managing invasive aquatic plants in a changing system...

Special Section

Managing Invasive Aquatic Plants in a ChangingSystem: Strategic Consideration of Ecosystem ServicesCARL HERSHNER∗ AND KIRK J. HAVENSCenter for Coastal Resources Management, Virginia Institute of Marine Science, College of William and Mary, 1208 Greate Road,Gloucester Point, VA 23062, U.S.A.

Abstract: Climate change is projected to increase stress for many coastal plant communities. Along large

portions of the North American coast, habitat degradation from anthropogenic changes to the environment

already threaten the community structure of tidal marshes and submerged aquatic grass beds. The potential

loss of ecological services historically provided by these communities has been a long-standing rationale for

aggressive control of invading plants such as Phragmites australis and Hydrilla verticillata. Increasing evidence

of ecological services provided by invasive species such as P. australis and H. verticillata suggest that, in the face

of increasing stress, it may be prudent to take a more pragmatic approach regarding the effect of these species

on coastal ecosystems. The notable resilience of these species to control efforts and their competitive success

and comparative vigor in stressed systems and capacity to provide at least some beneficial services combine

to suggest some invasive species may have a useful role in managed coastal ecosystems.

Keywords: climate change, ecosystem services, exotic species, Hydrilla, invasive aquatic plants, Phragmites

Manejo de Plantas Acuaticas Invasoras en un Sistema Cambiante: Consideracion Estrategica de los Servicios delEcosistema

Resumen: Se considera que el cambio climatico incrementara el estres para muchas comunidades de plantas

costeras. A lo largo de grandes porciones de la costa norteamericana, la degradacion del habitat por cambios

antropogenicos al ambiente ya es una amenaza para la estructura de la comunidad de marismas y de lechos

de pasto acuatico sumergido. La perdida potencial de los servicios ecologicos proporcionados historicamente

por estas comunidades por mucho tiempo ha sido la base logica para el control agresivo de plantas inva-

soras como Phragmites australis e Hydrilla verticillata. El incremento de evidencia de los servicios ecologicos

proporcionados por especies invasoras como P. e H. verticillata sugieren que, a la luz del incremento de estres,

puede ser prudente adoptar un enfoque mas pragmatico en relacion con el efecto de estas especies sobre los

ecosistemas costeros. La resiliencia notable de estas especies a los esfuerzos de control y su exito competitivo

y vigor comparativo en sistemas estresados y si capacidad para proporcionar por lo menos algunos servicios

beneficos se combinan para sugerir que algunas especies invasoras pueden jugar un papel util en ecosistemas

costeros.

Palabras Clave: cambio climatico, especies exoticas, Hydrilla, Phragmites, plantas acuaticas invasoras, servi-cios del ecosistema

Introduction

Much of the human population of the planet is concen-trated in coastal zones. The heavy use of these areas for

∗email [email protected] submitted July 12, 2007; revised manuscript accepted January 21, 2008.

homes, commerce, recreation, and waste assimilation hasresulted in engineered ecosystems in which many natu-ral conditions have been significantly altered. These cir-cumstances have made sustaining benefits of ecosystem

544Conservation Biology, Volume 22, No. 3, 544–550C©2008 Society for Conservation BiologyDOI: 10.1111/j.1523-1739.2008.00957.x

Hershner & Havens 545

services a continuing and growing challenge. Increasednutrient and sediment loads, altered hydrology, and mod-ified physical environments have all stressed the nativebiota, often to the limits of their tolerances. Such condi-tions are often favorable for successful invasions of non-native species.

Invasive species are generally viewed as threats to thehealth of native communities. There is an extensive litera-ture focused on the challenges of understanding the pro-cess of invasion, the consequences, and the possibilitiesfor control (Parker et al. 1999; Mack et al. 2000; Ricciardiet al. 2000; Simberloff 2003; Simberloff et al. 2005). Al-though much of this work focuses on the need to betterunderstand the biology of invaders and the value of effec-tive monitoring and communication, there is a commonrecognition that once established, invasive species can beintegral to the structure and functioning of an ecosystem(e.g., Posey 1988; Mack et al. 2000; Zavaleta et al. 2001;Sax & Gaines 2003).

