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WESTERN CORN ROOTWORM Ecology and Management prelims00.qxd 15/11/04 2:16 PM Page ii prelims00.qxd 15/11/04 2:16 PM Page iii
WESTERN CORN ROOTWORM Ecology and Management
Edited by Stefan Vidal
Georg-August University, Göttingen, Germany Ulrich Kuhlmann
CABI Bioscience, Delémont, Switzerland
and C. Richard Edwards
Purdue University, Indiana, USA
CABI Publishing prelims00.qxd 15/11/04 2:16 PM Page iv
CABI Publishing is a division of CAB International
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Library of Congress Cataloging-in-Publication Data Western corn rootworm : ecology and management / edited by Stefan Vidal, Ulrich Kuhlmann, C. Richard Edwards. p. cm Includes index ISBN 0-85199-817-8 (alk. paper) 1. Western corn rootworm--Congresses. I. Vidal, Stefan. II. Kuhlmann, Ulrich, 1964- III. Edwards, C. Richard.
SB945.W53W44 2005 633.1′5976′48--dc22 2004012843
ISBN 0 85199 817 8
Typeset by MRM Graphics Ltd, Winslow, Bucks Printed and bound in the UK by Biddles Ltd, King’s Lynn. prelims00.qxd 15/11/04 2:16 PM Page v
Contents
Contributors ix
Preface xiii
1 Invasive Alien Species – a Threat to Global Biodiversity and Opportunities to Prevent and Manage Them 1 Rüdiger Wittenberg
2 Monitoring of Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) in Europe 1992–2003 29 József Kiss, C. Richard Edwards, Harald K. Berger, Peter Cate, Mirela Cean, Sharon Cheek, Jacques Derron, Husnija Festic, Lorenzo Furlan, Jasminka Igrc-Barc˘ic´, Ivanka Ivanova, Wiebe Lammers, Victor Omelyuta, Gabor Princzinger, Philippe Reynaud, Ivan Sivcev, Peter Sivicek, Gregor Urek, Otmar Vahala
3 A Synopsis of the Nutritional Ecology of Larvae and Adults of Diabrotica virgifera virgifera (LeConte) in the New and Old World – Nouvelle Cuisine for the Invasive Maize Pest Diabrotica virgifera virgifera in Europe? 41 Joachim Moeser and Bruce E. Hibbard
4 Western Corn Rootworm, Cucurbits and Curcurbitacins 67 Douglas W. Tallamy, Bruce E. Hibbard, Thomas L. Clark and Joseph J. Gillespie
5 Natural Mortality Factors Acting on Western Corn Rootworm Populations: a Comparison between the United States and Central Europe 95 Stefan Toepfer and Ulrich Kuhlmann
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6 Movement, Dispersal and Behaviour of Western Corn Rootworm Adults in Rotated Maize and Soybean Fields 121 Joseph L. Spencer, Eli Levine, Scott A. Isard and Timothy R. Mabry
7 Within-field Spatial Variation of Northern Corn Rootworm Distributions 145 Michael M. Ellsbury, Sharon A. Clay, David E. Clay and Douglas D. Malo
8 Heterogeneous Landscapes and Variable Behaviour: Modelling Rootworm Evolution and Geographic Spread 155 David W. Onstad, Charles A. Guse and Dave W. Crowder
9 Sampling Devices and Decision Rule Development for Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) Adults in Soybean to Predict Subsequent Damage to Maize in Indiana 169 Corey K. Gerber, C. Richard Edwards, Larry W. Bledsoe, John L. Obermeyer, Gyorgy Barna and Ricky E. Foster
10 Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) and the Crop Rotation Systems in Europe 189 József Kiss, Judit Komáromi, Khosbayar Bayar, C. Richard Edwards and Ibolya Hatala-Zsellér
11 Application of the Areawide Concept Using Semiochemical- based Insecticide Baits for Managing the Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) Variant in the Eastern Midwest 221 Corey K. Gerber, C. Richard Edwards, Larry W. Bledsoe, Michael E. Gray, Kevin L. Steffey and Laurence D. Chandler
12 Genetically Enhanced Maize as a Potential Management Option for Corn Rootworm: YieldGard® Rootworm Maize Case Study 239 Dennis P. Ward, Todd A. DeGooyer, Ty T. Vaughn, Graham P. Head, Michael J. McKee, James D. Astwood and Jay C. Pershing
13 Is Classical Biological Control against Western Corn Rootworm in Europe a Potential Sustainable Management Strategy? 263 Ulrich Kuhlmann, Stefan Toepfer and Feng Zhang prelims00.qxd 15/11/04 2:16 PM Page vii
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14 Maize Growing, Maize High-risk Areas and Potential Yield Losses due to Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) Damage in Selected European Countries 285 Peter Baufeld and Siegfried Enzian
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Contributors
James D. Astwood, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Gyorgy Barna, Syngenta Seeds Kft., 1123 Budapest, Alkotas u. 41, Hungary Peter Baufeld, Federal Biological Research Centre for Agriculture and Forestry, Department for National and International Plant Health, Stahnsdorfer Damm 81, 14532 Kleinmachnow, Germany Khosbayar Bayar, Department of Plant Protection, Szent István University, 2100 Gödöllö, Hungary Harald K. Berger, Institute of Plant Health, Austrian Agency for Health and Food Safety, 1126 Vienna, Austria Larry W. Bledsoe, Department of Entomology, Purdue University, W. Lafayette, IN 47907-2089, USA Peter Cate, Institute of Plant Health, Austrian Agency for Health and Food Safety, 1126 Vienna, Austria Mirela Cean, Department of Entomology, Central Laboratory for Phytosanitary Quarantine, 077190 Bucharest, Romania Laurence D. Chandler, RRVARC, USDA-ARS, Fargo, ND 58105-5677, USA Sharon Cheek, Central Science Laboratory, Department for Food, Environment and Rural Affairs, Sand Hutton, York YO4 1LZ, UK Thomas L. Clark, USDA-ARS, 205 Curtis Hall, Department of Entomology, University of Missouri, Columbia, MO 65211-7020, USA David E. Clay, Plant Science Department, SD State University, Brookings, SD 57007, USA Sharon A. Clay, Plant Science Department, SD State University, Brookings, SD 57007, USA Dave W. Crowder, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL 61801, USA
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Todd A. DeGooyer, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Jaques Derron, Swiss Federal Research Station for Plant Production, 1260 Nyon, Switzerland C. Richard Edwards, Department of Entomology, Purdue University, W. Lafayette, IN 47907-2089, USA Michael M. Ellsbury, Northern Grain Insects Research Laboratory, USDA- ARS, Brookings, SD 57006, USA Siegfried Enzian, Federal Biological Research Centre for Agriculture and Forestry, Institute for Technology Assessment in Plant Protection, Stahnsdorfer Damm 81, 14532 Kleinmachnow, Germany Husnija Festic, Faculty of Agriculture, University of Sarajevo, 71000 Sarajevo, Bosnia-Herzegovina Ricky E. Foster, Department of Entomology, Purdue University, W. Lafayette, IN 47907-2089, USA Lorenzo Furlan, Department of Agronomy, Entomology, University of Padua, Legnaro, 2-35122 Padova, Italy Corey K. Gerber, Department of Entomology, Purdue University, W. Lafayette, IN 47907-2089, USA Joseph J. Gillespie, Holistic Insect Systematics Laboratory, Department of Entomology, Texas A&M University (TAMU), Rm 519, Minnie Belle Heep Bldg, 2475 TAMU, College Station, TX 77843-2475, USA Michael E. Gray, Department of Crop Science, University of Illinois, Urbana, IL 61801, USA Charles A. Guse, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL 61801, USA Ibolya Hatala-Zsellér, Csongrád County Plant and Soil Protection Service, PO Box 99, 6801 Hódmezövásárhely, Hungary Graham P. Head, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Bruce E. Hibbard, USDA-ARS, 205 Curtis Hall, University of Missouri, Columbia, MO 65211-7020, USA Jasminka Igrc-Barc˘ic´, Department of Agricultural Zoology, Faculty of Agriculture, University of Zagreb, 10000 Zagreb, Croatia Scott A. Isard, Department of Geography, 220 Davenport Hall, University of Illinois, Urbana, IL 61801, USA Ivanka Ivanova, Central Laboratory for Plant Quarantine, 1330 Sofia, Bulgaria József Kiss, Department of Plant Protection, Szent István University, 2100 Gödöllö, Hungary Judit Komáromi, Department of Plant Protection, Szent István University, 2100 Gödöllö, Hungary Ulrich Kuhlmann, CABI Bioscience Switzerland Centre, Rue des Grillons 1, 2800 Delémont, Switzerland Wiebe Lammers, Plant Protection Service, PO Box 9102, 6700 HC Wageningen, The Netherlands prelims00.qxd 15/11/04 2:16 PM Page xi
