A Transportation Model Assessment of the Risk to Native Mussel Communities from Zebra Mussel Spread

788

Conservation Biology, Pages 788–800Volume 12, No. 4, August 1998

A Transportation Model Assessment of the Risk to Native Mussel Communities from Zebra Mussel Spread

DANIEL W. SCHNEIDER,*†§ CHRISTOPHER D. ELLIS,* AND KEVIN S. CUMMINGS†

*Department of Urban and Regional Planning, University of Illinois at Urbana-Champaign, 111 Temple Buell Hall, 611 E. Lorado Taft Drive, Champaign, IL 61820, U.S.A.†Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, IL 61820, U.S.A.

Abstract:

When introduced species threaten native flora and fauna, protection requires an analysis of therisk that native species face from the spread of the introduced species. Models of invading species, however, of-ten do not include a dynamic component of risk. North American freshwater mussels are at risk of fouling bythe introduced zebra mussel (

Dreissena polymorpha

). Predictions of the risk to native mussel communities ofinvasion by the zebra mussel can help prioritize conservation efforts. We present a model for the spatial anal-ysis of zebra mussel spread that allows the characterization of relative risk of infestation to the high-qualitymussel communities of Illinois streams. A gravity model, constrained at origin and destination, was parame-terized with data on boat use at 120 boat landings in Illinois. The risk of spread of zebra mussels depends onthe number of boat trips from infested waters, which in turn depends on distance from an infested water,boat use at a site, and the position of a lake within a river system. Habitats supporting a high diversity of na-tive mussels and threatened and endangered species are at risk of infestation from reservoirs upstream. Inva-sion of inland lakes and reservoirs in Illinois is predicted to occur first at areas of high boat use close to cur-rently infested waters, including the Fox Chain of Lakes, followed by central Illinois reservoirs. Thesereservoirs will act as stepping stones, facilitating the invasion of lakes upstream of critical native mussel hab-itats. Efforts to protect waters that center on prevention of the initial invasion of stepping-stone lakes couldsignificantly reduce the risk of infestation. Education and inspection efforts are predicted to be more effectivethan quarantine because quarantines would displace boat traffic to critical habitats. The class of models pre-sented here is useful for predicting risk of invasion when complete data on vector movement are not available.

Evaluación de Riesgo de un Modelo de Transportación para Comunidades Nativas de Mejillones Debido a laDispersión del Mejillon Zebra

Resumen:

Cuando las especies introducidas amenazan la flora y fauna nativa, la protecci

ó

n requiere de unanálisis del riesgo que las especies nativas enfrentan debido a la disperción de las especies introducidas. Sinembargo, los modelos de invasi

ó

n de especies frecuentemente no incluyen un componente dinámico deriesgo. Los mejillones de Norteamérica se encuentran en riesgo de ser entampados por el mejill

ó

n zebra intro-ducido (

Dreissena polymorpha

). Las predicciones de riesgo para las comunidades nativas de mejillones po-drían ayudar a priorizar los efuerzos de conservación. Presentamos un modelo de análisis espacial de la dis-persión del mejillón zebra que permite la caracterización del riesgo relativo de infestación de comunidadesde mejillones de alta calidad de los arrollos de Illinois. Un modelo de gravedad, forzado en el origen y des-tino, se parametrizó

con datos de uso de botes en 120 arribos en Illinois. El riesgo de dispersión del mejillónzebra depende del número de viajes de boes a zonas infestadas, que a su vez depende de la distancia a la quese encuentran las aguas infestadas, el uso del bote en el sitio y la posición del lago dentro del sistema riverino.Los hábitats que soportan una gran diversidad de mejillones nativos y de especies amenazadas o en peligrose encuentran en riesgo de infestación por los reservorios aguas arriba. Se predice que la invasión de lagos yreservorios tierra adentro en Illinois, ocurriría primero en áreas con un alto uso de botes, cercanas a las zo-nas actualmente infestadas incluyendo la cadena de Lagos Fox, seguido de los reservorios del centro de Illi-

§

email [email protected] submitted January 24, 1997; revised manuscript accepted August 12, 1997.

Conservation BiologyVolume 12, No. 4, August 1998

Schneider et al. Risk Assessment of Zebra Mussel Spread

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nois. Estos reservorios actuarían como trampolines, facilitando la invasión de lagos rio arriba, con hábitatcrítico para especies de mejillones nativos. Los esfuerzos para proteger las aguas que se centren en la preven-ción de la invasión inicial de lagos-trampolín podría reducier significativamente el riesgo de infestación. Sepredice que la educación e inspección de esfuerzos serían mas efectivos que las cuarentenas, puesto que lascuarentenas podrían desplazar el tráfico de botes hacia hábitats críticos. La clase de modelos presentados eneste trabajo son de gran uso para la predicción de riesgos de invasión cuando los datos de vector de mov-

imiento no se encuentran a la mano.

Introduction

Introduced species can have profound effects on nativeflora and fauna, often leading to the extinction of nativespecies. For instance, the brown tree snake (

Boiga ir-regularis

), introduced to Guam from New Guinea, hascaused the extinction of 9 of the 11 forest bird specieson Guam (Gillis 1992). Spread of the cane toad (

Bufomarinus

), introduced into Australia for pest control,threatens a number of native species (Sutherst et al.1996). In freshwater habitats the zebra mussel (

Dreis-sena polymorpha

) threatens native unionid musselsthroughout eastern North America. Protection of nativespecies requires an evaluation of the risk native habitatsface from introduced species. Many models of the even-tual spread of invasive species rely on an evaluation ofclimatic characteristics and the requirements of the or-ganism (e.g., Daehler & Strong 1996; Sutherst et al.1996). Although these types of models may describe thefundamental niche of the organism and its potential dis-tribution, they do not take into account the dynamics ofthe invasion. Many suitable habitats may be too isolatedto be at substantial risk, and resources devoted to pro-tecting these sites might be better used elsewhere. Inplanning for invading species or trying to protect criticalhabitats, it is crucial to predict which habitats are atmost immediate risk of invasion (Carey 1996).

To evaluate risk, dynamic models of invasion areneeded that take into account the behavior of the vec-tors transporting the non-indigenous species. Methodsfor assessing the probability of species introductions,the rate or direction of range expansion, or the risks spe-cies pose once introduced are not well developed(Townsend & Winterbourn 1992; Suter 1993; Ruesink etal. 1995; Kareiva 1996). Most models of the range ex-pansion of introduced species incorporate mathematicaldescriptions of diffusion. These models, however, donot adequately address long-distance dispersal and dis-persal in heterogenous environments (Hastings 1996).The spread of species like the brown tree snake or zebramussel involves transport by human vectors between of-ten distant habitat islands. Diffusive spread may not bethe appropriate model for describing this type of rangeexpansion, particularly in the early stages of invasion(Hastings 1996). Instead, range expansion can be mod-eled by explicitly modeling the behavior and movement

of the human vectors (boating, shipping, air travel) forwhich a wide variety of transportation models havebeen developed (Thomas & Huggett 1980). We presenta model of zebra mussel risk to native unionids based ona common transportation model, and we demonstratehow the model can influence conservation plans for theprotection of threatened species.