Invasive species sometimes so dominate local systemsthat biodiversity and habitat heterogeneity are severelyaltered (Crooks 2002; Sax & Gaines 2003). These con-sequences are viewed as detrimental because ecologistsreason that diversity is a desirable characteristic in ecosys-tems. Diversity is believed to impart some system-levelemergent capacity to be resistant or resilient to stress.It is also viewed as an important aspect of the potentialto adapt to novel future conditions. For these reasons,aggressive control of invaders is a common managementobjective.

Tenets of Strategic Management of Invasive Species

The evolving understanding of invasive species andecosystem responses to their introduction is challengingthe simplistic argument that eradication or control are al-ways appropriate responses. Zavaleta et al. (2001) arguethat food-web and functional-role frameworks should beused to assess potential changes in ecosystems beforeinvasive species are eradicated. We suggest that the dif-ferential value of ecosystem services provided by nativeversus invasive species may not always justify manage-ment aimed at eradication or complete control. Recog-nizing that in some circumstances the cost and efficacyof control is limited (e.g., Pimentel et al. 2000), strategicmanagement of invasive species may involve acceptanceof their presence and value in the system. Several tenetssupport this perspective:

1. Environmental management exists to sustain ecosys-tem services valued by society.

2. The value of ecosystem services frequently dependson the location or time at which they are provided;therefore, management response to invaders shouldbe spatially or temporally variable rather than uniform.

3. Coastal ecosystems are undergoing changes thatcan alter their suitability for many aquatic plantspecies.

4. Invasive aquatic plants may be more capable of deal-ing with an increase in stressors that affect coastalenvironments.

Environmental Management and Ecosystem Services

Rogers and Biggs (1999) argue that an operational defini-tion of the desired system condition that reflects scien-tific rigor and broader societal value systems is essentialto any effective management scheme. In the case of inva-sive aquatic plants, this seems to argue for control of theseplants that goes beyond the simple objective of maintain-ing diversity. We suggest that the explicit identification ofimportant ecosystem services effectively establishes met-rics for evaluating effects of invaders that replace nativespecies. The capacity of invasive species to provide val-ued ecosystem services should be a consideration in thedesign of management responses. This is consistent withLindenmayer et al. (2007), who suggest effective manage-ment requires a clearly articulated vision and quantifiableobjectives that can be used for assessment.

Spatial or Temporal Management Response

Opportunity and capacity to provide ecosystem servicesare both factors in the valuation of those services. A wet-land with the appropriate physical and biological makeupto provide habitat services is most valued if it is locatedwhere the services are needed and can be used. Similarly,a wetland with only limited capacity to assimilate or se-quester nutrients is not of great value in areas with highnutrient loads. To the extent that invasive aquatic plantshave the capacity to provide certain ecosystem services,and when they are found in settings where those servicesare valued, the objective of environmental managementmay not be served by a commitment to eradication.

Coastal Ecosystem Changes

Anthropogenic impacts to coastal watersheds have in-creased nutrient and sediment loads, altered freshwa-ter inflows, modified local estuarine circulation pat-terns, altered physical substrates, and changed localenergy environments. Collectively, these stresses havechanged the quantity and quality of many aquatic-plantcommunities.

Current efforts to mitigate these effects are in a dy-namic balance with the negative effects of spreadingcoastal development. The result is a physical, chemical,and biological environment that is often more variableand more extreme than pristine systems. Evidence of ef-fects on native-plant communities can be found in de-creased distribution and vigor of tidal marshes and sub-merged aquatic-grass beds in many coastal areas.

Conservation Biology

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546 Invasive Plants in a Changing System

Current studies of climate change suggest that anthro-pogenic stresses will be compounded by climate-drivenstressors such as increasing temperatures, rising sea lev-els, and altered storm and precipitation patterns. Preciseforecasting of consequences to current plant communi-ties is not possible. But it is clear that the conditions thatexisted for the past several thousand years are changing.Although the relative stability of the past allowed thecurrent communities to establish themselves, there is lit-tle reason to presume that these communities will notchange under increasing stress.

Lindenmayer et al. (2007) observed that conservationoften aims at stasis and assumes an equilibrium state fornatural systems. They note that this approach is perhapsinappropriate given that landscapes are dynamic and maybecome more so with future climate variability.