Contributors xi
Eli Levine, Center for Economic Entomology, Illinois Natural History Survey, 172 Natural Resources Building, 607 E. Peabody Drive, Champaign, IL 61820-6917, USA Timothy R. Mabry, Holden Foundation Seeds, 503 S. Maplewood Ave., Williamsburg, IA 52361, USA Douglas D. Malo, Plant Science Department, South Dakota State University, Brookings, SD 57007, USA Michael J. McKee, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Joachim Moeser, Institute for Plant Pathology and Plant Protection, Entomological Section, Georg-August University, Grisebachstrasse 6, 37077 Göttingen, Germany John L. Obermeyer, Department of Entomology, Purdue University, W. Lafayette, IN 47907-2089, USA Victor Omelyuta, Institute of Plant Protection of Ukraine Academy of Agrarian Sciences, 03022 Kiev, Ukraine David W. Onstad, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL 61801, USA Jay C. Pershing, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Gabor Princzinger, Department of Plant and Soil Protection, Ministry of Agriculture and Rural Development, 1860 Budapest, Hungary Philippe Reynaud, Entomology Unit, INRA, 34060 Montpellier, France Ivan Sivcev, Institute for Plant Protection and Environment, 11080 Zemun, Serbia Peter Sivicek, Central Control and Testing Institute of Agriculture, 84429 Bratislava, Slovakia Joseph L. Spencer, Center for Economic Entomology, Illinois Natural History Survey, 172 Natural Resources Building, 607 E. Peabody Drive, Champaign, IL 61820-6917, USA Kevin L. Steffey, Department of Crop Science, University of Illinois, Urbana, IL 61801, USA Douglas W. Tallamy, University of Delaware, Department of Entomology and Wildlife Ecology, 250 Townsend Hall, Newark, DE 19717, USA Stefan Toepfer, CABI Bioscience Switzerland Centre, Rue des Grillons 1, 2800 Delémont, Switzerland Gregor Urek, Agricultural Institute of Slovenia, 1001 Ljubljana, Slovenia Otmar Vahala, Regional Division of the State Phytosanitary Administration, 62800 Brno, Czech Republic Ty T. Vaughn, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA Stefan Vidal, Institute for Plant Pathology and Plant Protection, Entomological Section, Georg-August University, Grisebachstrasse 6, 37077 Göttingen, Germany Dennis P. Ward, Monsanto Company, 800 N. Lindbergh Blvd, St Louis, MO 63167, USA prelims00.qxd 15/11/04 2:16 PM Page xii
xii Contributors
Rüdiger Wittenberg, CABI Bioscience Switzerland Centre, Rue des Grillons 1, 2800 Delémont, Switzerland Feng Zhang, CABI Bioscience Switzerland Centre, Rue des Grillons 1, 2800 Delémont, Switzerland prelims00.qxd 15/11/04 2:16 PM Page xiii
Preface
Since its first discovery in Europe in 1992 near Belgrade, Serbia, the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, which is often referred to as the most import pest insect of maize in the USA, has spread to more than 15 Eastern and Central European countries (Chapter 2). Increasing public and governmental concerns within the European Union regarding the impact of this pest on maize production has resulted in funding directed at determining potential management options for European countries. In 2000, the European Union funded a multi-country project on WCR Ecology and Management in Europe (QLRT-1999-01110). At the conclusion of this project, an international symposium was organized in Göttingen, Germany, in spring 2003. This symposium brought together WCR scientists and experts from Europe, and North and South America. Papers were presented on the current state of knowledge of the diabroticine beetles, including such topics as distribution, behaviour, natural enemies and management options. These papers and the discussions that followed formed the basis for this book. The 1986 J.L. Krysan and T.A. Miller book, Methods for the Study of Pest Diabrotica, is not necessarily considered comprehensive in regard to some of the recent issues in Diabrotica research. There is much to be learned from this invasive pest species, and recent research on WCR has added insight into evolutionary aspects of pest species, adaptation to changing environments, as well as to management options to counterbalance these adaptations. As such, this symposium-based book has been published with the aim of updating knowledge on the ecology and management of Diabrotica, which is equally relevant and applicable for management deci- sions in North America and Europe. Concepts and management options related to WCR are reviewed in this book to classify the invasion of Europe by WCR, and to place it into the context of best prevention and management practices of invasive alien species (Chapter 1). Specific management options for WCR should be adapted based on their behaviour in the field. The implications for
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alternative host plant use by WCR in Europe are numerous, and under- standing the nutritional ecology of this species (Chapter 3) and the phy- logenetic background of host plant use in different plant families (Chapter 4) may explain the rapid establishment of WCR in Europe. Detailed life table analyses (Chapter 5), carried out in Hungary, allow for the evalua- tion of the combined impact of mortality factors that occur naturally and that may serve as regulating factors of WCR in Europe. Since no compre- hensive information was available on natural enemies occurring in the area of origin of diabroticine beetles in Central America, a review of clas- sical biological control possibilities for WCR through the introduction of specialized natural enemies is provided (Chapter 13). Recently, WCR has changed its egg-laying behaviour in some regions in the USA due to intense selection pressure and adaptation to annual crop rotation systems, particularly in soybean and maize. On the other hand, the northern corn rootworm (NCR), Diabrotica barberi Smith and Lawrence, has adapted to this selection pressure by extending the dia- pause period in some individuals. The different adaptations of these two Diabrotica species, together with reports of insecticide-resistant popula- tions in some areas of the Midwestern USA, point to the importance of additional or alternative management options for the diabroticine beetles. Background on the phenomenon of egg laying in a non-host crop, such as soybean, has been elaborated in detail in Chapter 6. To understand the adaptation