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2005

Descriptive ecology of a turtle assemblage in an urban landscape Descriptive ecology of a turtle assemblage in an urban landscape

Conner C. A

B A. Douthitt

Travis J. Ryan Butler University, tryan@butler.edu

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Recommended Citation Recommended Citation Conner, C. A.*, B. A. Douthitt*, and T. J. Ryan. 2005. Descriptive ecology of a turtle assemblage in an urban landscape. American Midland Naturalist 153:426-435.

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Am. Mid!. Nat. 1.53:428-435

Notes aiid Discussion

Descriptive Ecology of a Turtle Assemblage in an Urban Landscape

cr.—We studied luixle populations inhabiting a canal and a lake (both mau-tnadc)within a hoavi!y disturbed, urban setting. Six aquatic and semi-aquaiic turtle .species werecollected in both liabitats: spiny softshell turtle (Afiohne apinifera). painted turtle (Chrysimyspkta), coninmn snapping tnil!e {Uielydra strfuniliua), common map turtle {Grnptpmysgungrtiphira). common musk turiJe (Sttmolhentf odimilus) and rod-caied siidcr {Trachemyssrripla). VVIiik' ('-. gpograpliiai WAS the must common species in the canal habit:it, 7! sniplfi WASmost (ommon in ihe lake habitat. We describe patterns of sexual size tlinioiphism and sexrdtio.s for the three most abundant species (fi. gengmphica, T. scripta and S. udoralus). Wediscuss our data in light of problems facing turtle assemblages in urban settings.

iNTRODlKmON

Habitat conversion and degradation is generally recognized as the most pervasive and important ofthe six major threats to biodiversity (other tJireats being invasive species, enviionmenlal poltutioii,disease/parasitism, unsustainable use and global climate change; Gibbons ft uL. 2t)00). The major effectof habitat conversion is the outright loss of critical habitats for essential life functions, including feeding{Vickery el «/., 2001), courting and nesting (Heckert et ai. 2(>03) and hibernation (Ball, 2002). Habitatconversion as the result of increasing urbanization, in particular, affect.s a wide aiTay of organisms, fromlarge carnivores (Reilty et al.. 2003) tn buticrflies (Collingc et al.. 2003) to plants (Frtinsisco-Ortgca et al.,20((0) in terrestrial situations and from salaniatiders (Willsoti and Dorcas, 2003) to fish (Paul and Meyer.20111) to algae (Fore and Gmle, 2002) in aquatic environments.

Tiuile populations have Ijecn significantly impacted by human activity, development andurbanization. Negative effects include (ragincntation of genetic structure (Rubin el riL, 2001).demographic effects (Garber and Burger, 1995; Lindsay and Dorcas, 20tll) and direct mortality {e.f(.,through collision witli autotnobiles, Gibbs and Sliriver, 2002). Nonetlieless. some turtle species may bevery resilient in the face of human activity and continue to exist In highly modified habitats when otheri\itdlife is extirpated (Mitchell, 1988). Data on the specific impacts of human activity on turtlel>opulations and assemblages, and how these effects may be amelionited, provide essential componentsro sound consen-ation practices in human-<iominateii landscapes. The pnr|)osf of the present study is lounderstand the basic ecology of a turtle assemblage living within au urban landscape. These descriptivepopulation and community ecology data can then sei"ve as a bas<'line For more thorough investigationsof the effects of urbanization.

MATEWALS AND METHODS

The Central Canal is a man-made riverine habitat created in the 1830s in Indianapolis, Indiana, the12"' largest city in tin; USA (2000 census population 791,90tVf residents). The remnant of a much laigeruncompleted canal sysietn, the Cetural Cunal originates from the White River and Mows south throughcommercial, residential and recreational areas for 11.2 km. At least a dozen roads cross the cajial.including fom' major thoroughfares and one iniriNtate highuay. At the scjuthem terminus, the canalenters a water treatment facility operated by the Indianapolis Water Company (IWt;). The canal trans-ports approxitnately 70% of the city's annual water use; water level, flow rate, submergent and emergentaquatic vegetation are all controlled in part by the IWC. The canal varies from 15 to 25 m wide and isusually less than 2 m at its deepest points. Shorelines aie practically non-existent in most places, withbanks 1—2 m high on either side. P'ragmented woodloLs border portions of the canal and fallen trees andsnags serve ;is basking sites; however, many of these basking sites are removed on a regular basis..Approximately ft.5 km of the canal (7ft%) is bordered by a greeniv'ay (the Central Canal Towpath)maintained by IndyParks, the ( ity of Indianapolis Deparunent of Parks and Recreation. Most of ourfield work f(»r this sttidy in the cuna! was in this 8..5 km section. In this section, the canal is never morethan 1 km from the White River and is as close a.s 2.'V-4O m at several points.

