
mresolution.Onalandscapelevel,thewebsarefairlyaggregated,but ACKNOWLEDGMENTS
thewebspecificAIssuggestthatwebsarerelativelydispersed.
Toourknowledge,theTuckernuck populationoccursincolonies This project was funded by a grant from the Nantucket
thataredenserthanotherreportedpopulations.Forcomparison,we Biodiversity Initiative, and equipment and logistical support was
compiled web numbers and, when available, the sampled area provided by the Nantucket Maria Mitchell Association. Wewould
reported by other researchers. Poteat (1889) studied a population like to thank the Tuckernuck residents and the Tuckernuck Land
thatcontained0.04webs/m^inNorthCarolina,whileHardy(2003, Trust(TLT)forallowingusaccesstoprivatepropertyandtheTLT
MCKENNA-FOSTERETAL.—DENSEPOPULATIONOFSPHODROSRUFIPES 173
fieldstation.WeareindebtedtothelateR.L. EdwardsandtoE.H. Hardy, L.M. 2003.Trees usedfortubesupport bySphodrosrufipes
Edwardsforpointing ustowardthiswork andcommentingon the (Latreille 1829) (Araneae, Atypidae) in northwestern Louisiana.
manuscript. L.M. Hardy and J. Goyette also provided helpful JournalofArachnology31:437^40.
comments that greatly improved this manuscript. R. Kennedy lent He, H.S., B.E. DeZonia & D.J. Mladenoff. 2000. An aggregation
ushisboattotraveltoTuckernuck. index (AI) to quantify spatial patternsoflandscapes. Landscape
Ecology 15:591-601.
McCook, H.C. 1888. Nesting habits of the American purseweb
LITERATURECITED spider. Proceedings of the Academy of Natural Sciences of
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AmericanMuseumNovitatesNo.2704:1-39. Manuscriptreceived5March2010, revised25October2010.