Freshwater mussels are at greater risk of extinctionthan many terrestrial species (Williams et al. 1993). Al-most 72% of the mussels in the families Margaritiferidaeand Unionidae are at risk in North America (Williams etal. 1993). Over the last two centuries, mussel popula-tions have been decimated by human activities. Many re-maining species are threatened by siltation, channelmodification, pollution, overharvesting, and dam con-struction. One of the most serious recent threats is theinvasion of the exotic zebra mussel. This introducedmussel attaches to native mussel shells and interferes withnormal movement, feeding, physiology, and growth(Haag et al. 1993; Baker & Hornbach 1997). EasternNorth America, the area with the highest zebra musselconcentrations, is also the region of greatest freshwatermussel diversity. Since the invasion of the zebra mussel,unionids have declined drastically in Lake St. Clair andLake Erie (Gillis & Mackie 1994; Nalepa 1994; Schloesser& Nalepa 1994), the St. Lawrence River (Ricciardi et al.1996), and the Hudson River (Strayer & Smith 1996), al-though native mussels may be able to survive in backwa-ter lakes and wetlands (Tucker & Atwood 1995).

Because of the expansion of the range of the zebramussel, it is important to identify populations of nativemussels at greatest risk. Boater education efforts andboat and trailer inspections can prevent transport of ze-bra mussels to those waters with populations at risk.Populations could also be prioritized for protection inholding basins protected from zebra mussels. If zebra mus-sel populations declined, the protected adults could be re-turned to the wild. Holding basins would be expensiveand could cause high mortality of native mussels (Cope &Waller 1995), so it would be appropriate for only thosepopulations at grave risk from the zebra mussel to be re-moved from their habitat and held in quarantine.

Zebra mussels may move between habitats via a num-ber of vectors (Carlton 1993), but the most likely meansappears to be through the activities of recreational boat-ers and fishermen (Johnson & Padilla 1996). Boater ac-

790

Risk Assessment of Zebra Mussel Spread Schneider et al.


tivity has been implicated in most colonizations of in-land lakes from the infested waters of the Great Lakes(Johnson & Carlton 1996; Padilla et al. 1996). Thus, anevaluation of risk of invasion should be based on an anal-ysis of patterns of boater movements. Although a num-ber of predictions concerning the eventual distributionof zebra mussels have been made, they were based onthe suitability of habitat for zebra mussels in terms oftheir physiological requirements (Strayer 1991; Ramcha-ran et al. 1992; Schneider 1992; Koutnik & Padilla 1994;Whittier et al. 1995) and lacked an evaluation of the rela-tive risk of actual invasion. Neary and Leach (1992)based a model of relative risk on distance from roads,with the assumption that lakes near roads would receivemore traffic that could potentially transport mussels. Pa-dilla et al. (1996) were the first to explicitly model thespread of zebra mussels based on data on boat traffic andthe behavior of boat users. Their study was conductedin Wisconsin, where overland transport represents thedominant mode of spread. We expand on their ap-proach in an environment where both overland trans-port between watersheds and downstream movementof veligers within a watershed are important. In Illinoismost lakes used for recreational boating and fishing areimpoundments and have connections to downstreamrivers and lakes. Once zebra mussels have establishedpopulations in a lake or reservoir, the transport of ve-ligers could rapidly establish new populations down-stream. We used data from creel surveys of boat use ofIllinois waters to estimate the potential boat traffic be-tween infested and uninfested waters. Studies of the des-iccation tolerance of adults suggest that in summer con-ditions, zebra mussels can survive 5 days attached to aboat hull or trailer (Ricciardi et al. 1995) making all ofthe inland lakes in Illinois potentially colonizable fromthe infested waters of Lake Michigan and the Illinois,Mississippi, and Ohio rivers.

Illinois historically supported 80 species of unionidmussels (Cummings & Mayer 1997). Six species arethought to be extinct, 11 have been extirpated from Illi-nois, 5 are federally endangered but still extant in thestate, 7 are former federal candidate species, 8 are stateendangered, 4 are state threatened, and 9 are state spe-cies of special concern. Of the remaining 30 species, 3are considered uncommon or limited in distribution,leaving only 27 species that currently have relatively sta-ble populations (Cummings & Mayer 1997). Overlandtransport followed by the downstream expansion of ze-bra mussels represent a serious threat to native unionidpopulations. It is important to assess the risk to the na-tive fauna posed by zebra mussels prior to infestation. Itis difficult to detect changes of less than an order ofmagnitude in populations of unionids because of theiraggregated distribution (Downing & Downing 1992). Bythe time mussel populations experience a measurabledecline, it might be too late to institute conservation

measures. To determine the relative risk faced by differ-ent native mussel populations in Illinois rivers, we over-lay the model of risk of infestation with detailed geo-graphic data on native unionid distributions. Efforts toprevent the introduction of zebra mussels range fromgeneral boater awareness campaigns to mandatory boatinspections, boat cleaning, or even quarantines in whichboats are forbidden access to key habitats. Our modelcan help evaluate the potential effectiveness of these dif-ferent management alternatives and direct protection ef-forts to the appropriate locations.

Methods

Modeling Spread

We assume that the most likely vector for zebra musselspread is transport by boats and trailers, and that thelikelihood of a zebra mussel being transported from aninfested water to an uninfested water will depend onthe number of boats moving between sites and the dis-tance between them. Boat use data were taken fromcreel surveys conducted at 13 boat landings on LakeMichigan and 55 inland Illinois lakes by the Illinois Natu-ral History Survey (Fig. 1). For boat access sites forwhich no creel information was available (access siteson the Illinois, Mississippi, and Ohio Rivers; 43% of allsites), boat use was estimated by contacting the officialresponsible for particular boat access sites. All data wereconverted to number of boats per year using an accesssite (data available from the author). Boat use at inlandlakes varied from 192 boats

/

year at Sterling Lake to29,524 boats

/

year at Fox Chain of Lakes and 29,977boats

/

year at Clinton Lake. Distances between lakeswere calculated by means of a geographic informationsystem in which the locations of all boat ramps werecoded as points.

