Forestry Department Food and Agriculture Organization of the United Nations

Forest Health & Biosecurity Working Papers

Alien Invasive Species: Impacts on Forests and Forestry

A Review

by Beverly A. Moore

November 2005

Forest Resources Development Service Forest Resources Division Forestry Department

Working Paper FBS/8E FAO, Rome, Italy

Background

This paper is one of a series of FAO documents on forest-related health and biosecurity issues. The purpose of these papers is to provide early information on on-going activities and programmes, and to stimulate discussion.

The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Further information on forest invasive species can be found at the FAO Alien invasive species Web site (www.fao.org/forestry/aliens) and information on forest health can be found at the FAO Forest health Web site (www.fao.org/forestry/site/pests).

Comments and feedback are welcome. For further information, please contact: Gillian Allard, Forestry Officer (Forest Protection and Health ) Forest Resources Division Forestry Department FAO, Viale delle Terme di Caracalla 00100 Rome, Italy Fax: + 39 06 570 55 137 Email: [email protected]

Pierre Sigaud, Forestry Officer (Forest Genetic Resources ) Forest Resources Division Forestry Department FAO, Viale delle Terme di Caracalla 00100 Rome, Italy Fax: + 39 06 570 55 137 Email: [email protected]

© FAO 2005

ALIEN INVASIVE SPECIES: IMPACTS ON FORESTS AND FORESTRY

A REVIEW

BY

BEVERLY A. MOORE CONSULTANT FOREST RESOURCES DEVELOPMENT SERVICE FORESTRY DEPARTMENT FAO NOVEMBER 2005

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ALIEN INVASIVE SPECIES: IMPACTS ON FORESTS AND FORESTRY TABLE OF CONTENTS

1. Introduction .........................................................................................................................................1 2. Alien invasive species affecting forests and forestry ...........................................................................2 3. Factors contributing to the introduction and spread of alien invasive species ..................................3 4. Impact of land use change and the forest sector on the introduction and spread of alien invasive species......................................................................................................................................................8 5. Positive impacts of introduced woody species on forests and forestry............................................. 11 6. Negative impacts of alien invasive species on forests and forestry .................................................. 13 7. Conflict species.................................................................................................................................. 17 8. Forest pest management options ...................................................................................................... 19 9. Conclusions........................................................................................................................................23 10. References ........................................................................................................................................24 Annex 1. Alien invasive species affecting forests and forestry ..............................................................34 Annex 2. Select alien invasive species affecting forests and forestry....................................................40 Annex 3. International and regional instruments and programmes.....................................................55

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1. Introduction

“We are living in a period of the world's history when the mingling of thousands of kinds of organisms from different parts of the world is setting up terrific dislocations in nature” Charles Elton, 1958. The ecology of invasions by animals and plants. An integral part of sustainable forest management includes measures to protect forests from natural threats such as fire, insects and diseases. Increasingly, an additional, and more severe threat, has been affecting the forest sector worldwide - alien invasive species. Alien invasive species are any species that are non-native to a particular ecosystem and whose introduction and spread causes, or are likely to cause, socio-cultural, economic or environmental harm or harm to human health. The increasing global movement of people and products, though beneficial to many people, is also facilitating the movement of alien species around world. These species may be unintentionally introduced to new environments in shipments of food, household goods, wood and wood products, new and used tires, animal and plant products, containers, pallets, internal packaging materials, and humans. In the absence of their natural predators, competitors and pathogens, they prosper in their new environments and spread at the expense of native species, affecting entire ecosystems. Not all invasive species have been inadvertently introduced, however. Particularly challenging to natural resource management are non-native species that have been intentionally introduced into an ecosystem to provide economic, environmental or social benefits. Many species of plants, trees and animals have been introduced outside their native ranges as ornamentals, for gardening, or for the pet industry, and these species have escaped to become serious problems in forests and other ecosystems. This is a considerable concern in the forest sector since many of the tree species used for agroforestry, commercial forestry and desertification control are alien or non-native to the area. It is vital to ensure that such species serve the purposes for which they were introduced and do not escape to cause negative effects on native ecosystems. While the definitions and impacts of invasive species on the forest sector are still debated and need reviewing in the context of forest management, a number of initiatives, programmes and activities have been initiated. Most programmes focus on damage caused to local forest ecosystems, or to particular species or group of species, by a given pest 1over a period of time. There is an overall lack of information on alien invasive species and the forest sector at the global scale. Information sharing is necessary in the planning and implementation of any strategy for the management of alien invasive species. This review concentrates, but not exclusively, on insects, diseases and woody species that have the potential to become invasive.

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Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products.

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2. Alien invasive species affecting forests and forestry Alien invasive species occur in all major taxonomic groups from micro-organisms to mammals. The Invasive Species Specialist Group (ISSG) of the Species Survival Commission (SSC) of the World Conservation Union (IUCN) has gathered information on 326 alien invasive species including 157 that negatively impact forests and the forest sector (www.issg.org/database [accessed on 23 August 2005]). Annex 1 provides a list of 204 species known to affect forests and the forest sector including the 157 species identified by ISSG. ISSG has also compiled a list of the One Hundred of the World’s Worst Invasive Alien Species (www.issg.org/database/species/search.asp?st=100ss&fr=1&sts=#SpeciesList) which aims to collectively illustrate the range of impacts caused by biological invasion. Although incomplete, this list is a first attempt to rank the impact of alien invasive species. Included are 62 alien invasive species - four fungi, one flatworm, 10 insects, two molluscs, two amphibians, one reptile, two birds, 13 mammals, two grasses, six plants and 19 trees and shrubs - that impact forests and forestry. Annex 2 provides a more detailed discussion of these species.