of Diabrotica to crop rotation, a model was developed (Chapter 8) that explains such phenomena in areas of the Midwestern USA. A field experiment designed to evaluate the possibilities of adapta- tion of WCR to crop rotation systems in Europe and the conclusions drawn from this experiment are outlined in Chapter 10. Diabroticine beetles are not evenly distributed in fields. The aggrega- tion of beetles makes sampling and quantification of populations within a field difficult. An evaluation of the spatial distribution of NCR larvae in the field (Chapter 7) and an evaluation of sampling devices and decision rules to predict subsequent maize damage (Chapter 9) both serve to clarify possible management strategies. The areawide management approach (Chapter 11) and the implementation of genetically modified maize culti- vars expressing a specific toxin targeted against WCR are both additional management options which could be implemented in Europe to keep WCR populations below threshold levels. Finally, as maize is grown less intensively and is more scattered in Europe compared to the USA, a quan- tification of high-risk areas prone to WCR damage may help to focus man- agement options within Europe. This book would not have been published without the continuous interest and help of Tim Hardwick, CABI Publishing. We therefore owe our thanks to him and his efforts.
Stefan Vidal, Ulrich Kuhlmann and C. Richard Edwards Göttingen, Germany; Delémont, Switzerland; and W. Lafayette, Indiana, USA – April 2004 chap01.qxd 12/11/04 10:56 AM Page 1
1 Invasive Alien Species – a Threat to Global Biodiversity and Opportunities to Prevent and Manage Them
Rüdiger Wittenberg CABI Bioscience Centre Switzerland, Delémont, Switzerland
Introduction
Changes in distribution of species are a natural phenomenon; ranges expand and retract and species colonize new areas outside their natural range by long-distance dispersal, for example reptiles on floating wood to new islands. However, these events are rare and restricted by natural bar- riers. The rather recent globalization of trade and travel has inadvertently led to the increased transport of organisms and introduction of alien species, tearing down these natural barriers. Alien species are not bad per se, in fact, many species are used for human consumption, e.g. most crop species are grown as aliens. Some of them may become harmful and pose threats to the environment and human populations, as will be discussed below. These so-called invasive alien species (IAS) are increasingly rec- ognized as one of the major threats to biodiversity. This global problem needs global reaction or, even better, proactive measures and solutions. Thus the Global Invasive Species Programme (GISP) deals with IAS to find the best practices to prevent and manage IAS in response to the undertaking in the Convention on Biological Diversity (Article 8h: ‘Each Contracting Party shall, as far as possible and as appropriate: Prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species’). This chapter will give a brief summary of the current knowledge on IAS, what IAS are, what their perceived impact is, how they travel to new places and what their characteristics are, and will focus on what can be done to prevent and manage IAS. The latter is based on best prevention and management practices derived from successful case studies. However, the chapter cannot cover all knowledge and options; this would not fit into an entire book, so only an overview and selected highlights can be covered. The complexity of the topic ‘IAS’ stems from the very different species involved, their diverse origin, the
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variety of pathways used, their varied impact on the new environment, their relationships with indigenous and other alien species, the ecosys- tem changes caused, their dependence on other factors such as global warming, their human dimensions, including the change in political and ethical views, and their ongoing evolution.
Historical Example
Besides habitat loss due to human destruction of natural ecosystems, IAS are the single most important threat to biodiversity. These negative effects are best documented in bird extinctions on islands where the majority of the bird extinctions since 1800 were caused by IAS (BirdLife International, 2000). Island birds often did not evolve with mammalian predators lacking natural defences to introduced predators. Therefore, it is of great concern that a quarter of all globally threatened bird species are currently affected by alien predators. The story of the Stephen Island wren (Xenicus lyalli (Rothschild)), which belonged to the family of the New Zealand wrens or Acanthosittidae, will serve as an example to these abstract figures. This is a very old bird family at the base of passerine radi- ation with only two remaining species (Chambers, 1989; Sibley and Monroe, 1990). The Stephen Island wren occurred only on Stephen Island, a small offshore island of New Zealand. Its demise started with the building of a lighthouse. One fine day the domestic cat (Felis catus (L.)) of the lighthouse keeper, which he kept as a companion, brought home a dead bird, which he did not recognize. So he gave it to ornithologists on the mainland and they confirmed that it was a species new to science (Quammen, 1996). While the lighthouse keeper, Mr Lyall, lent his name to the newly described species, his cat continued to feast on it until it was never found again. It is still unclear whether the Stephen Island wren was flightless or had some rudimentary flight ability – this knowledge was taken to the grave by the cat, the only one who would know. The light- house keeper saw the species only twice, running like a mouse in the evening in 1894 (Falla et al., 1993). Thus in this clear and special case it was only one specimen of an IAS that caused the extinction of a rare bird species. In most other cases it is not that obvious and it is more probably a combination of factors leading to extinction of a species rather than being monocausal. However, one subspecies of the bush wren (Xenicus longipes (Gmelin)), the other extinct species of the family, was restricted to Steward Island and some outliers. The demise of that subspecies is directly attributable to the arrival of the ship rat (Rattus rattus (Linnaeus)) and mustelids (Heather and Robertson, 1996). It is interesting to note that the four species of the New Zealand wren family were not adapted to mammalian predators in their natural environment and the two species extinct were ground dwellers with at most weak flight abilities, one in the forests and one on a small island, whereas the other two species live in more protected niches, one species in high alpine fields and the other chap01.qxd 12/11/04 10:56 AM Page 3
Invasive Alien Species 3
being a canopy species with good flight ability. This little example of the fate of four closely related species indicates a more general phenomenon. Small island populations without adaptation to mam- malian predators and breeding on or near the ground are most vulnerable to intentional or accidental introductions of alien opportunistic preda- tors.
