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Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis

Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis
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   DECEMBER   2002  Restoration Ecology Vol. 10 No. 4, pp. 703–713  703  © 2002 Society for Ecological Restoration  Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis  Carla D’Antonio   1,3,4  Laura A. Meyerson   2,5  Abstract  Exotic species have become increasingly significantmanagement problems in parks and reserves and fre-quently complicate restoration projects. At the sametime there may be circumstances in which their re-moval may have unforeseen negative consequences ortheir use in restoration is desirable. We review thetypes of effects exotic species may have that are im-portant during restoration and suggest research thatcould increase our ability to set realistic managementgoals. Their control and use may be controversial;therefore we advocate consideration of exotic speciesin the greater context of community structure and suc-cession and emphasize areas where ecological re-search could bring insight to management dilemmassurrounding exotic species and restoration. For exam-ple, an understanding of the potential transience ofexotics in a site and the role particular exotics mightplay in changing processes that influence the courseof succession is essential to setting removal prioritiesand realistic management goals. Likewise, a greaterunderstanding of the ecological role of introducedspecies might help to reduce controversy surroundingtheir purposeful use in restoration. Here we link gen-eralizations emerging from the invasion ecology liter-ature with practical restoration concerns, includingcircumstances when it is practical to use exotic speciesin restoration.  Key words: alien species, biological invasions, com-munity structure, disturbance, ecological resistance,invasibility, non-indigenous species, seed banks.  Introduction  ver the past two decades invasive non-native (here-after, exotic) organisms have come to be recog-nized as one of the most serious ongoing causes of spe-cies declines and native habitat degradation (Vitouseket al. 1997; Wilcove et al. 1998). For managers of parksand reserves exotic species are an ongoing threat to thepersistence of native assemblages because they can con-sume native species, infect them with diseases to whichthey have no resistance, outcompete them, or alter eco-system functions, making it difficult and expensive toreturn the ecosystem to its prior, often more desirable,condition (Vitousek et al. 1997). A major goal of restora-tion practitioners is to return a habitat to a more desir-able condition involving a particular species composi-tion, community structure, and/or set of ecosystemfunctions (Noss 1990).Invasive exotic species may play a role in the restora-tion process in the following ways. First, their presenceor dominance at the site may be part of the conditionleading to the assessment that restoration is needed. Inthe best-case scenario restoration may be as simple asremoving founding individuals of an exotic species.Second, exotic species may be the first species to recolo-nize after disturbances associated with removal. Third,exotic species may be the first to colonize after a planneddisturbance (powerline right-of-way, pipeline corridor,etc.) even if they were not present in the pre-distur- bance community and may interfere with restorationefforts or alter successional processes that would other-wise lead to a native assemblage. Fourth, they may leave behind a legacy after removal that makes long-term res-toration of the site difficult or challenges managementgoals. This legacy may be in the form of a buried seed bank or chemical or physical alteration to the habitat.Finally, exotic species may be used by managers to re-store particular functions if native species are not avail-able. This latter situation is prevalent in reclamationprojects where site conditions are badly degraded andnative species may not be able to survive or cannot de-liver the desired functions.Here we consider ways in which exotic species caninfluence ecological restoration and some of the issuessurrounding their occurrence and use. A large literature   1  Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, U.S.A.   2  Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912-9127, U.S.A.   3  Present address: Exotic and Invasive Weeds Research Unit, USDA-ARS, 920 Valley Road, Reno, Nevada 89512.   4  Address correspondence to C. D’Antonio, e-mail:   5  Present address: U.S. EPA National Center for Environmen-tal Assessment, 1200 Pennsylvania Ave., Washington, DC.  