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3. Factors contributing to the introduction and spread of alien invasive species Biological invasions are human-assisted – humans intentionally and unintentionally introduce species into new areas or alter ecosystems in ways that promote invasions. Global factors, both primary and secondary, that support the introduction and spread of alien invasive species include: ƒ land use changes including forest sector activities (see Chapter 4); ƒ economics and trade; ƒ climate change and changes in atmospheric composition; ƒ tourism; ƒ conflict and reconstruction; ƒ regulatory regimes; ƒ biological control of pests; ƒ public health and environmental concerns. It is important to note here, that not all introduced species become invasive. It is widely accepted that only a small proportion of introduced species establish and only a small proportion of these species spread and become pests. This is often termed the “tens rule” whereby approximately 1 in 10 introduced species escapes to the wild, 1 in 10 of these introductions become established in the wild, and 1 in 10 of these established species becomes a pest (Vander Zanden, 2005). In addition, species that are known to be invasive elsewhere may not necessarily become invasive in a new environment. Economics and trade The openness of a country’s economy and the composition of its trade routes enhance the vulnerability of nations to biological invasions (Perrings et al., 2002; Taylor and Irwin, 2004). Invasions are also enhanced by the national importance of agriculture, forest and tourism sectors. A high importance generally leads to increases in the resources allocated to quarantine and protection however it also increases the opportunities for introduction and spread (FAO, 2001a). Dalmazzone (2000) investigated data on established alien species in 26 countries in Africa, Australia, Europe, and North and South America from the early 1960s to the early 1990s and found a clear correlation between economic variables and disturbances associated with human activities and a country’s vulnerability to biological invasions. The influence of such activities becomes more apparent when considering the problem of invasions in island ecosystems. With particularly vulnerable native biodiversity and a higher percentage of imports in comparison to continental areas, islands exhibit both economic and ecological reasons for the incidence of alien invasive species (Dalmazzone, 2000). In South Africa, its long colonial history, well developed infrastructure, and prosperous agriculture and forest sectors have contributed significantly to the introduction, establishment and spread of invasive alien plants. About 8 750 tree and plant species have been introduced into South Africa and of these, 161 species are considered highly invasive and many more are likely to become weeds in the future (van Wilgen et al., 2001). Globalization has led to more and faster trade, new travel and trading routes, and increased trade in livestock, pets, nursery stock, agricultural produce and forest products; all of which can facilitate the introduction and spread of alien invasive species (FAO, 2001a). Weed seeds, plant pathogens, larval or adult arthropods and other invertebrates, and even some vertebrate species can be transported on such commodities. Sand, gravel, coal and metal ores, and other inorganic commodities can also be contaminated with seeds, arthropods and pathogens (Cox, 1999). Unprocessed wood, wood products and nursery stock are also a major source of forest pests and diseases having introduced a number of devastating species into the United States such as chestnut blight (Cryphonectria parasitica), Dutch elm disease (Ophiostoma ulmi sensu lato), and white pine blister 3

rust (Cronartium ribicola) (OTA, 1993). It is believed that Pineus boerneri, a pine woolly adelgid, entered Kenya and Zimbabwe on scion material (Diekmann et al., 2002). In addition to the possibility of the commodities themselves carrying alien invasive species, the containers and vehicles in which they are transported can also facilitate invasions. Wood packaging material made of unprocessed raw wood, including pallets, crates, drums, skids, cases, and dunnage, can be a pathway for the introduction and spread of pests, in particular forest pests (McNeely et al., 2001; IPPC, 2002). The Asian longhorned beetle (Anoplophora glabripennis), emerald ash borer (Agrilus planipennis) and brown spruce longhorn beetle (Tetropium fuscum) are alleged to have arrived in North America among packing materials from Asia (Keiran and Allen, 2004). The Sirex woodwasp (Sirex noctilio) probably entered Argentina, Australia, New Zealand and South Africa in wooden packaging from Europe or North Africa and the red turpentine beetle (Dendroctonus valens) may have been introduced into China’s pine forests through packaging made from infested North American wood (Keiran and Allen, 2004). The pinewood nematode (Bursaphelenchus xylophilus) the causative agent of pine wilt disease, has spread from its native North America to Asia and Europe in wooden packing materials (APHIS, 1999a; APHIS, 1999b). Containerized cargo can shelter alien species from microorganisms to reptiles and mammals and since inspecting such freight is very difficult and costly, many alien invasive species may enter a country undetected (OTA, 1993; Cox, 1999). No longer are the effects of alien species invasions being initially confined to areas around the ports of entry since containers are typically not unloaded until they reach their final inland destinations (OTA, 1993). Vehicles, including cars, trucks, trains, planes and ships, may also be contaminated with all types of pests and since the commercial and recreational movement of vehicles across international boundaries has increased, the threats are considerable (Cox, 1999). Railway sleepers have also been blamed for spreading many insect pests; it is suspected that Phoracantha semipunctata and P. recurva, both serious pests of eucalypts, entered South Africa in freshly-cut railway sleepers imported from Australia (EPPO, 2004). New trade activities, particularly in forest products, between China and the former Soviet Union and North America has led to a dramatic increase in pest and disease problems on both sides through accidental introduction of alien invasive species (McNeely et al., 2001; Normile, 2004). Climate change and changes in atmospheric concentration Human activities are releasing greenhouse gases such as carbon dioxide (CO2), methane, nitrous oxide, halocarbons and ozone into the atmosphere and the rising concentrations of such gases is changing climate (UNEP and UNFCCC, 2002). Global climate change has many environmental consequences including changes in species distributions and in their abundance within existing distributions as a result of direct physiological impacts on individual species and changes in abiotic factors, reproduction and recruitment opportunities, and interspecific interactions (McNeely et al., 2001). Climate change may produce more favourable conditions for alien invasive species. Once dominant species in native areas are no longer adapted to the environmental conditions of their habitat, it is likely that introduced species will displace them thus drastically changing successional patterns, ecosystem function and resource distribution (McNeely, 1999; Tilman and Lehman, 2001). For example, Oberhauser and Peterson (2003) investigated the possible effects of global climate change on distributions of migratory populations of monarch butterflies (Danaus plexippus) and concluded that predicted changes pose potentially significant threats to their survival. Climate, geography and other variables define the distribution limits of a species; however with changes in climate these limits are expanding, spreading species into higher latitudes and altitudes due to increased temperatures, humidity, precipitation and other climatic factors (McNeely, 1999; McNeely et al., 2001). Rouget 4