We used a production-attraction constrained transpor-tation model for the spatial analysis of zebra musselspread (Thomas & Huggett 1980; Sen & Smith 1995). Inthis model the likelihood of boat movement, and thuszebra mussel transport, between two sites is related tothe boat use at each of the two sites and the distance be-tween them as

where

T

ij

is the relative flow of boats between originand destination,

A

i

and

B

j

are scaling constants ensuringthat all boats arriving at a point also leave that point (be-cause there is assumed to be no storage of boats at anysite),

O

i

is the total number of boats leaving origin

O

i

,

D

j

is the total number of boats arriving at destination

D

j

,

c

ij

is the distance between origin

O

i

and destination

D

j

, and

a

is a power function related to the relative likelihood of

Tij AiOi Bj Dj( )/ cij( )α= ,



791

travel over short and long distances. This model is thusequivalent in form to Newton’s model of gravitational at-traction, in which the force of attraction between twobodies is proportional to the product of their masses di-vided by the distance squared. In this model, as the boatuse at a particular water body increases, its force of at-traction to other boats also increases. As the distance be-tween the water bodies increases, the force of attractiondecreases. Gravity models have been widely used inmodeling recreational behavior (Baxter & Ewing 1981;Haynes & Fotheringham 1984), including reservoir use(Rosenthal 1987). In addition, analysis of boater behav-ior supports the assumptions of this model. Perdue(1981) has shown that lake attractiveness is correlatedwith boat use. Further, crowding at Illinois reservoirshas not been shown to deter use or reduce attractive-ness (Gramann 1980).

The parameter

a

describes the effect of distance onforce of attraction for boats moving between lakes.Higher values of the power function suggest that boatersare unlikely to travel between lakes that are widely sepa-rated, whereas lower values suggest that distance doesnot greatly affect the likelihood of travel. At

a

5

1, aboater is 5 times more likely to visit a lake 20 miles awaythan one 100 miles away, whereas at an

a

5

5, theboater is 625 times as likely to visit the nearer lake. Theavailable data suggest that movement of boats betweennearby lakes (and thus a high value of

a

) is much morelikely. Studies of boater behavior in Wisconsin suggestthat boaters are much more likely to move betweenlakes that are within the same county (Padilla et al.1996). Studies within a single region, east central Illi-nois, suggest that distance from a lake within the regionis not a factor in predicting use of a lake by either fishers

Figure 1. Location of lakes and boat ramps used in gravity model. Shaded streams are biologically sig-nificant mussel streams in Illinois that have recent records of state or federal threatened or endangered species or high mussel diversity (Page et al. 1992). Rivers in bold show the current distribution of ze-bra mussels in Illinois streams.

792

Risk Assessment of Zebra Mussel Spread Schneider et al.


or boaters (Gahan 1970). For these reasons we feel justi-fied in using a value of 4 for this parameter.

The resulting matrix

T

ij

gives an estimate of the num-ber of boats traveling between all pairwise combinationsof origins

O

i

and destinations

D

j

. To convert these esti-mates of boat movement to the risk of zebra musselspread to a destination

D

j

, each row (

I

) value of

T

ij

wasmultiplied by a probability

P

i

that a boat leaving an ori-gin

O

i

could be carrying zebra mussels with it to destina-tion

D

j

. If an origin

O

i

is not currently infested, the prob-ability

P

i

would be zero. If the lake is infested, theprobability that a boat leaving the lake would carry a ze-bra mussel was set at 0.01. This value appears reason-able, given the available data on the transport of zebramussels by recreational boaters. Johnson and Carlton(1996) found that 0–31% of trailers in Michigan carriedmacrophytes with attached zebra mussels and 83% oflive wells contained veligers, whereas 0% of 1800 boatshad adults attached to their hulls. All boat landings onLake Michigan and on the Mississippi, Illinois, and OhioRivers were considered infested. Once a boat from an in-fested water arrives at an uninfested lake, there is someprobability that the transported zebra mussel will estab-lish a population. In this model the probability of estab-lishing a population is proportional to the total numberof infested boats arriving at a lake. The columns,

j

, of

P

i

T

ij

were added to obtain a measure of the number ofboats that could potentially infest any destination

D

j

,representing its risk of infestation. We have set at 2000the threshold number of intersite trips of infested boatsthat will result in infestation. Because the combinedprobability that a boat could transport zebra mussels andthat transported zebra mussels could establish a popula-tion in the new lake is unknown, this model evaluatesonly the relative risk of infestation among Illinois lakesand rivers, rather than the absolute risk.

Once a mussel population is established in a reservoiror lake, veligers can infest downstream sites even in theabsence of transfer by boats. The parameter

d

describesthe distance downstream per year that the populationwill move. We based the downstream distance on ve-liger growth rates (Stoeckel et al. 1997) and time oftravel data for Illinois rivers (Stall & Hiestand 1969). Ve-ligers require between 9 and 18 days to grow to settle-ment size in the Illinois River. Time of travel data sug-gest that veligers could move approximately 160 milesin most Illinois rivers. Horvath et al. (1996) suggest amuch slower downstream movement of zebra musselpopulations, so the value of 160 miles likely represents amaximum mean annual distance. The sensitivity of themodel to the value of

d

is evaluated below. Downstreammovements and distances were calculated by means ofthe Illinois Streams Information System (ISIS), a geo-graphic information system in which stream segmentsare encoded, allowing dynamic segmentation (Morin &Szafoni 1993).

Risk to Native Unionids

We assessed the risk of zebra mussel infestation to na-tive unionids by overlaying the predicted risk of zebramussel spread on maps of streams considered biologi-cally significant in terms of their mussel fauna (Fig. 1).These were defined as streams with state or federalthreatened or endangered species, more than 10 speciesof live mussels, or a diversity (Shannon’s

H

9

) greaterthan 2.5 (Page et al. 1992). The list of biologically signifi-cant mussel streams is conservative. It includes onlystreams with recent records (after 1980) of endangeredor threatened species or recent surveys indicating highdiversity.

Sensitivity Analysis

Sensitivity analyses were performed on three parameterswhose true values are unknown:

a

, the measure of theeffect of distance on the likelihood of travel betweensites;

t

, the threshold value of the risk function at whicha lake was considered infested; and

d

, the flow rate atwhich zebra mussels moved downstream following in-festation of an upstream lake. Varying these parametersalso allowed us to examine the sensitivity of the modelto the relative importance of the overland (

a

) versusdownstream (

d

) spread of zebra mussels. The model wasrun for varying values of these parameters (Table 1), andthe rank order in which bodies of water became infestedwas recorded. Sensitivity of rank to extreme values foreach parameter was tested by means of Kendall rankcorrelation. Significant correlation among the rank orderof lake infestation would indicate low sensitivity tochanges in that parameter. We evaluated the sensitivityof the model to altering all of the parameters simulta-neously by comparing eight runs of the model under ev-ery combination of extreme low and high values of eachof the three parameters. These ranks were compared bymeans of Kendall’s coefficient of concordance,

W

(Sokal& Rohlf 1981).