O   Exotic Plant Species and Restoration  704  Restoration Ecology  DECEMBER   2002  now exists on the ecology of plant invasions, but this lit-erature has not been well integrated into restorationand management. Thus, one of our goals is to begin tolink emerging generalizations in the field of plant inva-sions with restoration issues. We view this as part of a broader dialogue needed to begin developing strongerprinciples for restoration practitioners. Rather than ex-haustively reviewing the literature, we aim to raise im-portant issues about exotic plants in restoration settingsto stimulate further research and discourse. We focuson the role of plant invaders because plants are gener-ally an important part of restoration projects even if thetarget goal is an animal population.  Disturbance, Succession, and Invasion  Despite our increased acceptance of disturbance andchange as fundamental to all ecological systems, thepractical reality of restoration and management (i.e.,limited resources, short funding cycles) has requiredproducing particular target conditions within a shorttime and maintaining them for long periods. How doexotic species fit into our understanding of disturbance,succession, and ultimately restoration? Are there waysin which managers can anticipate which species will re-spond to disturbance and how long the responding spe-cies will persist in a community? A question fundamen-tal to restoration practitioners is this: Can aspects of planned disturbance or exotic species removal be ma-nipulated so as to maximize the likelihood that poten-tially undesirable species will not be a long-term part of the vegetation? Similarly, an understanding of the po-tential transience of exotics in a site and the role theymight play in changing processes that influence thecourse of succession is essential to setting priorities andrealistic goals.Both “natural” and direct human disturbances areknown to promote invasive exotic species (Hobbs &Huenneke 1992; Lozon & MacIsaac 1997; D’Antonio etal. 1999). Natural disturbances that promote invasioninclude small-scale ones such as rodent activity (e.g.,Peart 1989; D’Antonio 1993; Schiffman 1994) and largerscale ones such as fire (e.g., Richardson 1988; Richard-son et al. 1990), floods, and insect outbreaks (e.g., Peart1989; Maron & Connors 1996). In some situations exoticspecies are short-lived and successional to native spe-cies (see examples in D’Antonio et al. 1999). If this can be anticipated, then little money needs to be spent ontheir removal in the post-disturbance environment. Bycontrast, many exotic species are long-lived plants orpersistent annuals that set up feedbacks that perpetuatetheir own persistence (e.g., cheatgrass/fire cycles in theGreat Basin deserts; Whisenant 1990). Their invasionrepresents a long-term alteration in the successional tra- jectory of a site. Many of these exotic species defy sim-ple life history classification because they are both goodcolonizers after disturbance and persistent communitymembers (see Cronk & Fuller 1995). Their control should be a top management priority, as should research aimedat understanding the system attributes that promotetheir invasion or are altered by them as they establish.There are several reasons why disturbances will pro-mote invasive exotic species in plant communities, andan understanding of these may provide insight intomanagement options. Physical disruption of the soilsurface and exposure of soil to light and greater temper-ature fluctuation can increase nitrogen mineralization.Elevated resource levels should favor fast-growing spe-cies and can lead to their invasion or increased domi-nance (e.g., Huenneke et al. 1990; Maron & Connors1996). Several exotic species that take advantage of resource-rich post-disturbance environments are capa- ble of excluding native species for many years (e.g.,Hughes & Vitousek 1993; Busch 1995; Maron & Con-nors 1996). Disturbance often results in high soil nitratepools (e.g., Christensen 1973; Dahlgren & Driscoll 1994;Tardiff & Stanford 1998), which can directly increasethe germination of weed seeds (Baskin & Baskin 1998).Invasive non-native species often have very large per-sistent seed banks (Newsome & Noble 1986; Lonsdaleet al. 1988; Baskin & Baskin 1998). Indeed they oftenmaintain a much larger seed bank in their new homethan where they are native (Noble 1989). Hence, even if they were uncommon in the pre-disturbance vegeta-tion, their seeds may have accumulated in the soil overtime. For example, Drake (1998) found that exotic spe-cies made