428

2005 NOTES AND DISCUSSION 429

In addition to the canal, we also studied the turtle community inhabiting a man-made lake owned bythe Indianapolis Museum of .^rt {IMA Lake). The 14.7 ha lake is situated in close proximity to both thecanal (Iti5 m) and the W\mv River {30 m). Relictual woodlots surround about 75% of the lake'sshoreline and the lake is frequently used by recreational fisherman.

We captured 1044 individual turtles a total of 14119 times between April-October 2002 (<0.5% of thecaptures were made during a preliminary ir,^pping period in September-October 2001), Most captureswere made through the use of aquatic hoop traps {7tj..1 cm diameter hoops. 30 X 30 cm coated nylonmesh with a funned at one end and a closed bag at the other) although occasional capnires {<1 %) weremade by hand or with a dip net. Wliile no trapping method is without .species-.specific biases {seeGibbons, 1990a), the use of aquatic traps for turtle population and community studies ha.s gained wideacceptance (Bodie et ni. 2000; Smith and Iverson, 2002; Burv' and Crfrmano, 2003) when limitations areproperly acknowledged (see Results and Discussion, below). In the canal we deployed 6-20 traps spacedapproximately 10(1 m apart; spacing was considerably greater in the lake (>25() ml where trappiiig waslimited to September-Otober 2002. We baited traps with sardines and/or chicken livers {refreshedevery 4-5 d). checked traps daily and changed trap locations weekly in order to maximiw- coverage ofthe canal. Traps were submerged save for ihe top ,'>-20 cm. For each turde we recorded mid-linecarapace length (CL to the nearest mm) using calipers, mass (to the nearest g) using a benchtopelectronic balance, species, sex, location of capture and any notable damage. Each turtle was given anindividual mark by notching the marginal scutes in a unique pattern to allow for future identification(C'«igle. 1939). Turtles were processed and returned to the point of capiure within 24 h.

RESULTS AND DiscusstON

COMMUNtTy COMPOSITON

The same assemblage of six species w-as captured in both the canal and lake habitats: spiny suftshellturtle (Apolnne spinifera). painted turtJe {Onysemys pula). common snapping turtle {Chelydm serpentina),<ommon map turtle (Graptemys geogiapbira). common musk turtle (Stemotherus odoratus) and red-eared.slider {Trarhrmys srripla). However, relative abtindances of tJiese species dilfered significantlv in the twohabitats (Fig. 1). For the purposes of a quantitative comparison, we used data collected only duringSeptember-October 2002, the titne period when both habitats were sampled contemporaneously (TableI). Three species, A. spinifem. C. fnrtn and C. snpentiiuu collectively constiuited <20% of tlie totalcaptures in either habitat. In the Central Canal, C geuginphiia was most abundant with T. scriptarepresenling <I0% of all captures, whereas in IM. l.ake T. Am/*/ alone accounted for more (j5% of thecaptures and G. gengrapkirn represented <5%.

Differences in composition may be due in pan tn the unequal distiibution of aquatic mollusksbetween tliese habitaLs. Freshwater mollusks are the primar>' prey for Grnptemysgeographica (Gordon andMacCulloch, 1980; Vogt, 1981; White and Moll, 1992). The Central Ganal supports several species offreshwater snails (eg'., members of the genera Pipw/ncCT-fl, Goniohasis-and Vivapanis) and are found at highdensities locally; similar sampling efforts in IMA Lake have failed to detect the presence of aquaticsnails. The lake, thus, represents sui>optimal habitat for G. gengrapfuca due to a lack of preferred food.In contrast, 7 srripta is more omnivorous and opportunistic in feeding (Ernst et al., 1994) and is themost abundant species in the lake. It may be that the lack of G. gengraphicu in IMA Ijke due to theabsence of suitable food has allowed the Treirhnnys scriptn population lo grow tnore successftilly than inthe habitat where G. gpographirrj is abundant. The extent to which these two species interactcompetitively, however, is unclear and the differences in distribution may simply reflect microhabitatpreferences (icc Ernst etai, 1994). The relative consistency of the rest of the species abundances speaksto the overall similarity of the two sites despite the differences inherent in lentic and lotic habitats.