W

varies from zero, no concordance, toone, perfect concordance; it is tested by means of the

x

2

distribution. If

W

is significant, it indicates concordanceamong the ranks and thus low sensitivity of the model tothese parameters.

Table 1. Parameter values and ranges for model of zebra mussel spread.

ParameterValue usedin model

Range of valuesused in sensitivity

analysis Sensitivity

*

a

4 1, 5 12.40

p

0.01 0.01 —

t

2000 500, 2000 0.83

d

160 5, 160 0.99

*

Mean difference among lakes in rank order of infestation betweenvalues used in sensitivity analysis.



793

To examine the sensitivity of the rank order of individ-ual lakes rather than the overall pattern of risk, we com-pared the variability in rank of each lake under the eightextreme combinations of parameter values with the vari-ability expected if the rankings were random. Low vari-ability in rank would indicate low sensitivity to the pa-rameters used. We generated a random distribution bycalculating the standard deviation of eight rankingsbased on 1000 random combinations of possible rank-ings of 55 lakes. The standard deviation of the actualrankings for each lake was compared to this distribu-tion. Any lake whose standard deviation of rank wasgreater than the lowest 5% of the random rankings wasconsidered sensitive to the parameters used.

Results

Model Sensitivity

Overall, the model was not sensitive to changes in

a

,

t

,or

d

. The coefficient of concordance (

W

) was 0.765(

p

,

0.001) and indicated significant concordanceamong all runs of the model. Similarly, the rank order oflake infestation was similar for varying values of each pa-rameter analyzed separately. Rank order was signifi-cantly correlated (

p

,

0.000) for all 27 pair-wise com-parisons of extreme values of each parameter. Ingeneral, concordance was highest for the parameters re-lated to flow and threshold for infestation; it was lowerfor the power function, or force of attraction amonglakes. On average, changing

a

from 1 to 5 caused an av-erage change of 12.4 among all lakes in predicted rankorder of infestation. Extreme values of the other parame-ters produced an average change in rank of less than 1(Table 1).

The variability in rank of 44 lakes was lower thanwould be expected by chance (critical value of SD

5

10.5,

p

,

0.05), suggesting that the relative susceptibil-ity of these lakes to zebra mussel infestation was not sen-sitive to the values of the parameters used. Insensitivelakes clustered around both high and low risk. The re-maining 11 lakes, many of which had intermediate riskin all analyses, appeared to be more sensitive (Table 2).The sensitivity analyses thus suggest that overall pat-terns of risk were robust to changes in the key parame-ters. Individual lakes, however, could show a substantialchange in risk given different values of the parameters,with

a

being most important.

Zebra Mussel Spread

Infestation is expected to occur first in the heavily popu-lated northern portion of the state, followed by centralIllinois. The first lakes predicted to be infested are high-usage lakes relatively close to infested waters and in ar-

eas of high population density (Table 2). These lakes in-clude the Fox Chain of Lakes and Busse Lake, which arein the Chicago metropolitan area and close to the in-fested waters of Lake Michigan. Rivers downstream fromthese waters are predicted to be infested soon after theinitial colonization of the lakes. Shabbona Lake, LakeCarlton, and Argyle Lakes are predicted to be infestedrelatively early as well, although this prediction is moresensitive to the value of a used in the model. Followingthese lakes, the next most likely to be infested are high-use reservoirs in central Illinois, including Clinton Lake.Lakes in southern Illinois in close proximity to the Ohioand Mississippi Rivers are predicted to become infested,but colonization of lakes further from the infested wa-ters is predicted to occur later. Lower population densi-ties reduce boat use at these sites, making zebra musselinfestation less likely.

The high-use reservoirs in central Illinois act as step-ping stones for the spread of zebra mussels. ClintonLake and Lake Shelbyville are predicted to be infestedrelatively rapidly. Populations of zebra mussels estab-lished in these reservoirs then act as sources for furtherspread to other central Illinois lakes, both overland anddownstream. Lake Carlyle, on the Kaskaskia River, ispredicted to be infested via the downstream transport ofveligers established in Lake Shelbyville before it wouldotherwise be infested by overland transport.


Based on their mussel fauna, there are 38 biologically sig-nificant stream segments in Illinois (Fig. 1). The first ofthe streams predicted to be infested by the zebra musselis Salt Creek, at the outlet of Clinton Lake (Table 3). Theinfestation of Lake Sangchris is predicted to affect theSangamon River during the next period, followed by theinfestation of the Kaskaskia River from Lake Shelbyville.These stream segments are notable for having a high di-versity of mussels. The Wabash River supports the popu-lations of threatened or endangered species at greatestrisk. Infestation of the Wabash is predicted to occur viacolonization of Mill Creek Lake on Mill Creek, a tributaryof the Wabash. Probability of infestation of this lake in-creased rapidly following the infestation of Clinton Lake(Fig. 2).

We set infestation of Clinton Lake and Lake Shelbyvilleto zero to investigate the effect of disrupting stepping-stone colonization on the risk of infestation in the rest ofIllinois. If spread from these lakes were somehowstopped, the spread of zebra mussels in central Illinoiswould be significantly slowed (Fig. 3). Preventing infes-tation of the central Illinois reservoirs, Clinton Lake andLake Shelbyville, reduced the risk of invasion of biologi-cally significant mussel streams. The predicted time toinvasion was increased by 0–100%, depending on the lo-cation of the lakes (Table 3).

794 Risk Assessment of Zebra Mussel Spread Schneider et al.


Table 2. Rank order of predicted infestation of lakes by zebra mussels in Illinois.