et al. (2002) noted that the current distribution of stands of invasive trees in South Africa was largely influenced by climatic factors. Warming trends may also allow for longer breeding seasons for invasive species, as observed in populations of the collared dove (Streptopelia decaocto) in Europe (Crooks and Soulé, 1999). Since the development of insects is temperature dependent, it has been predicted that increasing temperatures will enhance the winter survival of insects and facilitate population increases and expansions in geographic range (Crooks and Soulé, 1999; Mooney and Hofgaard, 1999). Changes in climate and environmental factors may also allow existing introduced species to become invasive (Mooney and Hofgaard, 1999). Climatically induced stress on plants and other species also reduces their ability to resist invaders and thus make them more vulnerable to insect or pathogen damage (Mooney and Hofgaard, 1999; McNeely et al., 2001). Climate change may alter production patterns and trade in agricultural and forestry commodities by species being grown more competitively in higher latitudes and altitudes. Since alien invasive species establish more easily in habitats disturbed by human and other factors, such changes can provide more opportunities for them to invade (McNeely et al., 2001). Climate change also affects the frequency and intensity of extreme climatic events which may have the greatest influence on alien invasive species by disturbing ecosystems and thus providing increased opportunities for dispersal and growth of invasive species (McNeely et al., 2001). In addition to the effects of climate change, increasing concentrations of greenhouse gases can have significant effects on the success of alien invasive species as well. Higher concentrations of carbon dioxide increase photosynthetic rates and water use efficiency of plants and ecosystems (Vitousek et al., 1997; Mooney and Hofgaard, 1999). The resulting increased soil moisture has potential to provide habitat for late-season annuals which may be invasive. Distributions of spruce budworm in Oregon have been observed to change in relation to increased atmospheric CO2 concentrations (Vitousek et al., 1997). However, increased levels of CO2 affect plant species differently which is likely to result in substantial changes in the species composition and dynamics of terrestrial ecosystems (Vitousek et al., 1997). A large component of the nitrogen cycle is atmospheric nitrogen which must be fixed before it can be used by most organisms. As a result, it is this fixed nitrogen that controls the productivity, carbon storage and species composition of many ecosystems (Vitousek et al., 1997). Changes in the deposition of nitrates from the atmosphere through the burning of fossil fuels, has resulted in large changes in vegetation, as observed in Western Europe, which may favour the growth of some alien invasive species (Mooney and Hofgaard, 1999). Conflict and reconstruction Conflict and civil unrest can contribute to the introduction and spread of alien invasive species in a variety of ways (FAO, 2001a). ƒ Civil unrest leads to the breakdown of phytosanitary and animal health controls and management systems, the loss of supply lines for materials as well as to the displacement of substantial numbers of people. ƒ Areas where there is civil unrest or war are more vulnerable to the entry of pests and diseases because of the lack of inspections and border controls and also because of the increased unregulated movement of military personnel and refugees. ƒ Displaced people and their belongings can be a dispersal mechanism for, or the source of, alien invasive species. ƒ Increased smuggling can relocate alien species to new regions. ƒ Inflows of food aid may be contaminated with pests and diseases. ƒ Difficulties in obtaining access to border areas because of landmines and other hazards make these areas difficult to survey. 5

Military transport, equipment and supplies, often covered with dirt or mud from the field, are effective means of dispersal for many species which can be detrimental to new environments. For example, the puncture vine (Tribulus terrestris), native to the Sahara Desert, may have been introduced into North America on the tires of military aircraft and vehicles returning from Europe after World War I (Cox, 1999). The brown tree snake (Boiga irregularis), native to New Guinea and neighbouring areas, was accidentally introduced to the island of Guam in the late 1940s or early 1950s most likely in military shipments of fruit (OTA, 1993; Cox, 1999). This snake is an arboreal, nocturnal predator of birds, their eggs and young that has nearly eliminated all native forest bird species. Black rats were introduced to the Midway Islands by navy ships during World War II where they drove many species of wildlife to extinction (Cox, 1999). A desert shrub, African rue (Peganum harmala) was apparently introduced inadvertently into New Mexico and Texas at World War II airfields (Cox, 1999). The agricultural pests, witchweed (Striga asiatica) and the golden nematode (Globodera rostochiensis) are also believed to have entered North America on returning military equipment (OTA, 1993; Cox, 1999). Ceratocystis fimbriata, a fungus that causes canker stain of plane trees (Platanus spp.) or platanus disease, was introduced from the United States to several Southern European ports at the end of the Second World War and subsequently spread through Italy and France (EPPO, undated). Heterobasidion annosum, a root rot of pine trees, was inadvertently introduced into Italy by American troops during World War II where it has resulted in an unprecedented mortality rate of stone pines (Pinus pinea) on the Presidential Estate of Castelporziano (Pilcher, 2004). It is believed that the pathogen was transferred in transport crates, pallets or other military equipment made from untreated lumber from infected trees. Emergency relief, reconstruction efforts, and humanitarian assistance after wars and disasters may also contribute to the introduction and spread of alien invasive species. Though little information is available, particularly in regards to pests of forests and forestry, foreign food aid has been accused of introducing agricultural pests into a number of African countries such as the larger grain borer (Prostephanus truncatus), unintentionally introduced into the United Republic of Tanzania in a food aid shipment in 1979 (FAO, 2001a). Regulatory regimes A country’s lack of regulatory regimes, including resources for prevention and enforcement measures (i.e. a sustainable institutional framework) as well as attitudes and views regarding risks, make it more vulnerable to invasions. Regulatory systems for managing alien invasive species are heavily dependent on the actions of both the government and private sectors and the effectiveness of such systems is determined by the level of resources that governments can provide as well as the technical capacity that exists nationally (FAO, 2001a). Countries vary in their tolerance of the risk of alien species. Since countries are linked to others by the transboundary movement of people, goods and services, the level of control applied by one country will in turn affect the vulnerability of other countries (Perrings et al., 2002). Regulatory systems can also break down, or when faced with new challenges in alien species control can become inadequate to respond effectively either because of systemic deficiencies or because safety measures are evaded (FAO, 2001a). Tourism With approximately 650 million tourists crossing international borders every year, the opportunities for the introduction and spread of alien invasive species is profound and increasing (McNeely et al., 2001). Travellers can intentionally transport living plant and animal species that can become invasive or they can carry fruits and other living or preserved plant materials that contain potentially invasive insects and diseases that can have profound effects on agriculture, forestry and other sectors (McNeely et al., 2001). Travellers themselves can also be the vectors for parasites and diseases between countries. Parks and protected areas have experienced increased biological invasions due to human activities (Mooney and Hofgaard, 1999). 6

Biological control of pests Another source of alien invasive species is the intentional importation and release of insects, snails, plant pathogens and nematodes for biological control of pests. Such species can escape into other unintended areas and become pests themselves. The United States Office of Technology Assessment (OTA) (1993) noted that of a total of 722 non-indigenous species imported for biological control, 237 species had become established in the United States, some of which have become detrimental. Public health and environmental concerns Concerns about the effects of pesticides on the environment and human health can also promote the spread of alien invasive species by allowing such species to spread unchecked (FAO, 2001a).