Invasive Aquatic Plants Stress Adaptation

The success of invasive species is evidence of theirrelative fitness for the conditions they encounter. Theparadigm for plant invasion identifies 4 stages and a num-ber of conditions that can contribute to success (seealso Hellmann et al. 2008 [this issue]). These can in-clude species fitness (e.g., broad environmental toler-ances, phenotypic plasticity) and a variety of environmen-tal and ecosystem conditions that can mitigate potentialthreats to invaders (reduced competitive vigor in nativespecies, range shifts for predators, shifting resource avail-ability) (Theoharides & Dukes 2007). Whatever the un-derlying causes, successful invaders are well matched toextant conditions. Indeed, successful invasions are oftena result of disturbance in systems that approaches or ex-ceeds the capacity of native communities to persist. Theprospect of continued increases in coastal-system stres-sors from both development and climate changes makeswide environmental tolerances a competitive advantageand potentially a critical characteristic for long-termpersistence.

Case Studies

In the following we briefly review the cases of 2 inva-sive aquatic plants, Hydrilla verticillata and Phragmites

australis. These plants are the focus of management con-cern in many areas. In each case we highlight the modestto poor efficacy of current control efforts and reviewevidence that the plants have some capacity to providevalued ecosystem services in some settings. On the basisof these case studies, we argue for a paradigm shift fromfocus on the negative effects of invasive plants on existingstatic systems to recognition of potential, positive servicecontributions in a changing system. The correspondingpolicy shift would be from control and eradication tocontrol and ecosystem-service management.

Hydrilla verticillata

An invasive species of aquatic vegetation, H. verticillata,was originally seen as a severe nuisance. The dense standsof the grass can impede recreational and commercialboating and can affect numerous other uses of the watercolumn. Of particular fear was the potential eliminationof native, submerged aquatic vegetation (SAV) and thecascading ecosystem effects that the loss or alteration ofthat keystone community might produce.

In the mid-Atlantic region of the United States, muchattention was focused on Hydrilla when it first appearedin the Potomac River and Chesapeake Bay (U.S.A.) in the1980s. This was at the same time that significant regionalefforts were initiated as part of the Chesapeake Bay Pro-gram to reduce nutrient loads entering the bay and itstributaries. United States Geological Survey (USGS) scien-tists became interested in how Hydrilla was affecting therestoration of benefits targeted by water-quality manage-ment efforts.

Recent reports suggest that concerns for adverse ef-fects of Hydrilla on waterfowl, fish, and crabs may beunfounded, at least in the Potomac River. Informationfrom field and aerial surveys was used to document thedistribution of various SAV species throughout the riversystem over a 17-year period. Analysis of the data showedthat although Hydrilla was always more than 40% ofthe total abundance of vegetation, native species werenot eliminated. Indeed the proportion of native SAV in-creased over the period of investigation (Rybicki et al.2007).

The management of Hydrilla has proven problematic.Mechanical removal or harvesting rarely succeeds be-cause the plant reproduces effectively from pieces. Over-fertilization to enhance phytoplankton growth or appli-cation of dyes to reduce light transmission through thewater column are sometimes tried but generally are lim-ited to ponds. Biological control through introductionof grass carp (Ctenopharyngodon idella) can be effec-tive in ponds, but grass carp pose a problematic appli-cation of another non-native species in uncontained sys-tems. Chemical control through herbicide applicationsare commonly used, but the available products generallykill all plants. The cost of control even in selected areas issignificant. For aquatic weeds, with Hydrilla the predom-inate species, national annual control costs are estimatedat an order of magnitude greater than losses or damagesattributed to the plants (Pimentel et al. 2000). Even aftermany years of experience dealing with Hydrilla, controlremains an imperfect and expensive undertaking.

The USGS studies (Rydicki & Landwehr 2007) of Hy-

drilla populations in the Potomac River suggest that theplant has beneficial effects on the quality of habitat forother SAV species. Madsen et al. (2001) hypothesize thatimproved water clarity resulting from the stilling effect ofdense SAV beds, allows successful recruitment of native


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Figure 1. Waterfowl counts from National Audubon

Society Christmas Bird Count surveys (bars) and

submerged aquatic vegetation (SAV) coverage

summed for the upper and lower tidal Potomac River

segments (line) for 3 time periods. Myriophyllumspicatum was present, but SAV coverage was

unknown for 1959–1965. Figure reprinted with

permission from Rybicki and Landwehr (2007: 1203).