Map referencenumber a Lake

Time stepinfested Rank SD of rankb

2 Fox Chain of Lakes 2 1 0.0083 Braidwood Lake 7 2 4.3184 Shabbona Lake 8 3 2.3416 Busse Lake 11 4.5 4.4177 Lake Carlton 11 4.5 8.6723 Argyle Lake 12 6 9.9014 Clinton Lake 13 7 4.9618 Kinkaid Lake 14 9 2.6329 Baldwin Lake 14 9 5.5978 Weldon Springs Lake 14 9 5.4587 Pittsfield Lake 15 11 14.77*82 Sauk Trail Lake 16 13 12.29*93 Schuy-Rush Lake 16 13 16.31*94 Jones Lake 16 13 14.02*21 Cedar Lake 17 16 1.6476 Pierce Lake 17 16 6.0098 Mermet Lake 17 16 7.849 Lake Shelbyville 19 18 8.84

15 Carlyle Lake 20 20.5 10.2620 Otter Lake 20 20.5 7.4327 Carlyle Tailwater 20 20.5 10.2692 Sangchris Lake 20 20.5 8.79

107 Siloam Springs Lake 21 23 16.17*11 Rend Lake 22 26 4.5612 Mill Creek Lake 22 26 13.66*81 Lake Decatur 22 26 3.5791 Lake Jacksonville 22 26 3.71

110 Tampier Lake 22 26 14.17*19 Lake Murphysboro 23 29.5 6.1028 Lincoln Trail Lake 23 29.5 12.47*13 Dawson Lake 24 31.5 1.6022 Citizens Lake 24 31.5 15.11*30 Washington Co. Lake 26 33.5 6.6532 Coffeen Lake 26 33.5 11.39*80 Lake Le-Aqua-Na 27 35 7.387 Devils Kitchen Lake 28 36.5 2.25

33 Forbes Lake 28 36.5 8.7024 Lake Sara 29 38 6.8479 Lake Springfield 30 39 7.7831 Sam Parr Lake 32 40.5 7.1675 Dutchman Lake 32 40.5 2.0385 Frank Holten Lake 33 42 7.0610 Walnut Point Lake 34 43 4.231 Lake Charleston 35 44 2.35

17 Dolan Lake 36 45 2.846 Gillespie New City Lake 37 46 5.19

25 Beaver Dam Lake 38 47 4.625 Greenville New City Lake 39 48 3.908 Ramsey Lake 40 50 0.53

26 Lake Paradise 40 50 3.2389 Red Hills Lake 40 50 1.75

109 Sterling Lake 50 52 1.6886 Lake Mingo 71 53 12.24*90 Lake Vermilion 80 54 9.5688 Clear Lake 91 55 0.75

aSee Fig. 1 for location of lakes.bLakes whose sensitivity of rank in sensitivity analysis was more variable than expected by chance (SD . 10.5, p , 0.05) are marked by anasterisk.


Schneider et al. Risk Assessment of Zebra Mussel Spread 795

Discussion

This model shows significant differences in the likeli-hood of zebra mussel invasion in the inland waters of Il-linois and predicts only relative rather than absolute riskof invasion. As of December 1996 the only waters in Illi-nois reported to have zebra mussels were Lake Michi-gan, the Illinois River System (the Chicago River, theChicago Sanitary and Ship Canal, the Calumet Sag Chan-nel, the Des Plaines River, and the Illinois River and itsbackwater lakes), the Calumet River, the MississippiRiver, and the Ohio River (Biological Resources Divi-sion, U. S. Geological Survey, Non-indigenous AquaticSpecies Information Service, www.nfrcg.gov).

The site identified by this model to be at highest riskfrom zebra mussel invasion is the Fox Chain of Lakes.This site is close to the infested waters of Lake Michiganand receives heavy boat traffic. As of November 1996,officials of the Fox River Waterway Agency, who checktheir buoys for zebra mussels, had reported no sightings(K. Kabbes, personal communication). Researchers fromWisconsin Sea Grant, however, collected veligers fromBluff Lake in the Fox Chain in July 1996 (C. Kraft, per-sonal communication), and plankton samples from themouth of the Fox River (taken in September 1996 afterthe parameterization of this model) had a density of ze-bra mussels of 0.6 veligers/L, indicating that adults are

present at some location in the Fox River Watershed ( J.Stoeckel, personal communication). This is the first indi-cation of the spread of zebra mussels in inland waters ofIllinois. That it has occurred in the watershed identifiedby this model as most at risk lends support to the ap-proach of this model. Infestation of the Fox Chain ofLakes follows a similar pattern to that found in Wiscon-sin by Padilla et al. (1996) in which lakes near LakeMichigan that had a large number of direct trips by trail-ered boats were the first to be infested by zebra mussels.

The absence of zebra mussels from the vast majorityof inland waters in Illinois, however, including sites ofheavy boat traffic near infested waters, suggests that theabsolute risk of invasion may be relatively low. Recentsamples (September 1996) from the mouth of theKankakee River contained no veligers ( J. Stoeckel, per-sonal communication), and systematic sampling fromthe Wabash River (K. S. C., personal observation, August1996) also revealed no zebra mussels. Chemical factorsare not likely to play a large role in the absence of zebramussels from inland waters in Illinois. Calcium is high inalmost all Illinois waters. Turbidity may play some role,although the zebra mussel has successfully invaded thehighly turbid Illinois, Mississippi, and Ohio Rivers. If ze-bra mussels are most likely transported attached toaquatic vegetation (Johnson & Carlton 1996), however,the lack of abundant macrophytes in many Illinois wa-

Table 3. Risk of infestation by zebra mussels of streams in Illinois with threatened or endangered mussels or high mussel diversity.

Time stepsto infestation Stream

Upstream lakeresponsible

for infestation Mussel faunaa

Time step infested with prevention effortsb

15 Salt Creek Clinton Lake high diversity —20 South Fork,

Sangamon RiverLake Sangchris high diversity 22

Sangamon River Lake Sangchris high diversity 22Kaskaskia River

below ShelbyvilleLake Shelbyville high diversity —

22 Wabash River Mill Creek Lake Potamilus capax (FE)Cyprogenia stegaria (FE)Elliptio crassidens (ST)Plethobasus cyphyus (ST)high diversity

30

24 North Fork Salt Creek Lake Dawson Lasmigona compressa (ST) .5024 Kaskaskia River below

CarlyleLake Carlyle high diversity 25

28 Little Wabash River Forbes Lake Toxolasma lividus (SE) 3329 Lake Sara high diversity 5032 Embarras River below

NewtonSam Parr Lake high diversity 41

34 Embarras River below Walnut Point Lake

Walnut PointLake

Ptychobranchus fasciolaris (SE) Epioblasma triquetra (FC,SE) high diversity

43

36 North Fork of Saline River Dolan Lake high diversity 36aFederally listed endangered (FE), federally listed candidate (FC), state-listed endangered (SE), state-listed threatened (ST). Mussel distributionsfrom Page et al. (1992).bProtection of Clinton Lake and Lake Shelbyville from infestation by means of boater education, inspection, and other methods that do not dis-place boat traffic to other lakes.



ters may reduce the risk of zebra mussel transport. Pa-dilla et al. (1996) also noted the slow pace of invasion ofinland Wisconsin lakes and attributed it to the rarity ofdirect trips between Lake Michigan and an inland lake.