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4. Impact of land use change and the forest sector on the introduction and spread of alien invasive species Land use change has obvious direct effects on the world’s ecosystems through habitat destruction and alteration of competitive relationships but also indirect effects through alteration of fire frequency and nutrient and water balances (Mooney and Hofgaard, 1999). Land use change is a major contributor to the introduction and spread of alien invasive species which, in turn, are the second most important threat to biodiversity, behind habitat destruction. Activities of the forest sector can contribute to the introduction and spread of alien invasive species through forest utilization practices and the intentional introduction of species for commercial forestry, agroforestry and other purposes. Trade in forest products is also a contributor as was discussed in the section on trade in Chapter 3. Forest utilization, in particular practices such as timber harvesting, extraction of non-timber products, the construction of logging and transport roads and facilities for logging camps, and the conversion of natural forest to plantations, can have direct and indirect negative impacts on the ecological functions of forests and on forest biodiversity by promoting the invasion of alien species. For example, logging was found to be the prevailing factor influencing the establishment of alien plants areas in a national park in Madagascar and due to the dominance and persistence of the invasive species, the logged sites never recovered native species diversity (Brown and Gurevitch, 2004). As a result, the structure and diversity of the forest remained altered even long after logging operations has ceased. Clearcutting practices have also been shown to lead to the replacement of native ant communities with invasive nonindigenous species (Zettler et al., 2004). Forest roads provide essential access for timber extraction, management and monitoring of forest resources and as such are an important requirement for sustainable forest management and use. However when poorly designed and maintained, forest roads are often the cause of a variety of environmental problems associated with forest harvesting operations. Under some circumstances, forest roads may also initiate or accelerate the invasion of non-native species that ultimately displace native species. In addition, the increased levels of human activities in previously inaccessible areas, as facilitated by forest roads, cause many environmental problems including the possible introduction of alien species. Land use changes can promote the emergence or re-emergence of infectious diseases which degrade human health and that of other species (Morse, 1995; Vitousek et al., 1997; Bryan, 1999; Western, 2001). Infectious disease agents often, and perhaps typically, are alien invasive species since they are invaders over most of their range (McNeely et al., 2001). Activities of the forest sector contribute to the spread of infectious diseases in a variety of ways. Forest activities, such as clear-cutting and road building, may increase exposure of workers to infectious diseases such as human immunodeficiency virus (HIV), Ebola hemorrhagic fever, Marburg hemorrhagic fever, yellow fever, leishmaniasis, malaria and Ross River virus disease (Morse, 1995; Daszak, Cunningham and Hyatt, 2000; Chivian, 2002). The index case for the Ebola epidemic in the Democratic Republic of the Congo in 1995 is thought to be a charcoal maker that worked in the forest outside Kikwit (Sanchez et al., 1995). Logging can change the abundance, extent and quality of aquatic larval habitats for the Anopheles mosquito vectors of malaria and by disturbing the forest floor, creating depressions that catch and hold water, thus creating new sites for the development of more mosquitoes (Chivian, 2002). Deforestation can result in humans coming into closer contact with the vectors for such diseases as leishmaniasis, yellow fever, trypanosomiasis (both African sleeping sickness and Chagas disease), and 8

Kyasanur forest disease (Morse, 1995; Wilson, 1995; Chivian, 2001; Chivian, 2002; PAHO, 2004). Similarly, some animal reservoir hosts increase in abundance near the edges between forests and human settlements thus increasing the risk of human exposure to pathogens. The destruction of forest habitat may result in the removal, replacement or eradication of dominant vector species and sometimes, the replacement species are more effective vectors of disease as observed with loa loa (tropical eyeworm) and onchocerciasis (river blindness) (Chivian, 2002). Deforestation and desertification may also be accompanied by changes in the distribution of vectors such as ticks, blackflies, tsetse flies and Anopheles mosquitoes (Chivian, 2002). For example, forest degradation in the United States has led to the emergence of Lyme disease as a result of the loss of some predator and competitor species of white-footed mice which increased population densities of the mice in remnant forest fragments (Chivian, 2002). Higher population densities of the mice, which carry the bacteria (Borrelia burgdorferi) that cause Lyme disease, increase opportunities for ticks to acquire such infectious agents. Reafforestation activities can also affect the population dynamics of vectors and reservoirs which can promote the emergence of infectious diseases. Such activities in the United States and Europe have resulted in an increase in the population of deer and hence the deer tick, which is a vector of Lyme disease (Morse, 1995). The forest sector itself is a major source of alien invasive species. Many of the tree species used in commercial forestry in many parts of the world are alien or non-indigenous to the area. Hundreds of species have also been widely and successfully planted for a variety of purposes including afforestation, desertification and erosion control, and for the supply of fuelwood and other forest products. Such intentional introductions however, can bring about many unintended and costly consequences when these species escape cultivation and invade natural ecosystems. Alien tree plantations can have negative effects on the biodiversity and water resources of afforested areas (van Wilgen et al., 2001). Introduced species that spread from plantations to natural and semi-natural areas, and also into areas set aside for conservation and water production, have considerable impacts on ecosystem properties and functions (de Wit, Crookes and van Wilgen, 2001). In 2000, forest plantations covered 187 million hectares (FAO, 2000b). Pinus spp. (20 percent) and Eucalyptus spp. (10 percent) were the dominant species planted worldwide, followed by Hevea spp. (5 percent), Acacia spp. (4 percent), Tectona spp. (3 percent), other broadleaved species (18 percent), and other coniferous species (11 percent); the remaining percentage was unspecified. Globally, broadleaves made up 40 percent of forest plantation area with Eucalyptus the principal genus while coniferous species made up 31 percent of which Pinus was the principal genus; the remaining percentage was unspecified. Among the most widely planted trees, Acacia and Pinus species are the most prominent on weed lists and in reviews of invasive tree species (Richardson, 1999). This prominence is perhaps due to the fact that many species in both genera are highly capable of adapting to a wide variety of environments. Richardson and Rejmánek (2004), in a summary of data on conifer taxa from 40 countries, found that 80 alien taxa were known to be naturalized (13 percent of species) and 36 taxa were known invasives (6 percent). Twenty-eight of these known alien invasive conifer taxa belong to one family (Pinaceae) and 21 of these to one genus (Pinus). The Pinaceae has the highest proportion of invasive species than any other angiosperm family comprising mainly trees and shrubs. Considerable attention has been given to the effects of alien invasive trees from plantations in the southern hemisphere, in particular in South Africa. Plantations cover over 1.5 million ha of land area in South Africa and are an important part of the economy contributing US$300 million to the gross domestic product (GDP) and employing over 100 000 people (Richardson, 1998; FAO, 2000b). The forest sector has been one of the country’s major sources of alien species infestation; a large proportion (38 percent) of the area invaded by woody alien plants in South Africa is occupied by species used in commercial forestry, in particular Pinus and Acacia species (Richardson, 1998). Rouget et al. (2002) analyzed the distribution of forestry plantations and 9