The rarit>- of Chri.semys piita in Ixith habitats is unexpected, as it is one i>f the most common andabundant species throughout its lange and particularly in the Midwest (Anderson et ai, 2002; Bury and(iennano, 2003) and Southeast (Litidsayanci Dorcas. 2001). Moreover, it can be vei-y abtmdant in urbanhabitats; Mitchell {1988) estimated more than 500 individuals in a C/«r/« population inhabiting a smallcreek {6 m wide) and two small associated beaver ponds in urban Richmond, Virginia. We speculate thatthe Wliite River is tlie source of the turtle populations inhabiting the Central Canal and IMA Lake.

430 THE AMERICAN MIDLAND NATURALIST IS 3(2)

DCental Canal (n=873)

I IMA Lake (n=171)

0 0.2 0.4 0.6

Proportion of capturesF[(.. I,—Tbe proporiion of six aquatic atid st-ini-aquaLic luitle species captured in a man-made caiial

(Oniral Canal) aiitt lake (IMA Lake} within an urban landscape in Indianapolis. Indiana, USA. Thettual number of individuals collected between is indicaicd in the figure legend (see text for details).Abbreviations are as follows: \!i — Apohne spinifera. Cp - Chrysemys picta, Cs = {Chelydra serpmtina), Gg =Graptemys ^ographica. So - Stenuitherus odoratus, Ts = Tracliemys scripta

2005 NOTES AND DisajssioN 431

1.—Number and proportion of six aquatic and semi-aqtiaiic turtle species collected inSeptember-October 2002. B«-cause tbe sample size was low for Apolonf \pinifrra, Chelydra serpmtina andCJirysems picta in botli habitats, the goodnessof-Ht-tesi was conducted usinR only the ibree moreCO nun on species

Ap<»lo»f spiritjna

Chehdra snprntuia

Chrysrmys pic I a

Crraplfmys grogt'aphifa

Sttmothenis odtiraliuT^arhrmys sciipta

TOTAL

Otiiral (

nrnnbrr

1185

50436

126

.nal

proportion

0.0320.1430.0400.3970.3410.048

niimhci

3447

32102

152

IM.A hik<-

priipi-niiiiii

0.0200.0260.0260.0460.2110.671

tkHsdnessol-fit-tesi: %- = 115.8. df = 2, P < O.OOOl

Althougb C. picta is frequently abundant in punds and lake.*, it is notably less toinmon in rivers (Emst ftal., 1994) and. tliiw, the current low density may reflect a histonral low density in this region, It Ls woribnoting also that tlicst- populations may sufTcr from very low recniitnieni rales, as all 19 indi\iduak wecaptured in Ixjlh habitats were mature adults, whereas we have collected or observed batchlings for ibeother live .sjiecics.

SEXUAL SIZF. DIMORPHISM

We found significant sexual si/e dimoqihism for lhe three tnost frequently captured species, withfemales significantly larger iban ibe males in Iwth CI, and mass in each case (Table 2). Sexual si/edimorphism, with females lai-ger tban males, is tbe norm for most emydid turtles (Emst ft aL. 1994) andhas been dotumentcd in populations of Grfiplemys grographica and Trarhnnys scriptn throughout theirranges [e.g., Ciigle, 11)50; Vogt, 19K(); Ciibbons and Lovicb. 1990). as well as in (entral Indiana (Minton,2001). Sexual size dimorpbi.sm, however, is less cotnmon in kinosterid turtles, paniculai'ly in SUmollieriu

Tible 2.—Sex ratios and bod>' sizes of C grngraphica, S. odorauts. and 7 scriptn, the major species of tbefk-ntral Canal (ranal) and lM.-\ Lake (lake) turtle assemblages. For each population we report tbenumber, and mean (and St.) cuitpiice length {V.i.) and mass of eacb sex; we tised Chi-square goodness-of-fit tesLs tu detect skewed sex nuios and one-way analysis of variance on log-traiisformed CL and massto detect sexual size dimorphism

Nuiiiljer

(;. grtigrapbiea (canal)

fctiiale ni;ili"Ufi 15!)