The sensitivity analysis suggested that factors affectingoverland travel will have a more critical impact on rela-tive risk than factors affecting downstream transport.The rank order of predicted invasion for 11 lakes ap-peared sensitive to the model parameters used. The sen-sitivity of these lakes was due almost entirely to sensitiv-ity to the value of a used. In contrast, the order ofpredicted lake infestation was not appreciably altered bychanging downstream flow (d). Thus, actions taken todisrupt the overland flow of zebra mussels appear mostlikely to have an impact on the risk of invasion of a lake.Education efforts, both at lakes at high risk of infestation

and at currently infested lakes with high boat traffic,could be effective in slowing the spread of the zebramussel to key lakes.


Reservoirs represent potential sites of invasion of the ze-bra mussel that can lead to infestation of downstream ar-eas that support federal or state threatened and endan-gered species of freshwater mussels and areas of highmussel diversity. The construction of reservoirs through-out Illinois has increased the likelihood of transport ofnon-indigenous species. River regulation has been shownto facilitate invasion by non-indigenous species by alter-ing the hydrological characteristics of the river (Minck-ley & Meffe 1987). It also has the potential to facilitate

Figure 2. Predicted course of spread of zebra mussels after 29 time steps if no management actions are taken (a) or if the infestation of Clinton Lake and Lake Shelbyville is completely prevented by education or public awareness campaigns that do not divert boat traffic to other lakes (b). Size and shading of the points reflect the cumulative number of boats carrying zebra mussels to a particular lake.



overland dispersal by creating “hot spots” of recre-ational boating. The wide habitat requirements of the ze-bra mussel allow it to be transported to inland lakes andsubsequently to invade downstream habitats.

High-quality native mussel habitats in the Wabash, San-gamon, and Kaskaskia River systems are at risk from theoverland dispersal of zebra mussels to upstream recre-ational areas that attract a large number of boats. Otherriver systems, such as the Vermilion in east-central Illi-nois, are at less risk because reservoirs occur down-stream of areas with a high-quality mussel fauna. Risk toother rivers such as the Mackinaw could not be evalu-ated because of lack of boater data for upstream boatlandings. Other rivers, such as the Wabash, Kankakee,and upper Fox, have upstream lakes outside of Illinois,and their risk was not evaluated. This indicates the needfor a regional approach to assessing zebra mussel risk.

Potential Conservation Measures

The highest-risk populations, including state and federalthreatened and endangered species, occur in the Wa-bash River and its tributaries. The model suggests that alikely point of invasion to the Wabash system in Illinoisis Mill Creek Lake. Education or inspection efforts tar-geted at this lake could be effective in protecting thesepopulations. Mill Creek Lake and Mill Creek should bemonitored more carefully for zebra mussel infestation.Education efforts should be intensified at boat landingsat this lake, including signs warning boaters to examinetheir boats and trailers before launching and to removewater from live wells and other areas of the boat thatcould harbor veligers. Other habitats with state threat-ened or endangered species occur in the North Fork ofSalt Creek (Sangamon-Illinois River drainage) and the Lit-tle Wabash River. Similar efforts should be targeted atthe upstream lakes on these rivers.

A more general approach could also be taken to pro-tect populations of native mussels, particularly in south-ern and central Illinois. Central Illinois reservoirs appearto be likely stepping stones for invasion of other centraland southern Illinois lakes and streams. As the central Il-linois reservoirs became infested, colonization rates tonearby lakes increased substantially (Fig. 3). Preventingthe introduction of zebra mussels to Clinton Lake andLake Shelbyville could significantly slow the spread ofzebra mussels throughout the state. For instance, in asimulation in which infestation at Lakes Clinton andShelbyville was prevented, the predicted time to inva-sion of Mill Creek Lake was lengthened by over 33%,even though these lakes are in different watersheds.Thus, education programs aimed at visitors to Clintonand Shelbyville could be important in protecting nativemussel populations. This result suggests the perhapscounterintuitive strategy of aiming prevention effortsnot directly at the high-risk habitats but rather at the cur-

rently uninfested likely source of invasion, even if it isnot the most critical habitat for native mussels. Thisstrategy would have the benefit of reaching a greaternumber of boaters as well, increasing general awarenessof the zebra mussel problem among Illinois boaters.

The efficacy of slowing predicted time to infestationby preventing invasion of the high-use reservoirs de-pended on the location of the lakes (Table 3). Lakessuch as Mill Creek Lake and the lakes in the EmbarrasRiver system (Walnut Creek, Charleston, and Sam Parr)appear to be in the orbit of Shelbyville and Clinton, sopreventing infestation there reduces the flow of infestedboats elsewhere. Prevention of invasion of Lake Clintonhad a particularly strong effect on protecting DawsonLake, only 27 km away. Other lakes, such as Dolan Lakeon the North Fork of the Saline River in southern Illinois,appear to be more influenced by boat traffic from theOhio River sites, so halting invasion of central Illinoisreservoirs was ineffective in slowing the predicted timeto infestation of this lake.

The type of interdiction efforts used to prevent the in-festation of stepping-stone lakes can affect the predictedefficacy of management strategies. If zebra mussel inva-sion of stepping-stone reservoirs is prevented througheducation, inspection, or other efforts that don’t de-crease boat traffic to these sites, then infestation of lakessecondary to these reservoirs may be significantlyslowed (Table 3). But if methods such as quarantines areused, effectively shutting off access to these popularsites, boat traffic is predicted to move elsewhere, in-creasing the potential movement of zebra mussels toother lakes. Risk was always higher to the lakes up-stream of biologically significant streams following quar-antine of Lakes Shelbyville and Clinton rather than pre-vention that did not involve quarantine (Fig. 3). In somecases, for reservoirs close to the quarantined waterssuch as Lake Sangchris and Dawson Lake, quarantine ac-tually reduced the time to infestation over doing nothingat all, although this increase would not have been appar-ent if a higher threshold of infestation was used. Educa-tion and inspection are preferable to quarantine even ifthey are not 100% effective. If only 80% of the zebra mus-sels are interdicted using these techniques, infestation ofsecondary lakes is still slowed substantially (Fig. 3).

Inspection programs need to be designed in ways thatwill not significantly reduce the attractiveness of the res-ervoirs for boat use. Boater displacement in response torecreational setting has been documented for boaters us-ing central Iowa lakes (Robertson & Regula 1994). Thus,inspection efforts that reduced the attractiveness oflakes to boat use could also cause a displacement ofboaters to other, potentially high-risk lakes, and wemight expect a shift in patterns of risk somewhere be-tween educational efforts and strict quarantine. Thesecautions apply to efforts at currently uninfested waters.Once lakes are already infested, quarantine or rigid



inspection would not increase the risk to nearby lakes.Assessment of the impact of invasion prevention effortsmust include the overall movement of boats statewideand not simply movement to individual lakes.