invasive stands of Acacia mearnsii and Pinus spp. in South Africa and discovered that these two taxa account for 60 percent of the area under commercial plantations and 54 percent of the area invaded by alien trees and shrubs. Black wattle (Acacia mearnsii), introduced into South Africa from Australia in the mid-19th century, has been widely planted in the country and forms the basis of a small but significant industry. A highly invasive species, A. mearnsii has spread to an area of 2.5 million ha in South Africa where it negatively affects water resources, biodiversity, and the stability and integrity of riparian ecosystems (de Wit, Crookes and van Wilgen 2001). Due to their ability to fix atmospheric nitrogen, Australian Acacia species (A. cyclops and A. saligna), have radically altered nutrient-cycling regimes in nutrient poor fynbos ecosystems in South Africa (van Wilgen et al., 2001). The invasion of these species and some Pinus species into large areas of the tree-poor fynbos have caused many ecosystem level changes by altering factors such as biomass distribution, plant density and vegetation height, leaf-area index, litterfall and decomposition rates (van Wilgen et al., 2001). Such changes radically alter habitats resulting in major changes in the distributions of many animals. Invasive alien pines and acacias in South Africa can also reduce water tables (McNeely et al., 2001). Many countries are also reporting significant problems with introduced eucalypts from Australia which can be particularly harmful environmentally because their leaf litter contains chemical compounds that prevent other species from growing (McNeely et al., 2001). This can have significant consequences for the forest sector. Other notorious examples of invasive trees that negatively affect ecosystems include melaleuca and black cherry. The melaleuca or paper bark tree (Melaleuca quinquenervia), native to Australia, was introduced from South America into southern Florida in the United States as an ornamental in the early 1900s. Decades later, the population began to expand and the species is currently spreading at a rate of 11 000 ha a year throughout forests and grasslands of Florida’s Everglades National Park where it damages native vegetation and wildlife (van Wilgen, Cowling and Burgers, 1996; Crooks and Soulé, 1999; Pimentel et al., 2000; McNeely, 2004). The North American black cherry, Prunus serotina, was initially introduced into central Europe as an ornamental and over the years had been planted to produce valuable timber and to function as windbreaks, firebreaks and shelter for game (Starfinger et al., 2003). By the mid 1980s, the species was considered an aggressive invasive species, particularly in Germany, that formed dense thickets which threatened plant diversity and natural regrowth of indigenous forest tree species. Of the 2000 or so species used in agroforestry perhaps 10 percent are invasive (Richardson, 1999). While only one percent is considered highly invasive, these include many frequently planted species such as Casuarina glauca, Leucaena leucocephala and Pinus radiata (McNeely, 2004). The spread of introduced species from agroforestry plantings however are less of a threat than those from commercial forestry plantations since plantations are typically planted in much greater numbers over larger areas, afforested areas often adjoin natural vegetation, and spreading agroforestry trees will likely be held in check by agricultural practices and human use (Richardson, 1999). With the increase in afforestation and land use changes over the past few decades, the magnitude of such detrimental consequences has increased significantly (Richardson, 1998; de Wit, Crookes and van Wilgen, 2001). Generally, the species that cause the greatest problems are those that have been planted most widely and for the longest time and the areas most affected tend to have the longest histories of intensive planting (Richardson, 1998).

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5. Positive impacts of introduced woody species on forests and forestry As mentioned in the previous chapter, the forest sector (and other sectors) often depends on introduced species to provide a variety of socio-economic, environmental and human health benefits to the forest sector and to the rural communities that depend upon forests. Many introduced species are highly regarded because of their exceptional adaptability to a wide variety of sites, their rapid growth, and the multiple uses of their products. With growing concerns about the degradation and loss of natural forests, planted forests and trees outside forests, composed most often of introduced woody species, are becoming increasingly more important sources of products such as timber, fibre and fuelwood. In developing countries, fuelwood is the prime source of energy, representing over 80 percent of the wood harvest (FAO, 1999). They also provide nonwood forest products, such as fruits, leaves, roots, honey, fibres, oils, resins, cosmetics and medicines, either from the planted trees themselves or from other elements of the ecosystem that they help to create. Such products contribute to the livelihoods of rural communities by providing food, medicine and employment as well as income from their sale. In countries with low forest cover, planted forests and trees outside forests constitute the main source of wood and non-wood forest products. In addition to providing valuable forest products, alien tree species planted in forest plantations and other areas help provide many vital ecosystem services such as: ƒ combating desertification; ƒ protecting soil and water; ƒ rehabilitating lands exhausted from other land uses; ƒ diversifying the rural landscape; ƒ maintaining biodiversity; ƒ enhancing carbon sequestration; ƒ providing amenity and shade. When planted in riparian areas, trees provide spawning beds for fish and molluscs and shade which aids in the reduction of eutrophication. Trees planted on farms help to increase soil fertility by providing organic matter through litter decomposition at the soil surface or through atmospheric nitrogen fixation (nitrogen-fixing trees), both of which contribute to improvements in food production. Along roads and highways, trees and plants not only add beauty to cities and towns but also provide shade and control outdoor noise and traffic pollution. Trees also play a major role in preserving the social and cultural values attached to forests, particularly as natural forests decrease in size through deforestation or are designated for other purposes. The forest sector also employs introduced species in biological control programmes to help combat pest problems with considerable success (McNeely, 1999). Practical reasons for using introduced species instead of native species in the forest sector include the following (Richardson, 1998; Richardson, 1999). ƒ Introduced trees often grow much faster than native species, particularly in the first years after planting. ƒ Native species are more difficult to manage silviculturally, in part because the biology of these species is often poorly known including information on how to collect, store and germinate seeds, how to produce seedlings in a nursery and how to manage them in a forest. Introduced species, on the other hand, have been well studied in a variety of settings. ƒ Seeds from introduced species are more readily available and easier to handle than seeds of native tree species. 11

ƒ In order to improve the trade balance by reducing the need for imported wood products, it is often necessary to develop local forest industries. Knowledge of markets and manufacturing technology is more readily available for introduced species such as pines and eucalypts. Positive impacts may be best achieved by careful selection of species and risk assessment prior to introduction and large-scale planting, coupled with management of introduced species in order to prevent them from becoming invasive.