SAV species. Once established the native species are ableto compete with modest success, sustaining and evenexpanding their presence in the SAV community.

In addition to the plant habitat services, Rybicki andLandwehr (2007) also compared waterfowl abundancebetween periods when no SAV was present and oneswhen Hydrilla dominated. Waterfowl abundance doesrespond positively to Hydrilla abundance (Fig. 1). Thisresult suggests the non-native SAV provides some ecolog-ical services of value to the Potomac River system.

Phragmites Australis

P. australis, commonly referred to as Phragmites, is acosmopolitan species found throughout most of NorthAmerica. It has relatively wide tolerances for environ-mental conditions, and numerous researchers have de-scribed its success invading disturbed habitats, in partdue to its ability to modify disturbed habitats to condi-tions suitable for itself (Maryland Sea Grant 2002; Cham-bers et al. 2003). There is evidence from peat samplesthat Phragmites has been a part of wetland communitiesin North America for several thousand years, but has be-come much more prevalent during the 1900s (Rice et al.2000). Recently the aggressive expansion by Phragmites

has been attributed to non-native clones, genetically dif-ferent from native clones (Saltonstall 2002). This line of

investigation has led to the recognition that there are mul-tiple clones of Phragmites that exhibit a large degree ofphenotypic plasticity in the species. This variable char-acteristic enables the species to compete very effectivelyover a wide variety of environmental conditions (Hansenet al. 2007).

Recent work in the Chesapeake Bay region byR. Chambers et al. (unpublished data) examined theoccurrence of Phragmites along tidal shorelines inMaryland and Virginia. They compiled data from ashoreline inventory program conducted by the Vir-ginia Institute of Marine Science (http://ccrm.vims.edu/gis data maps/shoreline inventories/index.html). Distri-butions of Phragmites and many other characteristics ofthe intertidal and riparian areas are mapped for more than8850 km of shoreline in Virginia and Maryland. We usedthe data compiled by R. Chambers et al. to examine therelationship between the percentage of shoreline withtidal marshes containing Phragmites and the percentageof developed land in the riparian area for 12 sixth-order(12-digit) hydrologic units. The relationship detected fortributaries in Virginia is shown in Fig. 2. The areas’ in-ventories covered a wide range of human disturbance ofriparian lands in which tidal wetlands are still dominatedby native plants and where there was an evident positiverelationship between Phragmites occurrence and levelof human disturbance. Although this analysis falls wellshort of a documented cause-and-effect relationship, ourintent was a first-order test of the hypothesis that Phrag-

mites is comparatively well adapted to persist in the faceof the multiple stresses created by human occupation ofthe landscape.

King et al. (2007) document that Phragmites abun-dance increases sharply in wetlands where developmentcovers more than 15% of the surrounding area. They

Figure 2. Relationship between percentage of tidal

marshes in Virginia with P. australis and percentage of

riparian area with human disturbance (developed

and agricultural land uses) in 12. 12-digit hydrologic

units (R2 [adj] = 63.2%).


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Figure 3. Overall bird densities calculated for the 3

habitat types Peltandra virginica (n = 11), Spartinacynosuriodes (n = 11), and Phragmites australis (n =10) during the summer breeding season.

conclude that urbanization near coastlines appears toplay an important role in the invasion success of Phrag-

mites in Chesapeake Bay and probably much of easternNorth America.

Phragmites control has been a management objectivefor many years. The rapid spread of the plant into wet-land communities led to immediate concerns for loss ofdiversity and the ecosystem characteristics generally re-lated to diversity. The response was to attempt eradica-tion or at least control. Practitioners quickly learned thatthe plant had a remarkable regeneration and propagationcapacity. Physical disruption merely increased the num-ber of regenerating sites. Burning needed to be carefullytimed and repeated and still failed to provide effectivecontrol. Herbicides also needed carefully timed and re-peated applications, generally with only modest successand significant collateral risk. Biological controls have yetto prove effective.