These simulation results suggest that critical Illinoismussel populations are at risk from invasion of the zebramussel. Currently, zebra mussel education efforts areconcentrated at waters already infested by zebra mus-sels, such as the Great Lakes, and sites identified as pos-sessing high-quality mussel faunas at risk of infestation,such as the St. Croix Scenic River in Wisconsin and Min-nesota (University of Wisconsin Sea Grant Institute 1993)or water supply reservoirs (University of Wisconsin SeaGrant Institute 1992). This model shows that educationefforts targeted at uninfested, high-use lakes in Illinoiscould help reduce the risk of infestation of critical habi-tats, even if the fauna at or downstream of the lakes them-selves are not at grave risk from zebra mussel invasion.

Dynamic models of invasion that can evaluate relativerisk will be useful in prioritizing interdiction and conser-vation efforts. Transportation models that explicitly

model the behavior of boat traffic, shipping, or airline traf-fic are ideal for analyzing the potential spread of specieswhose main vector is human activity. The models andtechniques presented here can be applied to a number ofinvasive species and vectors. The relative risk of invasionby the brown tree snake to islands of the Pacific is analo-gous to the threat of zebra mussels. Islands with frequentair service from infested islands would be most at risk. Inaddition, there may be critical stepping-stone islands. Rel-ative risk could be modeled with data on airline trafficamong islands. The risk of invasion of other aquatic spe-cies via ballast water (e.g., Pierce et al. 1997) can also bemodeled with data on boat traffic. The gravity models pre-sented here are especially useful when complete data onvector movements are not available.

Acknowledgments

D. Padilla first suggested using boater data to predict thespread of zebra mussels. We thank D. Padilla, D. Johnston,

Figure 3. Cumulative zebra mussel colonization of lakes expected with either no risk reduction measures, with quarantines in which boat traffic shifts to unquarantined lakes, and with educational measures that are 80% and 100% effective in preventing zebra mussel colonization and that do not decrease boater use of lakes, for Mill Creek Lake, Dawson Lake, Lake Sara, and Sangchris Lake.



L. Hopkins, and B. Aten for helpful comments duringthis study. S. Sobaski and W. Brofka provided access tocreel data; R. Hallberg helped locate boat data for otheraccess points; and D. Szafoni, K. Hunter, and C. Mayerhelped with access to geographic information systemdata. We thank D. Soluk, C. Phillips, C. Kraft, C. Ramcha-ran, E. Main, R. Noss, and an anonymous reviewer forcomments on the manuscript. This study was supportedby grant CR 820574 from the U.S. Environmental Protec-tion Agency and grant PCFR-97 from the Illinois Depart-ment of Natural Resources to D. W. S.

Literature Cited

Baker, S. M., and D. J. Hornbach. 1997. Acute physiological effects ofzebra mussel (Dreissena polymorpha) infestation on two unionidmussels, Actinonaias ligamentina and Amblema plicata. Cana-dian Journal of Fisheries and Aquatic Sciences 54:512–519.

Baxter, M., and G. Ewing. 1981. Models of recreational trip distribu-tion. Regional Studies 15:327–344.

Carey, J. R. 1996. The future of the mediterranean fruit fly Ceratitiscapitata invasion of California: a predictive framework. BiologicalConservation 78:35–50.

Carlton, J. T. 1993. Dispersal mechanisms of the zebra mussel (Dreis-sena polymorpha). Pages 677–697 in T. F. Nalepa and D. W.Schloesser, editors. Zebra mussels: biology, impacts, and control.Lewis Publishers, Boca Raton, Florida.

Cope, W. G., and D. L. Waller. 1995. Evaluation of freshwater musselrelocation as a conservation and management strategy. RegulatedRivers: Research and Management 11:147–155.

Cummings, K. S., and C. A. Mayer. 1997. Distributional checklist andstatus of Illinois freshwater mussels (Mollusca: Unionacea). Pages129–145 in K. S. Cummings, A. C. Buchanan, C. A. Mayer, T.Naimo, and T. Rice, editors. Conservation and management offreshwater mussels. II. Initiatives for the future. Proceedings of aUMRCC symposium, 16–18 October 1995. Upper Mississippi RiverConservation Committee, Rock Island, Illinois.

Daehler, C. C., and D. R. Strong. 1996. Status, prediction and preven-tion of introduced cordgrass Spartina spp. invasions in Pacific es-tuaries, USA. Biological Conservation 78:51–58.

Downing, J. A., and W. L. Downing. 1992. Spatial aggregation, preci-sion, and power in surveys of freshwater mussel populations. Cana-dian Journal of Fisheries and Aquatic Sciences 49:985–991.

Gahan, L. W. 1970. An analysis of factors affecting water-oriented rec-reation consumer patterns in east-central Illinois region. Ph.D. dis-sertation. University of Illinois, Urbana.

Gramann, J. H. 1980. Social-psychological applications to the struc-tural modelling of crowding in outdoor recreation. Ph.D. disserta-tion. University of Illinois, Urbana.

Gillis, A. M. 1992. Keeping aliens out of paradise. BioScience 42:482–485.

Gillis, P. L., and G. L. Mackie. 1994. Impact of the zebra mussel, Dreis-sena polymorpha, on populations of Unionidae (Bivalvia) in LakeSt. Clair. Canadian Journal of Zoology 72:1260–1271.

Haag, W. R., D. J. Berg, D. W. Garton, and J. L. Farris. 1993. Reducedsurvival and fitness in native bivalves in response to fouling by theintroduced zebra mussel (Dreissena polymorpha) in western LakeErie. Canadian Journal of Fisheries and Aquatic Sciences 50:13–19.

Hastings, A. 1996. Models of spatial spread: a synthesis. Biological Con-servation 78:143–148.

Haynes, K. E., and A. S. Fotheringham. 1984. Gravity and spatial inter-action models. Sage Publications, Beverly Hills.

Horvath, T. G., G. A. Lamberti, D. M. Lodge, and W. L. Perry. 1996. Ze-

bra mussel dispersal in lake-stream systems: source-sink dynamics?Journal of the North American Benthological Society 15:564–575.

Johnson, L. E., and J. T. Carlton. 1996. Post-establishment spread inlarge-scale invasions: dispersal mechanisms of the zebra musselDreissena polymorpha. Ecology 77:1686–1690.

Johnson, L. E., and D. K. Padilla. 1996. Geographic spread of exoticspecies: ecological lessons and opportunities from the invasion ofthe zebra mussel (Dreissena polymorpha). Biological Conserva-tion 78:23–33.

Kareiva, P. 1996. Developing a predictive ecology for non-indigenousspecies and ecological invasions. Ecology 77:1651–1652.

Koutnik, M. A., and D. K. Padilla. 1994. Predicting the spatial distribu-tion of Dreissena polymorpha (zebra mussel) among inland lakesof Wisconsin: Modeling with a GIS. Canadian Journal of Fisheriesand Aquatic Sciences 51:1189–1196.