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6. Negative impacts of alien invasive species on forests and forestry Alien invasive species negatively impact the forest sector in economic, ecological and environmental, and social and health terms, though these impacts are almost never mutually exclusive. Economic Alien invasive species generate substantial costs to the forest sector in lost revenues, in expenses for their control and in lost conservation values and ecosystem services. Alien invasive species, in particular insect pests and diseases, can damage trees in all stages of development and affect the ability of both natural and planted forests to meet their management objectives (FAO, 2001b). The most direct economic impact of alien invasive species on the forest sector is related to the loss or reduced efficiency of production. Approximately US$4.2 billion in forest products are lost each year to alien insect pests and pathogens in the United States (Pimentel et al., 2000). In Canada, the damage resulting from past introductions of harmful invasive plant pests on agricultural crops and forestry has been estimated at CAD$7.5 billion annually and in the Canadian province of Manitoba alone, economic losses due to Dutch elm disease (Ophiostoma ulmi sensu lato) have been estimated at approximately CAD $30 million (Environment Canada, 2004). The detection of Asian longhorned beetle (Anoplophora glabripennis) in Canada poses a significant threat to both the hardwood products industry and the maple syrup industry, whose products were valued in 1997 at CAD$480 million and CAD$130 million respectively (Environment Canada, 2004). Such infestations of alien invasive species directly affect the quantities of forest products demanded or supplied thereby impacting global prices and markets (FAO, 2001a). Although quantitative estimates of the economic impacts are not readily available for other countries, alien invasive species no doubt significantly impact productivity. Possibly outweighing direct production costs, the introduction and spread of alien invasive species can have major implications for trade which will depend on the policy response of trading partners to news about outbreaks, the importance of the traded commodities, the extent of the damage, and the demand and supply elasticities (FAO, 2001a). In addition to these direct production and trade costs, the associated control costs, including the costs of inspections, monitoring, prevention, and response, of even just a few species can be enormous. The United States Forest Service currently spends approximately US$11 million annually on control of the gypsy moth (Lymantria dispar), a major pest of forest and ornamental trees (Pimentel et al., 2000). Removal of elm trees affected by Dutch elm disease (Ophiostoma ulmi sensu lato) costs approximately US$100 million per year (Pimentel et al., 2000). In South Africa, the cost of controlling alien plant invasions has been estimated to be approximately US$1 200 million although some have challenged this estimate (Nyoka, 2003). Ecological and environmental The full economic costs of invasions include not only the direct damage and control costs but also the effects on the ecosystems themselves. The ecological and environmental impacts of alien invasive species can be felt by all levels of organization including the gene, species, habitat and ecosystem level. Genes. If introduced or spread into habitats with closely related species, alien invasive species could interbreed with native species resulting in changes to the genetic makeup of either species (Secretariat of the Convention on Biological Diversity, 2003a). The possible negative consequences of such alterations include reduction in the survival of either species, creation of a more successful invader, or the creation of hybrids that could be more susceptible to certain pests and pathogens. Of recent concern to the forest sector is the 13

impact of possible introduction of new tree genotypes (non-local provenances or genetically improved planting stock) resulting in the creation of hybrids and the resulting loss of gene pools that may have acquired specific characteristics through local adaptation (FAO, 2005). The issue has not been extensively studied in forest trees, except possibly in the European black poplar (Populus nigra) (Cock, 2003). Species. Alien invasive species can influence species diversity, richness, composition and abundance. At the species level, direct effects of alien invasive species occur through processes such as the predation of, competition with, and pathogen and parasite transmission to individual organisms, eventually leading to population declines and species extinctions (Loehle, 2003; Secretariat of the Convention on Biological Diversity, 2003a). Through direct impacts on species or through alterations of habitats, alien invasive species are responsible for placing 762 forest species at risk (IUCN, 2005). The loss of such species is leading to a more homogenous world which is perhaps the biggest threat to global biological diversity, behind habitat loss (Perrings, Williamson and Dalmazzone, 2000; McNeely et al., 2001; Richardson and Rejmánek, 2004). Some examples of species level impacts in forests include the following. ƒ Miconia (Miconia calvescens), a tropical American tree introduced to French Polynesia in 1937, has significantly altered the forests of French Polynesia and other Pacific islands by shading out all native plants and promoting erosion and landslides with its shallow roots (Denslow, 2002). ƒ The invasive weeds, Scotch broom (Cytisus scoparius) and gorse (Ulex europaeus), are repressing the regrowth of the commercially important Douglas fir (Pseudotsuga menziesii) in British Columbia, Canada (FAO, 2000a). ƒ The Asian shrub, Amur honeysuckle (Lonicera maackii), has invaded many eastern North American areas where it impacts the natural regeneration of forests by reducing tree seedling density and growth and fecundity of perennial forest herbs (Gorchov and Trisel, 2003; Miller and Gorchov, 2004). Habitats. Through their impacts on species and ecosystem processes, alien invasive species can result in the fragmentation, destruction, alteration or complete replacement of habitats which in turn, has cascading effects on even more species and ecosystem processes (McNeely et al., 2001; Secretariat of the Convention on Biological Diversity, 2003a). Some examples of these impacts in forests include the following. ƒ Alien invasive diseases and pests have caused major changes in the composition of forests in eastern North America over the past century, including the decline of species like chestnut, elm and hemlock (McNeely et al., 2001). ƒ Kizlinski et al. (2002) investigated the direct impacts of an invasion of the hemlock woolly adelgid (Adelges tsugae) and the indirect impacts of the removal of the infested trees in hemlock-dominated forests and found profound changes in the structure, composition and ecosystem function. The loss of hemlock trees in these forests has also resulted in significant changes in the composition and distribution of bird populations (Tingley et al., 2002). ƒ When alien invasive insect species threaten native insect species, they can also have cascading effects on insectivorous birds and on plants that rely on insects for pollination or seed dispersal (McNeely et al., 2001). Ecosystems. The impacts of alien invasive species at the ecosystem level include changes to trophic structures, changes in the availability of resources such as water and nutrients, and changes in the disturbance regimes (McNeely et al., 2001; Secretariat of the Convention on Biological Diversity, 2003a). Some examples of the impacts of alien invasive species on forest ecosystems include the following. ƒ Invasive alien plants and trees have decreased water supplies for nearby communities and increased fire hazards in South Africa (McNeely et al., 2001; van Wilgen et al., 2001; Petit et al., 2004). ƒ Invasive grasses that are particularly fire-prone may lead to a permanent loss of forests (Mooney and Hofgaard, 1999). ƒ Australian Acacia species, such as A. cyclops and A. saligna, have radically altered nutrient cycling regimes in nutrient poor ecosystems due to their ability to fix atmospheric nitrogen (van Wilgen et al., 2001).