Who Is Who
Who and what are IAS? They are species introduced by human interven- tion, either intentionally or accidentally, into a new environment, a new area, where they are able to adapt and thrive, causing negative impacts on the native ecosystem, species or humans. It needs to be stressed, as men- tioned above, that not all alien species cause problems. In fact, most of the crop species used worldwide are grown as non-indigenous species, for example the wide use of American maize (Zea mays L.) in Europe and Africa. Bearing in mind that creating lists of native and introduced species and ascertaining the impact of IAS are not flawless, as a rule of thumb the tens rule can be employed; this states that 10% of introduced species are able to establish in their new environment and 10% of those established become invasive (Williamson, 1996). IAS are found in virtually every taxonomic group. The following examples will attest this statement. The West Nile virus causing encephalitis hitched a ride to the New World in an infected bird, mos- quito or human (Enserink, 1999). The bacterium Vibrio cholerae, the causal organism of the human disease cholera, is a member of brackish water communities and is frequently found in ballast water of ships (McCarthy and Khambaty, 1994), by which means some new highly viru- lent strains have been redistributed leading to epidemic outbreaks of cholera. Some fungal pathogens are amongst the IAS with the most dis- ruptive impact on ecosystems, a well-known example being fungi attack- ing trees, e.g. chestnut blight (Cryphonectria parasitica (Murrill) Barr) was introduced with alien chestnuts to North America, where it virtually eradicated the American chestnut (Castanea dentata (Marsh.) Borkh.), which was a dominant tree in eastern forests, thereby changing the entire ecosystem and composition of the forests (Hendrickson, 2002). Many marine phytoplankton species cause harmful algal blooms, in particular dinoflagellates, which can kill fish and especially sessile organisms and also produce potent toxins that can find their way to humans through consumption of seafood (Weidema, 2000). One of the worst invaders in the Mediterranean Sea is the macroalga Caulerpa taxifolia (Vahl) C. Agardh, which escaped from an aquarium and is replacing the native seaweed beds and thus altering large tracts of coastal ecosystems. Weeds are the predominant group of IAS known to cause economic problems as well as deleterious effects on the environment. The giant reed (Arundo donax L.) is used in many countries, for example, as wind-breaks and is chap01.qxd 12/11/04 10:56 AM Page 4
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readily invading natural areas, and the small herb common ragweed (Ambrosia artemisiifolia L.) is swiftly expanding its exotic range in Europe causing severe allergic problems for the human population. An almost global problem, disrupting, or at least altering, the natural succes- sion is bushes such as broom (Cytisus scoparius (L.) Link) and gorse (Ulex europaeus L.), introduced as ornamentals and soil-enhancing plants; another successful invader of natural forest is the vine old man’s beard (Clematis vitalba L.), which smothers remnant forest tracts in New Zealand, and finally trees such as pines (Pinus spp.) and eucalypts (Eucalyptus spp.) escape from their plantations and may replace native vegetation and deplete water sources (Van Wilgen et al., 1997). Many dif- ferent kinds of worms found their way to new areas with human assis- tance, especially parasitic worms from the Platyhelminthes and Nemathelminthes, and also in the marine environment, for example the polychaete Marenzelleria viridis (Verrill) introduced into the North and Baltic Seas (Zettler et al., 2002). A spectacular disaster caused by an intro- duced snail was the introduction of the carnivore rosy wolfsnail (Euglandina rosea (Férussac)) to many subtropical and tropical islands, destroying the diverse endemic snail faunas. Another example of an introduced mollusc in Europe is perhaps the most aggressive freshwater invader worldwide, the zebra mussel (Dreissena polymorpha (Pallas)), inflicting not only huge economic costs but also severe biotic changes as it functions as an ecosystem engineer species (Karateyev et al., 2002). Small introduced crustaceans dominate the fauna of many rivers and lakes worldwide due to the increased ship traffic transporting organisms in their ballast water to new areas and also due to the creation of canals connecting formerly insurmountable natural barriers between water- sheds. Thus, alien species (mainly crustaceans and molluscs) dominate the Rhine in total abundance and biomass by more than 80% (Haas et al., 2002). Interestingly, introduced insects, despite their diversity, have not shown a high potential for causing environmental problems, albeit they can be devastating pests in agriculture and forestry. However, several ant species destroy the native faunas, especially on islands – but also, for example, the Argentine ant (Linepithema humile (Mayr)) in southern Europe. The infamous cane toad (Bufo marinus (L.)) is quickly spreading over Australia feeding on everything smaller than itself and poisoning the bigger predators, such as quolls (Dasyurus spp. E. Geoffroy St-Hilaire). One of the most devastating introduced reptiles is the brown tree snake (Boiga irregularis (Merrem)) on Pacific islands; it arrived on Guam with military help and brought the silent spring to the island by feasting on the bird species. Moreover, it is causing frequent power cuts and is a danger to babies because of its venom. The Nile perch (Lates niloticus (L.)), intro- duced into Lake Victoria to improve fisheries, caused the extinction of more than 100 fish species of the cichlid family, most of them endemic to the lake – before the predator came it was called an evolutionary labora- tory, afterwards an ecological disaster. The American ruddy duck (Oxyura jamaicensis (Gmelin)) was introduced to England as an addition chap01.qxd 12/11/04 10:56 AM Page 5
Invasive Alien Species 5