up 67% of the soil seed bank in an undis-turbed Hawaiian woodland even though they com-prised less than 12% of the aboveground cover and lessthan 5% of the annual seed rain. Altered environmentalconditions associated with disturbance, including ele-vated soil nitrate, and increased light and temperaturefluctuations should promote seed germination formany species, including exotics (Baskin & Baskin 1998).Several investigators have shown that species composi-tion after disturbance is somewhat predictable from thepre-disturbance seed bank (e.g., Smith & Kadlec 1985;van der Valk & Pederson 1989). Hence, in the case of planned disturbance, sampling of the pre-disturbanceseed bank can provide insight into whether exoticsmight become abundant at a site. Manipulation of fac-tors that stimulate germination may be a means of dis-couraging exotic species.Aspects of the disturbance regime can be manipu-lated to try to promote native over exotic species. Forexample, if it is necessary to disturb an area of vegeta-tion where most of the native species are fire tolerant,piling and burning the removed plant material on sitemay stimulate regeneration of native species from thesoil seed bank while decreasing the seed bank of fire in-   Exotic Plant Species and Restoration  DECEMBER   2002  Restoration Ecology  705  tolerant exotics. Fire has been successfully used to reducethe seed bank of the exotic forb Centaurea solsticialis  (yellow star thistle) in California (Hastings & DiTomaso1996). However, even within fire-tolerant vegetationfire can promote exotic species. For example, on Van-denberg Air Force Base in central California controlled burning of maritime chaparral was conducted to en-courage regeneration of declining endemic shrub spe-cies. One site with a listed rare plant became heavily in-vaded by the South African succulent Carpobrotus edulis  (Hottentot fig) after burning (Zedler & Scheid 1988). Itis now known that seeds of C. edulis  are abundant insoils throughout this region, and although they do notsurvive fire well, discontinuities in fuel distribution cre-ate microsites where they do not experience killing tem-peratures during controlled burns (D’Antonio et al.1993). In addition, locally abundant deer transport via- ble C. edulis  seed to burned areas, and soil conditions af-ter fire are highly conducive to growth of C. edulis  seed-lings (D’Antonio et al. 1993). These problems were notrecognized until well after fire had been used to man-age chaparral on the Base and C. edulis   became abun-dant in all burns conducted during the 1980s (Hickson1988). Although it can be an aggressive competitoragainst native species (D’Antonio & Mahall 1991), C.edulis  is relatively easy to control if plants are removedwhen young. In addition, manipulation of the distribu-tion of fuel before fire could help eliminate those micro-sites where seeds are otherwise not experiencing lethaltemperatures. An important lesson from this is thatmanagers must be broad thinking when consideringpossible responses to their planned activities and mustrespond quickly to surprises.  Removal of Exotics in Nature Reserves as aRestoration Practice  Managers of many reserves estimate they spend anenormous amount of their annual operating budget oncontrol of non-indigenous species. For example, at Ha-waii Volcanoes National Park, Resources Managementdirector Tim Tunison estimates that 80% of their annual budget is spent controlling exotic species. Likewise, atGolden Gate National Recreation Area and Point ReyesNational Seashore, two California parks within a Medi-terranean climate region, over 60% of the ResourcesManagement budget is spent controlling exotic species(S. Farrell, GGNRA, 1999, personal communication).More broadly, exotics pose a serious threat in at least194 of the 368 National Park Units in the United States(NPS 1997). System-wide management plans called formore than 535 species to be managed between 1996 and2000 at a cost of more than $80 million dollars, but ac-tual funding for that period allowed less than 10% of the projects to be conducted ($8 million available) (NPS1997). In 1998 for those plants and animals listed underthe Endangered Species Act, management costs rangedfrom $32 to $42 million annually, 90% of which was dueto invasive species (Wilcove & Chen 1998).Because funding for invasive species management ef-forts is typically limited, it is essential to prioritize thosespecies and populations that are most important to con-trol. Prioritization should be based on potential impactsof invaders and potential for control. Yet determiningthe potential or real impact of an exotic species is diffi-cult, particularly if the species is new to a region andhas not been well studied in similar environments.Also, pre-invasion baseline data describing the ecosys-tem may be unavailable (Parker et al. 1999). Lag timesassociated with population explosions of invasive spe-cies (Hobbs & Humphries 1995; Kowarik 1995; Crooks& Soule 1999) also complicate assessment.  