Minckley, W. L., and G. K. Meffe. 1987. Differential selection by flood-ing in stream-fish communities of the arid American southwest.Pages 93–104 in W. J. Matthews and D. J. Heins, editors. Commu-nity and evolutionary ecology of North American stream fishes.University of Oklahoma Press, Norman.

Morin, M. D., and D. L. Szafoni. 1993. Using GIS to integrate a state-wide streams database. Illinois GIS and Map Notes 11:43–45.

Nalepa, T. F. 1994. Decline of native unionid bivalves in Lake St. Clairafter infestation by the zebra mussel, Dreissena polymorpha. Cana-dian Journal of Fisheries and Aquatic Sciences 51:2227–2233.

Neary, B. P., and J. H. Leach. 1992. Mapping the potential spread ofthe zebra mussel (Dreissena polymorpha) in Ontario. CanadianJournal of Fisheries and Aquatic Sciences 49:406–415.

Padilla, D. K., M. A. Chotkowski, and L. W. Buchan. 1996. Predictingthe spread of zebra mussels (Dreissena polymorpha) to inland wa-ters using boater movement patterns. Global Ecology and Biogeog-raphy Letters 5:353–359.

Page, L. M., K. S. Cummings, C. A. Mayer, S. L. Post, and M. E. Retzer.1992. Biologically significant streams. An evaluation of the streamsof Illinois based on aquatic biodiversity. Illinois Natural History Sur-vey, Center for Biodiversity, Champaign.

Perdue, R. R. 1981. Spatial behavior in recreational boating: a disaggre-gate analysis of factors influencing lake-use. Ph.D. dissertation.Texas A&M University, College Station.

Pierce, R. W., J. T. Carlton, D. A. Carlton, and J. B. Geller. 1997. Ballastwater as a vector for tintinnid transport. Marine Ecology ProgressSeries 149:295–297.

Ramcharan, C. W., D. K. Padilla, and S. I. Dodson. 1992. Models to pre-dict potential occurrence and density of the zebra mussel, Dreis-sena polymorpha. Canadian Journal of Fisheries and Aquatic Sci-ences 49:2611–2620.

Ricciardi, A., R. Serrouya, and F. G. Whoriskey. 1995. Aerial exposuretolerance of zebra and quagga mussels (Bivalvia: Dreissenidae): im-plications for overland dispersal. Canadian Journal of Fisheries andAquatic Sciences 52:470–477.

Ricciardi, A., F. G. Whoriskey, and J. B. Rasmussen. 1996. Impact ofthe Dreissena invasion on native unionid bivalves in the upper St.Lawrence River. Canadian Journal of Fisheries and Aquatic Sci-ences 53:1434–1444.

Robertson, R. A., and J. A. Regula. 1994. Recreational displacementand overall satisfaction; a study of central Iowa’s licensed boaters.Journal of Leisure Research 26:174–181.

Rosenthal, D. H. 1987. The necessity for substitute prices in recreationdemand analyses. American Journal of Agricultural Economics 69:828–837.

Ruesink, J. L., I. M. Parker, M. J. Groom, and P. M. Kareiva. 1995. Re-ducing the risks of nonindigenous species introductions. Bio-Science 45:465–477.

Schloesser, D. W., and T. F. Nalepa. 1994. Dramatic decline of unionidbivalves in offshore waters of western Lake Erie after infestation bythe zebra mussel, Dreissena polymorpha. Canadian Journal of Fish-eries and Aquatic Sciences 51:2234–2242.



Schneider, D. W. 1992. A bioenergetics model of zebra mussel, Dreis-sena polymorpha, growth in the Great Lakes. Canadian Journal ofFisheries and Aquatic Sciences 49:1406–1416.

Sen, A., and T. E. Smith. 1995. Gravity models of spatial interaction be-havior. Springer-Verlag, New York.

Sokal, R. R., and F. J. Rohlf. 1981. Biometry. 2nd edition. W. H. Free-man and Company, New York.

Stall, J. B., and D. W. Hiestand. 1969. Provisional time-of-travel for Illi-nois streams. Illinois State Water Survey, Urbana.

Stoeckel, J. A., D. W. Schneider, L. A. Soeken, K. D. Blodgett, and R. E.Sparks. 1997. Larval dynamics of a riverine metapopulation: impli-cations for zebra mussel recruitment, dispersal, and control in alarge-river system. Journal of the North American Benthological So-ciety 16:586–601.

Strayer, D. L. 1991. Projected distribution of the zebra mussel, Dreis-sena polymorpha, in North America. Canadian Journal of Fisheriesand Aquatic Sciences 48:1389–1395.

Strayer, D. L., and L. C. Smith. 1996. Relationships between zebra mus-sels (Dreissena polymorpha) and unionid clams during the earlystages of the zebra mussel invasion of the Hudson River. Freshwa-ter Biology 36:771–779.

Suter, G. 1993. Exotic Organisms. Pages 391–401 in G. W. Suter, editor.Ecological risk assessment. Lewis Publishers, Boca Raton, Florida.

Sutherst, R. W., R. B. Floyd, and G. F. Maywald. 1996. The potentialgeographical distribution of the cane toad, Bufo marinus L. in Aus-tralia. Conservation Biology 9:294–299.

Thomas, R. W., and R. J. Huggett. 1980. Modeling in geography.Barnes and Noble Books, Totowa, New Jersey.

Townsend, C. R., and M. J. Winterbourn. 1992. Assessment of the envi-ronmental risk posed by an exotic fish: the proposed introductionof channel catfish (Ictalurus punctatus) to New Zealand. Conser-vation Biology 6:273–282.

Tucker, J. K., and E. R. Atwood. 1995. Contiguous backwater lakes aspossible refugia for unionid mussels in areas of heavy zebra mussel(Dreissena polymorpha) colonization. Journal of Freshwater Ecol-ogy 10:43–47.

University of Wisconsin Sea Grant Institute. 1992. Water supplyreservoirs closed to boaters. Zebra mussel update 13. Madison,Wisconsin.

University of Wisconsin Sea Grant Institute. 1993. Boating restrictionsimplemented. Zebra mussel update 17. Madison, Wisconsin.

Whittier, T. R., A. T. Herlihy, and S. M. Pierson. 1995. Regional suscepti-bility of northeast lakes to zebra mussel invasion. Fisheries 20:20–27.

Williams, J. D., M. L. Warren, Jr., K. S. Cummings, J. L. Harris, and R. J.Neves. 1993. Conservation status of freshwater mussels of theUnited States and Canada. Fisheries 18:6–22.

A Transportation Model Assessment of the Risk to Native Mussel Communities from Zebra Mussel Spread

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