to the wildfowl fauna, where it ostensibly does no harm, but then it spread to Spain where it readily endangers the native closely related white- headed duck (Oxyura leucocephala (Scopoli)) by hybridization. Feral mammals introduced to islands brought many bird species to the brink of extinction or beyond by feeding on their eggs and chicks (e.g. Long, 2003). However, some taxonomic groups seem to include more invasive species than others do. Mammals are a major threat to island faunas and floras. Whereas rats, mongooses, mustelids and feral cats devastate the local bird and reptile fauna of islands, feral goats (Capra hircus) can diminish the native flora drastically. Islands, due to their size and other characteristics, which cannot be summarized here (see, for example, Loope and Mueller- Dombois, 1989), are particularly vulnerable to these invaders. These intro- duced mammalian plagues are due chiefly opportunistic predators, with high abilities for adaptation to different circumstances and food items as a precondition for bringing native species to extinction. It is perhaps not sur- prising that the largest angiosperm families supply the largest percentages in the world of invading species (Heywood, 1989). The high number of invasive mammals and members of the Asteraceae, which are regarded as some of the most advanced groups (amongst classes of vertebrates and fam- ilies of angiosperms, respectively) from the evolutionary point of view, is also an indication that these groups are currently in radiation, evolutionar- ily successful at these times, and have developed biological features which ensure both survival under extreme conditions and high reproductive rates. This suggests that, for example, in the era of the dinosaurs, the reptiles would have been the most damaging invaders. IAS are recorded from every part of the globe, emphasizing the world- wide impact of human disturbance, leaving almost no ecosystem untouched and pristine. All continents and habitats seem to be vulnera- ble to invasions, though islands are particularly at risk, as mentioned above, and some patterns between continents seem to arise. In the highly populated area of Central Europe IAS seem to be of less importance to biodiversity than on other continents with large tracts of more natural habitats. The smaller reserves in Central Europe are easier to manage and control of alien species in these places is often more practical. The long association between introduced species and the human population in Europe is a very different situation compared to other continents, as all habitats are highly altered and human-made habitats dominate. These human-made habitats are regarded as a valuable heritage of Central Europe and are often based on alien species introduced long ago. There are also broad differences between the numbers of introduced species. Kowarik (2002) states that in the German flora 22.4% are alien species, whereas New Zealand has 1579 established alien angiosperms to 2212 native ones (Clout and Lowe, 2000). In the case of New Zealand the numbers of native to alien land mammals is of even greater concern, with a ratio of two indigenous bats to 34 alien species. Comparing the numbers of introduced to established and invasive alien species, one has to bear in mind the long lag phases, which often chap01.qxd 12/11/04 10:56 AM Page 6
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occur. Most introduced species will need some time before they become invasive, i.e. enter the lag phase. Kowarik (2003) shows that for woody alien plants in a part of Germany the average time-lag between first intro- duction and their expansion is about 147 years. The reasons for the lag phase can be manifold and will be different from case to case. The founder population needs some time to adapt to the local environment, genetic changes could also play a role with a new mutation better adapted, ongoing releases could also be a crucial factor in the establish- ment, or a change in human disturbance can be of additional help for an alien species. Whatever the reason, the fact that often time-lags occur makes predictions on invasiveness of alien species very difficult. A species showing no harm today can still be an invasive of tomorrow, especially in combination with other global changes. Figure 1.1 summa- rizes some important factors influencing the invasiveness of species.
Human dimension Extrinsic factors
Abiotic factors, such as Attractiveness to climatic conditions and soil humans: pathways, use of species, number of introductions No. natural enemies
No. competing Variation: species: native and species traits exotic such as seed dispersal, Invasiveness of species number, size Interactions with native and exotic species: pollination, dispersal, food Intrinsic factors source, change of ecosystem
Ecosystem characteristics, e.g. disturbed (natural or human-induced) Control of this and Land use other IAS change Global climate change
Fig. 1.1. Factors affecting the bioinvasion process and the invasiveness of alien species. chap01.qxd 12/11/04 10:56 AM Page 7
Invasive Alien Species 7
There are three major factors which determine the ability to become inva- sive:
1. Intrinsic factors or species traits, such as the ability to adapt to differ- ent conditions, a wide amplitude to abiotic factors, pre-adapted to differ- ent climatic zones, and a high reproductive rate. 2. Extrinsic factors or relationships between the species and abiotic and biotic factors, such as the number of natural enemies, the number of com- peting species (native and alien), other interactions with native and alien species (pollination, dispersal, food source, ecosystem engineers), cli- matic conditions, soil conditions, degree of disturbance (natural and human-induced), global climate change, change in land use patterns, and control and eradication of other IAS. 3. Human dimension. The attractiveness and importance for humans influence the pathways, vectors, the numbers of specimens introduced, the numbers of introductions and the potential to eradicate or control the species.
Pathways and Vectors
The ways in which alien species are introduced are almost unlimited. Some important pathways, for example to an island, are shown in Fig. 1.2. The increasing boat traffic also accelerated the introductions of
Aqua- culture
Fig. 1.2. Some important pathways into the terrestrial, freshwater and marine environments of a given island. chap01.qxd 12/11/04 10:56 AM Page 8
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species by transport both within and upon the vessels. One of the most important pathways for marine and freshwater species is transport in ballast tanks. The species are either in the ballast water or live in the ballast sediment. In former days when the ships took soil as ballast, even terrestrial species were carried, e.g. plants. Another important way for introductions of aquatic species is the hull-fouling organisms, which can reproduce and colonize new areas from the travelling ship. The cargo of ships and other transport vehicles also harbours species. Moreover, people carry hitchhiking species around on their dirty boots and tents. All these pathways transport species accidentally. Some other groups are deliberately introduced, such as species used in aquaculture, for fisheries, as forest trees, for agricultural purposes, as species for hunting, plants for soil improvement and just for the pleasure of humans as ornamentals. It can be summarized that most aquatic species and invertebrates in general mainly hitchhike accidentally, whereas most plants and vertebrates are deliberately introduced. Minchin and Gollasch (2002) and Carlton and Ruiz (2004) give excel- lent overviews on pathways and vectors in more depth. The latter divide pathways into cause (why a species is transported), route (the geographi- cal path) and vector (how a species is transported).