Melaleucaquinquenervia  (melaleuca) and Schinus terebinthofolius  (Brazilian pepper) were introduced to Florida in the late1800s (Ewel 1986), but their populations did not beginto “explode” until the 1950s. Under these types of sce-narios exotic species currently colonizing a natural areacould potentially be disregarded until populations haveexploded and removal has become more difficult. Thisis especially likely to occur during the early stages of aninvasion in vast natural areas that are not heavily moni-tored (Crooks & Soule 1999). Hence, it seems vital thatmanagers of reserves are alert for potential threatswithin their region and that vigilant monitoring andrapid response teams are a part of management plans.Management plans that include removal of harmfulexotic organisms are often not linked to a post-removalrevegetation plan. Yet the two should go hand in hand because removal frequently results in soil disturbanceand subsequent regeneration by the same or other un-wanted exotic species. For example, Luken and Mat-timiro (1991) found that removal of Lonicera maackii  (Amur honeysuckle) from temperate forest understoryenvironments stimulated resprouting from basal stemsand honeysuckle reestablishment from seed. The extentto which post-removal revegetation is needed dependson the biology of the invader, the spatial extent of theinvasion, and the length of time the invader has beenpresent. The longer the time the invader has beenpresent, the more it might have contributed to the seed bank, disrupted the input of natives to the seed bank,and affected the soil so that restoration might not be easy(see Legacies under Removing Exotics: Approaches,Concerns, and Legacies, below). For example, Holmesand Cowling (1997a,b) found a stronger decline in na-tive species richness, diversity, and abundance and anincrease in the abundance of exotics in the seed bank of historically invaded compared with recently invadedfynbos vegetation in the Cape region of South Africa.   Exotic Plant Species and Restoration  706  Restoration Ecology  DECEMBER   2002  An important consideration when planning a restora-tion project is the condition of the surrounding region.In many countries fast-growing human populations atthe edges of reserves and natural areas, in “buffer zones,”and inside reserves themselves put increasing pressureson the reserve’s biological resources (e.g., Meyerson1998). Pressures include human traffic through the re-serve, which increases the likelihood of exotic propagulescoming in and the amount of disturbed land for theirestablishment; human harvesting of species within re-serve boundaries; increasing isolation of the reservefrom other “intact habitats”; and increasing fragmenta-tion of the reserve itself. Sites with a history of humanimpact and/or sites adjacent to long-impacted areashave been shown to have a high representation of exoticspecies in their soil seed banks (reviewed in Luken1997). Even natural areas without adjacent populationpressures are subject to introduction of invasive “hitch-hikers” both on animals that cross park boundaries and by human visitors (e.g., Chaloupka & Domm 1986;Hobbs & Huenneke 1992; Lonsdale & Lane 1994). Allthese pressures will affect the success of restoration.Nonetheless, it may be useful to set restoration goals based on where reserves fall on an urban to rural gradi-ent because reserves adjacent to urban areas are likelyto be heavily affected by exotic species and control oreradication of exotics may not be a financially viableoption.  Management of Exotics after Planned Disturbance  Predicting species responses to disturbance has been amajor focus of ecological research. Thus there is a basicliterature that should be useful for predicting how bothnative and exotic species should respond to planneddisturbances. Planned disturbances typically are differ-ent from historic disturbances. Perhaps the most signifi-cant difference is that planned disturbances often dis-rupt the soil profile over a large scale. Examples of disturbances that have been left to “revegetate” natu-rally are abundant on our landscape. In some cases,such as the eastern deciduous forests of the UnitedStates, natural regeneration after farmland abandon-ment has led to the