The combined management experience has led to therealizations that eradication will not happen and large-scale control is both expensive and probably not feasi-ble. Targeted efforts to limit establishment and spreadof Phragmites into vigorous native plant communitiescan and have succeeded, but they are always an effortwithout end.

A number of studies document the ecological servicesprovided by Phragmites stands, and a variety of com-parisons of these stands with native marsh communitieshave been made. The results of these investigations con-sistently show that Phragmites is not bereft of value inthe ecosystem. In fact it can provide some ecologicalservices at levels comparable to native plants. In thesestudies, particular attention was paid to water qualityand habitat services.

Phragmites is well known for its capacity to colonizeand stabilize disturbed sediments, modifying the edaphicenvironment (Windham 2001), but it also plays a role in

nutrient uptake and retention. Tanner (1996) comparesnutrient uptake in a number of wetland plants and doc-uments that aboveground tissue concentrations of nitro-gen are highest in Phragmites. Chambers et al. (1999)report similar findings. They examined the expansion ofPhragmites into tidal wetlands along North Americancoasts. They conclude that ecosystem services in tidalwetlands dominated by Phragmites are not diminished.Specifically, they suggest that nitrogen retention may becomparatively greater in Phragmites wetlands (Figure 3).

In North America the use of Phragmites by fauna variesand in some cases equals or exceeds use of other robust,emergent plant communities (Meyerson et al. 2000). Pax-ton and Watts (2003) compared bird use of different plantcommunities on 2 large marshes in Virginia. Their resultsindicate comparable performance among plant commu-nities in terms of avifaunal habitat.

This small sampling of research on Phragmites pro-vides evidence of the significant capacity of the speciesto provide water quality and habitat services in some set-tings. This capability may make the plant a desirable com-ponent of some managed systems, particularly in light ofits comparative vigor under high sediment and nutrientloadings.

Conclusions

Hydrilla and Phragmites are 2 aquatic plants long con-sidered to be detrimental invaders of native plant com-munities. In each case, managers have mounted aggres-sive control efforts that have been expensive and onlymodestly effective. Research into the ecology of these 2species suggests they are both very capable of establish-ing and maintaining themselves in areas under significantstress from anthropogenic activities. Sediment and nu-trient loads detrimental to native plant communities arewell tolerated by these 2 species, and each has demon-strated a capacity to modify its local environment in waysthat help it persevere. In the case of Hydrilla, the modi-fications have sometimes actually allowed other speciesto reestablish themselves as part of the local plant com-munity. In the case of both Hydrilla and Phragmites,water-quality improvement and habitat services can belocally significant.

We suggest that in a system experiencing sustainedand often increasing stress from human occupation, andin the face of climate-driven changes to many aquatichabitat parameters, species with wide tolerance limitsmay be important components of sustainable systems.We do not suggest abandoning control efforts that seekto preserve vigorous native communities. But, we do be-lieve informed consideration of the role these non-nativeplants can play in sustaining valued ecosystem servicesshould be central to management planning. This practice


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might result in a decision to tolerate Phragmites estab-lishment and expansion in industrialized tributaries, suchas the Elizabeth River system in Virginia or the Patap-sco River in Maryland. In these settings, shoreline plantcommunities are particularly valuable for water-qualityservices. Phragmites can tolerate the multiple stressorsin these settings and can provide sediment retention andnutrient uptake services that rival any of the other plantspotentially found in the area.

Numerous authors note that improving the efficacy ofresource management, specifically with respect to sus-taining ecosystem services, will require improved com-munication among researchers, managers, and policymakers and a broad appreciation of the links betweensociety and ecosystem services (Palmer et al. 2005; Groff-man et al. 2006). An important first step is recognitionthat in a variable and changing environment, manage-ment responses to invasive aquatic plants need to bedriven by objective consideration of both risks and ben-efits. As Ludwig et al. (2003) state, “Effective, consistentmanagement decisions . . . can only be made on the basisof site-specific findings, a consideration of technologieswith their needed level-of-effort, and explicit expositionof the human values driving the management options.”

Acknowledgments

Virginia Institute of Marine Science contribution number2930. This manuscript benefited from the comments ofB. Bierwagen and 2 anonymous reviewers.

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