Consequences
The impacts of IAS are often considerable, as ecosystem functioning can be altered and species can be brought to extinction. There is no way back to the former state in the latter case. The environmental impacts can be divided into four major factors: (i) competition; (ii) predation (including herbivory); and more subtle interactions such as (iii) hybridization and (iv) transmission of diseases. All these factors alone or in concert with other factors can decrease biodiversity and cause extinction. The most obvious examples for competition are between introduced and native plants for nutrients and exposure to sunlight. Resource competition has led to the replacement of the native red squirrel (Sciurus vulgaris L.) by the introduced American grey squirrel (Sciurus carolinensis Gmelin) in almost all of Great Britain. The latter forages more efficiently for food and is stronger than the native species (Williamson, 1996). Impacts due to pre- dation and herbivory are very extensive on island fauna and flora, as men- tioned above. The brown tree snake eliminated most of the bird species on Guam, and feral goats are a menace to native island vegetation, where they were often released as a living food depot. A well-known hybridiza- tion example from Europe is the ruddy duck, which hybridizes with the native white-headed duck, as mentioned before. In some cases IAS can harbour diseases and be a vector for the diseases to native species. This is the case with American-introduced crayfish species to Europe, which are carriers of the crayfish plague (Aphanomyces astaci Schikora) without many symptoms, but the native noble crayfish (Astacus astacus (L.)) is chap01.qxd 12/11/04 10:56 AM Page 9
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highly susceptible to the disease, and thus is struggling to coexist with populations of the American crayfish species. Introduced species can interact with natives in a variety of ways and indirect effects can be very difficult to demonstrate. Direct and indirect effects can lead to very complex interactions and a combination of effects can cause complex impacts. In addition to the biodiversity impacts, many IAS also impose enor- mous economic costs. These costs can arise through direct losses of agri- cultural and forestry products and through increased production costs associated with control measures (US Congress, 1993; Pimentel et al., 2000). One often-cited example is the costs caused by D. polymorpha to water plants by clogging water pipes and other structures in the Great Lakes in North America. Costs for environmental problems are more dif- ficult to calculate than costs imposed in the agricultural sector. A North American study calculated costs of US$138 billion per annum to the USA caused by IAS (Pimentel et al., 2000). This takes into account all costs associated with the IAS, not only the direct costs such as loss of harvest, costs of control, etc. Some of the costs calculated in the paper are rather estimates; however, even giving or taking a number of such magnitude, it is a very impressive number and shows the importance of IAS. Some IAS also have implications for human health. Giant hogweed (Heracleum mantegazzianum Sommier et Levier) was introduced from the Caucasus to Europe as an ornamental plant. It produces copious amounts of a sap which is phototoxic and can lead to severe burns of the skin. Regularly, in particular children are hospitalized after contact with the plant, especially when they play with the hollow stems and petioles. The racoon dog (Nyctereutes procyonoides Gray), introduced as a fur animal, can, like the native fox, be the vector of the most dangerous par- asitic disease vectored by mammals to humans in Central Europe, i.e. the fox tapeworm (Echinococcus multilocularis Leuckart) (Thiess et al., 2001). Although the racoon dog is only an additional vector, this can have effects on the population dynamics of the parasite and lead to an increase in the disease in humans. To address these impacts, populations of existing invasive species need to be managed. New introductions must be assessed as to the threat they may present and only introduced on the basis of a risk analysis, and new invasions must be minimized.
Prevention
The rapid global increase in trade, travel, transport and tourism is leading to an increase in introductions of non-indigenous species (Peck et al., 1998). Prevention is the first and most cost-effective line of defence against IAS. An ounce of prevention is worth a pound of cure – this maxim of medicine, dictating such measures as quarantine and inocula- tion, is equally valid for dealing with biological invasions. It is essential chap01.qxd 12/11/04 10:56 AM Page 10
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to prevent as many alien species as possible from entering the country, in addition to following a planned risk assessment process for species and pathways. Although there are significant costs associated with preven- tion, failure to prevent a single highly invasive species from colonizing, such as the zebra mussel in North America or the western corn rootworm (Diabrotica virgifera virgifera LeConte) in Europe, might lead to enormous costs outweighing all prevention efforts. One cost of prevention, and the most obvious, is the expense of maintaining the exclusion apparatus (salary and training of interception personnel, and facilities such as fumi- gation chambers, inspection apparatus and quarantine quarters). A second cost is that affecting individuals who are not allowed to profit from bringing in alien species (which may or may not be intended for release to the environment), and equally the public that might have ben- efited from a planned introduction disallowed by the prevention proce- dures. These costs are offset by the benefits that accrue to society from potential invasions that are prevented. In many cases, the benefit from an introduced species lies with the importer, whereas control costs of IAS are borne by the public. However, prioritizing support for a costly pre- vention system is often difficult, because the impact of alien species and the potential costs of invasions cannot be reliably predicted because of the complexity and calculated against the actual costs of the apparatus. Most prevention measures are focused on certain species known to be pests elsewhere, since this is the most reliable feature of invasive species. However, most of these quarantine species are economically important species for the agriculture, forestry or human health sectors. Prevention of entry of species on these black lists is the rather conservative goal for quarantine and other measures taken at present. A more recent approach, in order to incorporate all potentially dangerous organisms, not only in an economic view but also in terms of saving the world’s biodiversity, is a move to using white lists (e.g. US Congress, 1993; Panetta et al., 1994). The approach is also often called ‘guilty until proven innocent’ (Ruesink et al., 1995). A proposed intermediate step is the use of ‘pied lists’, which are more realistic to implement when constrained by a lack of facilities, staff and funding. The ‘pied list’ would contain a section of known pest species (equivalent to black lists) with strict regulations and measures to ensure pest-free imports. Another section of the list would describe species cleared for introduction (white lists) – organisms declared as safe. All species not listed on either list would be regarded as potential threats to biodiversity, ecosystems or economic sectors. A stakeholder proposing an intentional introduction would have to demonstrate beyond reason- able doubt the safety of the proposed introduction in a risk assessment process. Species assessed for their likely invasiveness would be moved to the white or black list depending on the outcome of this investigation. There are three principal strategies to reduce further introductions:
1. Interception based on regulations and their enforcement with inspec- tions and fees. This approach involves inspection, decontamination and chap01.qxd 12/11/04 10:56 AM Page 11
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potential constraints to specific trade commodities rated as high-risk. The illegal importation of prohibited items – smuggling – is also of high concern, since the items are not inspected and smugglers are unlikely to pay attention and take precautions against hitchhiking species. In order to meet the precautionary principle a risk assessment process should be the basis for every proposed intentional introduction unless the species is already on the white list. 2. Treatment of goods or their packaging material suspected to be con- taminated with alien organisms, including biocide applications (e.g. fumigation, immersion or spraying), heat and cold treatment, pressure, and irradiation (Sharp and Hallman, 1994). 3. Trade prohibition. Under the World Trade Organization Sanitary and Phytosanitary (WTO SPS) Agreement (WTO, 1994), member countries have the right to take sanitary and phytosanitary measures to the extent necessary to protect human, animal or plant life or health provided these measures are based on scientific principles and are not maintained without sufficient scientific evidence.