widespread occurrence of foreststhat are largely similar (although younger) in structureto pre-disturbance forests. By contrast, in other settingsanthropogenic disturbance has led to persistent weedycommunities that in no way resemble those native to aregion (e.g., Conn et al. 1984; Brandt & Rickard 1994;Stylinski & Allen 1999). The problem seems particularlysevere in arid and semiarid habitats or highly enrichedfarmlands. Managers in Everglades National Park foundthat to restore native plant communities on a formerfarmland park in-holding, the highly altered enrichedagricultural soils had to be removed (J. Ewel, 1999, per-sonal communication). The environmental conditionsunder which severe anthropogenic disturbances reveg-etate readily with native species need to be exploredsystematically.Planned disturbances are now often accompanied byrevegetation plans that include minimizing disturbanceto the soil profile and reconstruction of the soil profileafter disturbance. Topsoil is frequently collected andstored, to be replaced at the surface during revegeta-tion. Although this practice is important for restoringthe native seed bank and soil microbial community,there have been few studies of storage duration. It isknown that the seed bank of native species declinesover time when new seed input is disrupted (e.g.,Holmes & Cowling 1997a,b; van der Valk & Pederson1989). Hence, the longer the time topsoil is stockpiled,the more likely it is that viable seeds of native specieswill decline. Because many harmful exotic species havepersistent seed banks (e.g., Holmes 1988; Lunt 1990;Baskin & Baskin 1998; Drake 1998), it is likely that theproportional representation of exotics in the seed bankwill increase with time in storage.Can planned disturbances be designed to have nega-tive effects on exotic species? Because disturbances typ-ically increase resource levels and stimulate germina-tion, it seems unlikely they can be planned to eliminateall exotics. Yet in circumstances where native and exoticspecies at a site are adapted to different types of distur- bance, it may be possible to select against particular ex-otic species during restoration. The success of such ap-proaches will be site specific, and practitioners shouldgather detailed information on conditions associatedwith their disturbances (e.g., fire temperature, moisturecondition of the fuel bed) to gain a mechanistic under-standing of successes and failures.  Removing Exotics: Approaches, Concerns, and Legacies  A variety of techniques can be used to remove exoticspecies from reserves or restoration sites. These mostcommonly include hand removal, mechanical removal,herbicides, fire, or some combination of the above (e.g.,Masters & Nissen 1998). Biological control is practicedprimarily in rangeland or agricultural settings and hasnot been used greatly in habitat restoration, perhapswith the exception of Lythrum salicaria  (purple loosestrife).The technique used in a given site depends on both the biology of the species and socioeconomic, political, andcultural factors. For example, fire is used to control Gen-ista monspessulana  (french broom) in areas of GoldenGate National Recreation Area (California) that are dis-tant from houses. In other sectors of the same parkcloser to urban lands, Genista  is controlled using handor mechanical removal. Herbicides are not used be-cause of the scale of the invasions, proximity to hous-  Exotic Plant Species and Restoration  DECEMBER   2002  Restoration Ecology  707  ing, and environmental considerations (S. Farrell 1999,personal communication).Selective manipulation of soil fertility might allow forcontrol of some undesired species. Although applica-tion to natural areas may be difficult, this approach ispotentially useful in a restoration project where the par-ticular nutrient requirements of an invader are known.Where high N-demanding exotic species are present,several investigators have suggested the addition of sawdust or a carbon “cocktail” to decrease soil-avail-able N (Wilson & Gerry 1995; Arthur & Wang 1999;Reever-Morghan & Seastedt 1999). The underlying rea-soning behind this idea is that labile C will stimulatemicrobial population growth and that increased micro- bial populations will then immobilize soil N. The result-ing lower soil N will differentially affect the faster grow-ing more N-demanding plant species, decreasing theircompetitive advantage over native species for at least a brief window of time. Wilson and Gerry (1995) andReever-Morghan and Seastedt (1999) demonstrated thatadding carbon to soil can decrease standing mineral Npools, but neither study