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Social and health The full costs of invasions also include the social and health impacts of alien invasive species on humans, in particular to the rural communities depending on forests. As a result of the negative impacts of alien invasive species on native forest biodiversity, a loss of food sources and traditional medicines may be experienced thereby compromising not only the health of local people but also the livelihoods of those dependent on the collection and sale of such items for income. For small-scale landowners, alien invasive species can also decrease the value of their land. Forest workers, as part of their jobs, and people living in and around forests are more exposed to alien invasive species such as the reservoirs and hosts of many emerging infectious diseases. Examples of such diseases include Lyme disease, human immunodeficiency virus (HIV), Ebola and Marburg hemorrhagic fevers, malaria, yellow fever, leishmaniasis, trypanosomiasis and Kyasanur forest disease (Morse, 1995; Sanchez et al., 1995; Wilson, 1995; Daszak, Cunningham and Hyatt, 2000; Chivian, 2001; Chivian, 2002; Cinco et al., 2004). Chapter 4 has a more detailed discussion of the connections between the forest sector, emerging infectious diseases and human health. People living in and around invaded forest areas may also suffer allergic or other negative reactions to the alien invasive species themselves or to the measures used to control them such as chemical and biological pesticides. A commonly planted tree for land restoration and as a source of forest products, mesquite (Prosopis juliflora) is a major cause of allergies in India, Kuwait, Mexico, Saudi Arabia, South Africa, the United Arab Emirates and the southwestern United States (Killian and McMichael, 2004). Sensitivity to mesquite pollen has been shown to result in asthma, rhinitis and conjunctivitis (Killian and McMichael, 2004). In Mongolia, children living close to areas infested with Dendrolimus sibiricus have experienced significant allergic reactions to the hairy caterpillars that have entered their homes. The hairs on larvae and egg masses of gypsy moth (Lymantria dispar) also cause allergies in some people (Allen, Miller and Tyler, 1991; ISSG Global Invasive Species Database, 2005). In the United States, forest workers working in areas heavily infested with the tussock moth caterpillar (Orgyia pseudotsugata) experienced itching of the skin and eyes, nasal discharge, cough and respiratory difficulty (Press et al., 1977). Estimating the costs of alien invasive species Estimates of the full costs of biological invasions are rare because of the difficulty in approximating the costs of a problem with so many components, many of which are difficult if not impossible to quantify such as the impacts of alien invasive species on biodiversity, ecosystem functions, human health and other indirect costs such as the impacts of control measures. No estimates of the costs to the forest sector on a global scale have been made. A few attempts at estimating the costs of alien invasive species have been made. ƒ Pimentel, Zuniga and Morrison (2005) estimates that the 50 000 alien species in the United States cost almost US$120 billion in environmental damages and losses yearly. Pimentel et al. (2000) gave an estimate of US$137 billion per year. ƒ Pimentel et al. (2001) looked at over 120 000 alien species of plants, animal and microbes that have invaded Australia, Brazil, India, South Africa, the United Kingdom and the United States causing significant economic losses in the agriculture and forest sectors and negatively affecting ecosystems. They estimated that the total cost in the six countries was US$314 billion in damages per year - Australia ($13 billion), Brazil ($50 billion), India ($116 billion), South Africa ($7 billion), the United Kingdom ($12 billion) and the United States ($116 billion). ƒ OTA (1993) concluded that about 4 500 exotic species occur in the United States and that about 20 percent of them have caused serious economic and environmental harm. The cumulative loss caused by 79 of these species was estimated at almost US$97 billion for the period 1906 to 1991.

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While these estimates do not take all components into account, they nonetheless illustrate the enormity of the costs of alien invasive species.

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7. Conflict species While many introduced species are highly regarded because of the benefits they can provide, these same species have in some cases become serious threats to forests and the forest sector. Such conflict species are a considerable problem from a management perspective requiring a clear and unbiased analysis of the costs and benefits of their use. Some examples of conflict species include the following. ƒ Pinus and Eucalyptus species are the most important introduced species used in commercial forestry enterprises worldwide and most particularly in the tropics and subtropics (Richardson, 1998; Richardson and Higgins, 1998; FAO, 2000b). Many introduced commercial species such as rubber (Hevea brasiliensis), coconut (Cocos nucifera) and oil palm (Elaeis guineensi) are becoming important sources of wood and fibre (FAO, 2000b). Several of these alien forest trees have spread beyond the areas in which they were planted with devastating impacts. The main impacts are considered to be caused by reduced structural diversity, increased biomass, disruption of existing vegetation dynamics and altered nutrient cycling (Richardson, 1998; Richardson and Higgins, 1998). ƒ Many species of Australian Acacia have been introduced into the Cape Floristic Region of South Africa for timber, fuelwood and building materials (A. mearnsii); for tannins which are used by leather industries (A. saligna, A. mearnsii); and for sand stabilization (A. cyclops, A. saligna) (McNeely, 1999; de Wit, Crookes and van Wilgen, 2001; Matthews and Brand, 2004). Such species have radically altered habitats for wildlife resulting in major changes in the distribution of species, particularly birds, and nutrient cycling regimes in nutrient poor ecosystems due to their ability to fix atmospheric nitrogen (van Wilgen et al., 2001). They have also decreased water supplies for nearby communities and increased fire hazards (McNeely et al., 2001; van Wilgen et al., 2001; Petit et al., 2004). ƒ Leucaena leucocephala has been widely introduced as a source of timber, fuelwood, fodder and shade and is also used to restore degraded lands, improve soils and stabilize sands. Leucaena is a fast-growing, nitrogenfixing tree that is tolerant of arid conditions and saline soils and as such is highly regarded in arid regions in Asia and Africa (Matthews, 2004; Matthews and Brand, 2004). In areas where it has been introduced however, this species tends to form dense impenetrable thickets and readily invades forest margins, roadsides, wastelands, riparian areas and agricultural lands (McNeely, 1999). Also, the toxicity of its seeds and foliage decreases its value as a source of fodder (McNeely, 1999). ƒ Prosopis juliflora, introduced 70 years ago in the Thar Desert of India, has dense green vegetation which is very useful in controlling soil erosion, reducing the aridity of the area, and providing a source of fuelwood as well as fodder and shelter for both wild and domesticated animals (McNeely, 1999). Such benefits however are being overshadowed by the negative impacts of this species. P. juliflora displaces native flora resulting in reduced biodiversity and reduced diversity of products available to rural communities (McNeely, 1999). Its dense impenetrable thickets also render invaded lands useless for agricultural purposes (Richardson, 1998). ƒ Salt cedar (Tamarix spp.), introduced from central Asia to the southwestern United States nearly 200 years ago to control erosion along river banks, now forms dense thickets on more than 400 000 ha of riparian habitat severely impacting hydrological systems (McNeely et al., 2001). ƒ Australian brush-tailed possums (Trichosurus vulpecula), introduced into New Zealand for a successful fur industry, have caused considerable damage to native forests by changing forest composition and structure through the defoliation and eradication of preferred food plants (McNeely, 1999).