Public education is an essential part of prevention and management programmes. In fact, some scientifically well-devised projects have been interrupted or stopped because of public disapproval. Besides these extreme cases, public awareness and support can greatly increase the success of projects to protect and save biodiversity. Travellers are often unaware of laws and regulations to prevent introductions of alien species, and the reasons for them. Education should focus on raising the aware- ness of the reasons for the restrictions and regulatory actions, and the environmental and economic risks involved. In addition to printed mate- rial, e.g. posters and brochures, video presentations and announcements on aeroplanes are a promising approach. The public as well as industry should perceive prevention measures not as an arbitrary nuisance but rather as a necessity for travel and trade. The most common approach for prevention of invasive organisms in the past was to target individual pest species. However, a more compre- hensive approach is to identify major pathways that lead to harmful inva- sions and manage the risks associated with these. Pathways such as ballast water can only be targeted on the entire pathway and should be analysed for risks involved. IAS are not just species brought into a country from another country, but also within-country movements must be considered. Political bound- aries do not make sense in the definition of alien species, which should rather be based on eco-regions and natural boundaries, for example fish species transported between unconnected watersheds. Some species translocated within the same country can be as disruptive to ecological systems as a species from a different continent. Human-made structures may enhance subsequent spread of alien species formerly restricted to one area. The completion of the Welland Canal between Lake Ontario and Lake Erie enabled invasive chap01.qxd 12/11/04 10:56 AM Page 12
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organisms, such as the sea lamprey (Petromyzon marinus L.), to bypass the Niagara Falls and subsequently spread to other lakes and river systems (Simberloff, 1996). The opening of the Suez Canal initiated a remarkable influx of hundreds of Red Sea species into the oligotrophic Mediterranean Sea, outcompeting and replacing indigenous species (Galil, 1999). In conclusion, prevention is the most cost-effective control. However, it is difficult to mobilize a sufficient budget before an IAS causes prob- lems and inflicts costs. Exclusion methods based on pathways rather than on individual species are a more efficient way to concentrate efforts. Three major possibilities to minimize further invasions are interception, treatment of suspect imported material and prohibition of particular commodities under international regulations. Deliberate introductions should all be subject to an import risk assessment.
Early Detection
The longer an alien species goes undetected, the higher will be the popu- lation (approaching the lag phase), the fewer options will remain for its control and in particular eradication, and the more expensive any inter- vention will become (Mack et al., 2000). Eradication options will swiftly fade with the building up of the alien population. However, during the lag phase, it can be difficult to distinguish doomed populations from future invaders. Since not all alien species will necessarily become invasive, species known to be invasive elsewhere under similar conditions are pri- orities for early detection. The possibility of early eradication or getting a new colonizer under effective early control makes investment in early detection worthwhile. Surveys for early detection should be carefully designed and targeted to answer specific questions as economically as possible. Some invasive species are easily seen, while others are cryptic and require special efforts to locate or identify them, particularly when they are low in numbers. Traps can be very effective for the more cryptic species. Surveys by experts should be made for certain groups of pests to enable a rapid response before the invasive species becomes well established. A contingency plan is usually a carefully considered plan of the action that should be taken when a new invasive species is found or an invasion is suspected. The plan may be just a simple paper document that all staff, selected volunteers or relevant organizations have written, are aware of and will act on in a contingency situation. Alternatively the plan may be expanded to include comprehensive kits of tools that are stored in a ‘ready-to-use’ condition at appropriate locations. Contingency funding must also be immediately accessible to deal with species colo- nization in an early stage. chap01.qxd 12/11/04 10:56 AM Page 13
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Management
Even the most effective prevention and early warning systems will have leaks and thus new introductions have to be anticipated. For those species and the ones already present in the country at the onset of pre- vention measures, control options have to be investigated. In a manage- ment project dealing with invasive species, several important issues have to be addressed, including planning, budgeting, monitoring, analysis, recording, reporting, follow-up and dissemination of the results. Adequate funding needs to be secured for all steps until the project goal, set prior to the beginning of the project, is met. The first step is to determine the management goal for any project for the management of invasives. The target area needs to be defined. It may be an entire country, all or part of an island or all or part of a reserve or conservation area. In some instances regional projects will include more than one country and need good coordination between countries. In par- ticular in cases of management of IAS in national parks, which protect high biodiversity and an important habitat of the area, the management of IAS should be incorporated into a strategy for the national park. The strat- egy needs to state the goals of the protected area and the objectives of how to conserve the native biodiversity. The management of IAS is only one part of the bigger picture to conserve or restore the natural processes. In many cases IAS will need to be addressed. Thus, the final goal will be the preservation of the unique ecosystems and the development of sustain- able use of ecosystem services. The management area, as defined in the management goal, has to be surveyed for alien as well as native species to assess the potential loss of natural habitat and to estimate the impacts. These surveys include litera- ture search, collection records and actual surveys in the area. The docu- mentation has to include the best available knowledge about the abundance and distribution of alien species, their impact on the habitat and, when justified (e.g. based on experience in neighbouring areas), a prediction of future impact. If there are earlier data available, a compari- son between past and current species composition and distribution of single alien (and native) species can reveal the status and spread of species in that area (and their impact). Past control actions and their success or failure should be summarized too. The next step would be to examine the management options for each target species, using local knowledge, information from databases and published and unpublished sources. Local circumstances, such as cul- tural, religious and socio-economic features may affect the suitability of different options. Options for eradication, containment or control and needs for further surveys, experimental investigations and other research should all be evaluated. Priorities should be set, with the highest priority given to existing infestations that are expanding most rapidly, are most disruptive and affect the most highly valued areas of the site. The prior- ity-setting process can be difficult, partly because many factors need to be chap01.qxd 12/11/04 10:56 AM Page 14
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considered and the limited budget needs to be spent where it has the highest impact for the management goals. Important factors for the prior- itization process are the current extent of the species on or near the site, the current and potential impacts of the species, the value of the habitats that the species infests or may infest in the near future, and the practical- ity of control. In the long run, it is usually most efficient to devote resources to preventing new problems and immediately addressing incip- ient infestations. All IAS populations need monitoring, because many species not yet regarded as invasive may in fact be ‘sleeping’ organisms passing through their lag phase of invasion and will become invasive later on when entering the lag phase. The three main strategies for dealing with alien species that have already established populations in the area under concern will be pre- sented below, i.e. eradication, containment and control.