was able to demonstrate thatthis affected interactions among native and exotic spe-cies. The efficacy of this approach is currently question-able (Corbin et al. in press).Biological control is currently being explored for sev-eral exotic plant species that affect wildland habitatvalue (for a review see DeLoach 1997). To date, thereare very few examples of its use in wildland manage-ment except for control of rangeland and aquatic weeds.Sometimes reserves benefit from biocontrol agents re-leased on weeds in nearby agricultural settings. For ex-ample, biocontrol agents were shown to decrease theabundance of the noxious weed Senecio jacobaea  (tansyragwort) in Redwoods National Park, but the agentshad been released in the region for control of S. jacobaea  on rangelands and not in the Park. Release of biocontrolagents in U.S. Parks and reserves has been controver-sial. Advocates of weed biological control contend thereis little evidence that control agents cause unexpecteddamage. However, Simberloff and Stiling (1996) pointedout that only a small percentage of all biocontrol re-leases have been carefully studied. Rather than arguingto abandon biological control, they insist that controlagents not be pronounced safe until research supportsthis conclusion (Simberloff & Stiling 1996). We advocategreater research in this area.Monitoring and maintenance after invasive speciesremoval efforts and subsequent restoration is essential but is often overlooked. Species with buried seed banks,such as  Alliaria petiolata  (garlic mustard) or Genista mon-spessulana  and Cytissus scoparius  (scotch broom), or ex-tensive and persistent rhizomatous networks, such as  Phragmites australis  (common reed), require repeatedfollow-up treatment, sometimes for several years. Al-though notoriously difficult to fund, continued mainte-nance may save time and resources over the long term.For example, the cost of initial treatment on a site in-vaded by Tamarix  or retreatment of an invaded site notmaintained can run $675 an acre in the first year (1997constant dollars, Lake Mead National Recreation Area,cited in Wilcove & Chen 1998). However, these costsdrop to $10 an acre in the second year and to less than$10 an acre every 2 to 3 years thereafter (Wilcove &Chen 1998). In other cases it may be that regular main-tenance of a restored site does not make financial sense.Long-term management costs should be considered ona case by case basis.  Legacies  Restoration of a site colonized by an invasive speciescan present a unique challenge because some speciescan continue to affect a system after their removal, pre-venting attainment of the desired restoration outcome(Cronk & Fuller 1995). Several researchers have foundthat exotic plant species are capable of controlling par-ticular aspects of ecosystem biogeochemistry. For ex-ample, the invading actinorrhizal N fixer  Myrica faya  (Canary Islands faya tree) has colonized young volcanicsoils in Hawaii (Vitousek et al. 1987; Vitousek & Walker1989) where it can fix N at a rate four times as high asall other sources of fixation combined. Currently, it is being killed by an introduced leafhopper (  Sophonia rufo- fascia  ), leaving behind a legacy of high soil N. Introducedperennial grasses that pose high fuel danger appear to be benefiting from this die-off, complicating efforts atrestoration (Adler et al. 1998). Similarly,  Acacia  spp. inSouth Africa have been shown to fix large amounts of N, and soil N has increased significantly after their in-vasion (Witkowski 1991; Stock et al. 1995). Kourtev etal. (1999) reported a higher density of European earth-worms in the northeastern United States beneath theexotic species  Microstegium vimineum  (Japanese stilt-grass) and Berberis thunbergii  (Japanese barberry) andan associated increase in N-mineralization and nitrifica-tion, as well as decreased litter thickness. These factorscombine to create very different soil conditions thanwould occur if only native species were present. The re-versibility of these conditions and their impacts on res-toration warrant further study.In addition to altering N availability, exotic speciescan alter soil salinity (Vivrette & Muller 1977). These ef-fects can persist long after the species have been re-moved and can make it difficult for native species to re-colonize an area. For example, Vivrette and Muller(1977) found that the annual South African species   Mesembryanthemum crystallinum  (crystalline iceplant)concentrates salt from throughout the soil profile intotrichomes on the leaf surface. Each summer after the
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