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ƒ The flatworm Platydemus manokwari has been introduced into many areas where it successfully controls populations of another alien invasive species, the giant African snail, Achatina fulica (McNeely, 1999). Although successful as a biological control agent, P. manokwari is now considered a significant threat to native gastropod species, including rare and endemic species, in the areas where it was introduced. It is vital to ensure that such species serve the purposes for which they were introduced and do not escape to cause negative consequences on native ecosystems.

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8. Forest pest management options Two sets of complementary strategies for dealing with alien invasive species are: prevention and early detection; and response, which includes eradication (rarely achieved), containment, control and mitigation. Many agencies have been advocating the concept of biosecurity which is a strategic and integrated approach that encompasses the policy and regulatory frameworks (including instruments and activities) that analyse and manage risks in the sectors of food safety, animal life and health, and plant life and health, including associated environmental risk (FAO, 2003). It covers the introduction of insect pests and diseases, the introduction and release of genetically modified organisms and their products, and the introduction and management of alien invasive species and genotypes (FAO, 2003). Unless otherwise noted, the information presented in this section is taken from Wittenberg and Cock (2001). Prevention and early detection Prevention is the first line of defence against biological invasions and is also the most cost effective since once an alien invasive species becomes established, it is extremely difficult and hence costly to eradicate it. An important first step in prevention is identification of the species capable of becoming invasive, the possible susceptible sites and more importantly, the pathways in which they can be introduced. The more comprehensive approach of identifying pathways rather than individual species results in a greater concentration of effort where pests are more likely to enter a country which not only avoids wasting resources elsewhere but also helps in the identification of more species, vectors and pathways. Once pathways are identified then potential prevention tools and methods can be more specifically developed. The International Plant Protection Convention (IPPC) is an international treaty with a purpose to secure action to prevent the spread and introduction of pests of plants and plant products, and to promote appropriate measures for their control. This convention specifically governs transboundary movement of all plants and plant products (including forest products) and the scope is not limited to agricultural plants. It is governed by the Interim Commission on Phytosanitary Measures (ICPM) which adopts International Standards for Phytosanitary Measures (ISPMs). All of these ISPMs have direct relevance to the forest sector including guidelines for the export, shipment, import and release of biological control agents and other beneficial organisms (ISPM No. 3), guidelines on pest risk analysis (ISPM No. 2, 11 and 21), guidelines on pest eradication programmes (ISPM No. 9), guidelines on pest status and reporting (ISPM No. 8 and 17), and guidelines for regulating wood packaging materials in international trade (ISPM No. 15) (IPPC, 2005). Some of the important tools used to prevent the entry and establishment of alien invasive species include: ƒ public information and education; ƒ risk assessments and environmental impact assessments for intentional introductions; ƒ national and international regulations on prevention and quarantine measures and their enforcement with inspections and fees; ƒ treatment of imported commodities, including through fumigation, immersion, spraying, temperature treatment, ultraviolet sterilization, and pressure; ƒ technically justified trade restrictions; ƒ emergency measures or actions if a pest should be detected before establishment (McNeely et al., 2001). Early detection of alien species should be based on a system of regular surveys – general, site-specific or species-specific – to identify newly established species. Although not all alien species become invasive, the costs of those that do become invasive suggest that a precautionary approach to the issue is best. If alien species are identified early, the chances for eradication will be high in particular because for some invasive species there can be a long lag period between initial introduction and subsequent population explosion

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(Crooks and Soulé, 1999; Parker, 2004). The longer species go undetected the fewer the options for its control or eradication and the more expensive any intervention will become. The IPPC’s ISPM No. 5 provides the international standard to which general surveillance should be conducted for plants (IPPC, 2005). Early detection is highly dependent on the capacity of individuals to recognize both native and alien species (See Box 1). As a result, a large component of this step is training, not only of national professionals responsible for surveying but also for any persons that spends time in the natural environment such as farmers, gardeners, forest workers, ecologists, tourism workers, photographers and hikers. Trained professional national workers should be able to not only recognize native and alien species and the ecological effects of alien species but they also should be able to use databases, keys, manuals and other identification sources. Early warning systems which include lists and datasets of recorded or potentially invasive species, in given countries, time sets and conditions are important tools in this regard. Finally a contingency plan outlining the actions that should be taken once an alien invasive species has been identified or an invasion is suspected, should be developed. Box 1. Attributes of potentially invasive species and susceptible ecosystems Though much is known about the factors that contribute to the introduction and spread of alien invasive species, little is known about the attributes of alien invasive species and susceptible habitats. Isolated ecosystems, evolutionarily and geographically, are particularly vulnerable to biological invasions while deserts, semi-deserts, tropical dry forests and woodlands, and arctic systems appear to be the least susceptible (McNeely et al., 2001; Perrings et al., 2002). Ecosystems with low species diversity, especially if predators and competitors are absent, seem to be more susceptible than species-rich systems with well-established species interactions (McNeely et al., 2001; Perrings et al., 2002). However, such species-rich ecosystems may be at risk to a greater range of invaders because of their greater diversity of habitats (McNeely et al., 2001). Frequent disturbance, slow recovery rate and fragmentation of communities promote plant invasions (Rejmánek, 1999). Some ecological factors that may allow introduced species to spread include the following (Pimentel et al., 2000). ƒ Lack of predators, competitors and parasites. ƒ Ability of an alien parasite to switch to a new host. ƒ Ability to be an effective predator. ƒ Availability of artificial or disturbed habitats. ƒ High adaptability to novel ecosystems. ƒ Efficient dispersal. While the success of alien invasive species is often explained by simply placing an introduced species in a favourable environment, some have suggested that the lack of enemy pressure actually results in a reallocation of resources from defence, as it is no longer needed in enemy-free habitats, to growth and reproduction (Petit et al., 2004; Withgott, 2004). Others have suggested that this may not be the case for all circumstances and there is likely not a simple trade-off between defence mechanisms and growth (Withgott, 2004). Identification of the life-history traits of potentially invasive species has been attempted with some success, primarily for woody species. For conifer species, invasiveness is associated with: small mean seed mass (