Eradication
Eradication is the elimination of the entire population of an alien species, including any resting stages, in the managed area. When prevention has failed, an eradication programme, as a rapid response to the early detec- tion of an alien species, is often the key to a successful and cost-effective solution. However, eradication should only be attempted if it is feasible to eradicate the species with the budget and the methods available. A careful analysis of the costs (including indirect costs) and likelihood of success must be made (rapidly) and adequate resources mobilized before eradication is attempted. However, if eradication of the invasive species is achieved it is more cost-effective than any other measure of long-term control. Eradication programmes can involve several control methods on their own or a combination of these. The methods vary depending on the inva- sive species, the habitat and the circumstances. Successful eradication in the past has been based on mechanical control (e.g. hand-picking of snails or hand-pulling of weeds), chemical control (e.g. using toxic baits against vertebrates), spot spraying of plants, biopesticides (e.g. Bacillus thuringiensis (Bt) sprayed against insect pests), habitat management (e.g. grazing and prescribed burning), or hunting of invasive vertebrates. Some groups of organisms are more suitable for eradication efforts than others. The best option is to base an eradication programme on a suc- cessful case study, although the circumstances may vary slightly and lead to a different outcome. Each situation needs to be evaluated to find the best method(s) in that area under the given circumstances. Plants can be eradicated best by a combination of mechanical and chemical treatments, e.g. cutting woody weeds and applying a herbicide to the cut stems. Many successful eradication programmes have been carried out against land mammals on islands. The methods most frequently used were bait sta- tions where toxic substances were offered to the invasive species, e.g. rat chap01.qxd 12/11/04 10:56 AM Page 15
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control. Bigger animals can be hunted, provided the ecosystem is of an open kind with less cover in which to hide. A particular issue with erad- ication programmes against land vertebrates may be adverse public opinion, especially that of animal rights groups. It can be very difficult to try to eradicate a fluffy and cute animal. Amongst land invertebrates only snails and insects have been successfully eradicated on occasion. Snails can be hand-picked, whereas the commonest options to eradicate insects are based on the use of insecticides or biopesticides, usually by wide- spread application, or using baits or traps or a combination of methods. The use of sterile male releases, often in combination with insecticide control, has been effective on several occasions against insects, such as fruit flies and the screw-worm fly. There are two published successful eradications of invasive species in the marine environment to date. An infestation of a sabellid worm in a bay in the USA was eliminated by hand-picking of the host (Culver and Kuris, 1999); the other case was the eradication of a mussel species in Australia using pesticides (Bax, 1999). Foreign freshwater fish species have been eradicated in the past by using toxins specific to fish (Courtenay, 1997). Pathogens of humans and domesticated animals have been eradicated by vaccination of the respec- tive host. In general, it seems more feasible to apply methods for eradica- tion to the hosts (cf. mosquitoes) rather than directly to the pathogens. If an eradication programme is feasible, it is the preferred choice for action against an IAS. The advantage of eradication as opposed to long- term control is the opportunity for complete rehabilitation to the condi- tions prevailing prior to the invasion of the alien species. There are no long-term control costs involved (although precautionary monitoring for early warning may be appropriate) and the ecological impacts and eco- nomic losses diminish to zero immediately after eradication. This method is the only option that totally meets the management goal, because the invasive species is completely eliminated. The major drawbacks of eradication programmes are that they are very costly and their success cannot be guaranteed. The programme needs full commitment and attention until its successful completion; no eradi- cation programme should be started unless an assessment of the available options and methods has shown that eradication is feasible. Thus, eradi- cation should only be pursued when funding and commitment of all stakeholders are secured. Public awareness of the problems caused by the invasive species should be raised beforehand and public support sought. A well-designed and realistic eradication approach has to be developed to achieve the required goal. Many failed attempts were highly costly and had side-effects on non-target species, as in the case of the attempt to eradicate South American red fire ants (Solenopsis invicta Buren) in the southern states of the USA (Simberloff, 1996). The insecticide initially used proved disastrous to wildlife and cattle. The ant bait subsequently developed also had non-target effects, and proved to be more effective against native ant species than against the intruder. This in fact enhanced the populations of the alien species due to a decrease of interspecific com- chap01.qxd 12/11/04 10:56 AM Page 16
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petition with native ant species. Finally, the eradication efforts had to be abandoned. Eradication efforts have been especially successful in island situa- tions. The term ‘island’ is in this case not restricted to real islands but can also include ecological islands isolated by physical or ecological barriers, e.g. forest remnants surrounded by agricultural fields. However, the target species may survive in small populations outside an ecological island and depending upon the degree of isolation could rapidly re-invade the ecological island after an eradication campaign. Although eradication methods should be as specific as possible, the rather rigorous nature of concentrated efforts for eradication will often inflict incidental casualties on non-target species. In most cases these losses can be seen as inevitable and acceptable costs in achieving the management goal and can be balanced against the long-term economic and biodiversity benefits. However, the potential non-target effects should be evaluated beforehand. When attempting eradication using toxins, it should be ensured that these are as specific as possible and that their persistence in the ecosystem is of short duration. How- ever, some toxins unacceptable for use in a long-term control programme might justifiably be used in an eradication campaign over a short period of time. Eradication (or control) of well-established non-indigenous species that have become a major element of the ecosystem will influence the entire ecosystem. Predicting the consequences of the successful elimina- tion of such species will be difficult. The relationships of the invasive species to indigenous and alien species have to be considered. A strong carnivore–prey relationship between two invasive species points to the need to investigate the potential for combined methods to eliminate both species at the same time. Control of one species in isolation could have drastic effects on the population dynamics of the second species. Elimination of the normal prey may eliminate the carnivore or it may cause it to change its behaviour and feed on native species. Elimination of an introduced carnivore is likely to allow the introduced prey to increase greatly in numbers and may cause more damage than when both were present. By way of synthesis, basic criteria for a successful eradication pro- gramme are summarized as follows: