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:
•land use changes including forest sector activities (see Chapter 4);
• economics and trade;
• climate change and changes in atmospheric composition;
• conflict and reconstruction;
• regulatory regimes;
• biological control of pests;
• 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 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 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).
• 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.
• 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.
• Displaced people and their belongings can be a dispersal mechanism for, or the source of, alien invasive species.
• Increased smuggling can relocate alien species to new regions.
• Inflows of food aid may be contaminated with pests and diseases.
• Difficulties in obtaining access to border areas because of landmines and other hazards make these areas difficult to survey.
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).
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).
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).
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).