X" = 0.554

P = 0.457

S. «Mi

tein;.l<.-122

i

aulf (ranal)

malt'170

' - 7.89= 0.005

T. SI

45

)/

•nptri (t.iiinll

k- mak-.t6

' = 0 . 3 1 7

y: »Tipi,i (lake)

It-nuilf aiiili-5(1 4H

X' = 0.041

/ ' = 0.840

CL (mm)

Mass (g)

168,7

(4..')4)

P<

800.6

(51.9)

f^ l.MU

P<

102.7

(1.03)

= 209.7

0.0001

137.3

(.S,45)

= 235.2

O.(K)O1

107.1

(0.81)

P<

212.4

(4.34)

P<

98.6

(0.82)

0 = 46.9

0.0001

157.7

(3.36)

„ = 76.1O.O(K)1

I7.'i.l

(6.98)

P =

988.0

(130.0)

f'' 1,7«

P =

] 51.0(•1.90)

; = 5.040.028

542,0(46,6)

, = 9.820.002

178.2(6.36)

P =

972.8(78.0)

;^<

148.6(4.61)

s = n.o0.001

522.3(44.4)

fi = 14.40.0001

432 THE AMERICAN MIDLAND NATCIRAUST

(Gibbons and Lovich. 1990; Ernsi etai.. 1994). Lovich and (libbons (1992) promoted a straightforwardindex lor quanlitying these differences, the sexual dimorphism index (SDl):

SDI = (mean length of larger sex/mean length of smaller sex)+ 1 when males > females

or

SDI = (mean length of larger sex/mean length of smaller sex)- 1 when females > males

with the value of size raiio being positive when females are the larger sex and negaiive when males arethe larger sex. The SDI in 14 populations (if SuniQl/urus otloratus ranged between -0.068 and 0.127, withA mean absolute vuliie of 0.039 (Gibbons and LoWch, 1990). In oiher words, on average there is less thana 4% difference in body size between ihe sexes of S. odoraius in most population.s. In our study, the SDIrdlue is 0.086. more ihan twice ihe mean difference. The CL ot \, oilimitus adtiks in our population is(•(impanihle to other populations in natural habiiaLs throughoui its range {see Gibbons and Lovich,1990). Tinkle (i9(il) found a comparable degree of sexual si/e dimorphism in S. odoratiLs. but only inextreme southern populations. The absence of" sexual size dimorjihisni in Slej-nothinns is generallyaiiribuied to the early age of maturation both male.s (,V4 y) and females (4-8 y) (Mahmoud, 1969;Mitchell. 1988). The significance of sexual dimorphism in our population of .S. odoratus is at this pointunclear and in need of further study.

The SDI values for Graplemys geofrraphica (0.64S) and Trachems scripta (0.160 and 0.199 for the canaland lake habiuiLs. respeciively) in our study are comparable to, but generally less than, values reportedtor these species elsewhere throughout their range (G. ffOf^aphiai mean SDI - 0,810 for threepopulatiotis: T. scripta mean SDI =0.251 for five populations in the United States; Ciibbons and Lovich.1990). The smaller SDI may he due to smaller adult female hody si^e. In the populations summaiized byGihbons and L(3vich (1990). adult females are ahout 17% larger than (he females iu our population(mean CL = 196.8. .st. - 11.7; compare with Table 2), whereas the difference between maJes in our aiidother populations is 8.5%, half the difference (mean CL = 112.3. SE = 3.7; compare with Table 2).

SEX RATIOS

We observed parity in sex ratios for Graplemys geographira and for both the canal and lake populationsof Tmcbemys saipln (Tahle 2). Male-bias is the norm in South Carolina 7; scripta populations studied byGibbons and Lovirh (1990), but not in Midwestern populations (Bodie and Semlitsch. 2000: Andersonel al., 2002). Likewist;. sex ratios are variable among populations of C. ^graphica. For example,a riverine population in Pennsylvania had a non-significant male bias (maleifemale ratio — 1:0.82; Philoand Bellis. 1986). hut other population are notably male-biased {1:0.59 in Quebec. Gordon ajidMacCollouch 1980; 1:0.5.3 in Wisconsin, Vogt, 1980).

In our siudy, only the Slrmolhmu, wloratm population inhabiting the Ont ra l Canal exhibitsa significant bias, with a male:female ratio of 1:0.67. A significant female hias (Dodd. 1989). male bias(Edmondand Brooks, 1996; Smith and Ivei-son, 2002) and equal sex ratios (Bancroft el al., 1983; Enist.1986; Mitchell. 1988). have been documeiued in other populations of S. tMliiratm. Smith and Iverson(2002) advanced several potential explanations for a consistent (>20 y) male-bias in a north-centralIndiana population of .V. odoraius, including differential mortality, higher rates of activity among males,differential hahitat use. temperature-dependent sex detemiination and sampling techuique-bias. Eachof these explanation are plausible in our population, however, we ciu-rently lack the data to test thesehypotheses.

CONSERVATION IMPLICATIONS

The Central Canal-IMA Lake habitats support a robust turtle assemblage, despite the challenges ofnesting in an urban landscape and the heightened risk of collision with motor vehicles during terrestrialmovement. Recruitment is a critical for the persistence of populations, especially for long-lived specieswith a relatively old age at first reproduction (C«ngdon el al., 1993). Access to suitable nesting sites is

2005 N O T E S AND DISCUSSION 4SS

a critical precursor lo successful recruitmeni and Bnding such sites is potentially difficult in highlyurbanized landscapes. For example, the Central Canal is suiTounded predominantly by impervioussurfaces (roads and parking lots in the commercial districLs), scattered woodloLs and residential lawns.Of ihese, the latter is ihc most likely to be used by nesting females because of reduced cover andrelatively uncompacted soils. We have anecdotal data on the ase of lawns as nesting site.s in some areasaround the canal (unpubl. daia). A further complicatiim is that nesting females and hatchlingsfrequently have to cross roads when moving to and from tbe canal; this is a definite and persistent threat((iibbs and Shriver, 2002) thai mav result in male-biased populations (.Sieen and (Jibbs, 2004). Forexample, in May 2002, we collected the carca.sse.s of five Graptnnys ge.ographita nestlings on a majorthoroughfare, approximately 35 m from the Central Canal. In addition, we have noted shell damageconsistent with automobile collision and have collected roadkill G. geographica females duringjtme andJuly, the height of nesting sea.son. We have docunienied individuals moving between aquatic habitai.s(including the White River) ihrough mark-recapture and radiotelemetry studies (Ryan et aL. in re\'iew).Risk of collision wilh motor vehicles is likely less between the two aquatic habitats as few roads (if any)usually need to be crossed. Death associated with terrestrial mnvt-ment liktrly represents one of tbegreatest sources of mortality in our commuaiiy. Removal (il individuals by fishemian and turtle"enthusiasts" is also a threat.

The Central C^nal is more than 100 y old, however the histor>- of the turtle community inhabiting iiand surrounding man-made aquaric habitats (e.g., IMA Lake) is unclear. Minton (2001) observed "largenumbers" of Graptemys geogtaphica in the canal in the 1950s. He described a general decline in theabundance of G. gEographica-^nA other specie.s (Minton, 1968) between the 1950s and the 1990s (Minion.2001). Unfortunattly, he did not conduct regular sampling to detect population trends with anyconfidence. We have initiated a long-tenn mark-recapture study (with associated studies on movenienLsand habitat use) to monitor turtle population trt-nds in an area of considerable human activity. Thefiirreni data set. thus, serves not only as a description of the ecology of turtles in ati urban setting, but alsoas a baseline for long-term monitoring efforts. In the future, we will use recapture data to devt-Iopestimates of population sizes. Although tliero are a few notable long-term studies turtle communityecology in natural environments [ .g., at the Savannah Ri%'erSite in South Carolina (Gibbons, 1990b) andthe E. S. George Reserve in Michigan (C^ngdon and Gibbons. 1996)], sucb efforts are also needed toensure the persistence of wildlife in highly modified urban habitats.

AcknowUd^nents.—We would like to thank the Biitlei University Ecology classes (2001 and 2002),B. Bastain and .^. l^mbert for help in ihe field, the Indianapolis Water Company and IndianapolisMuseum of Arts for pennission ro collect turtles on their properties. Mike Dorcas and Kiistiuf Graysonand two anonymous reviewers made ihonghtliil suggestions that improved tbe quality tif themantiscript. Our research was supported by a grant from the Butier University Holcomb AwardsCommittee and is A publication of the Urban Turtle Ecology Research Project (U-TERP) and the Centerfor Urban Ecology at Butler University.

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CHRISTOPHER A. CONNER,' BROOKE A. DOUTHnT^ AND TR/ \V1S J. RYAN,' Departmentof Biological Science, Butler University, Indianapolis, Indiana 46208. Submitted 19July 2003; accrfited 28July 2004.

Current address: Di\ision of Biological Sciences, University of Missouri, Columbia, Missouri^ Current address: School of E^nvironmental and Public Afiairs, University of Indiana, Bloomington,

Indiana 47403^ Corresponing author: Telephone: (317) 940-9977; e-mail: tryan@butler.edu