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.
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 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.
• 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).
• 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).
• 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.
• 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).
• 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).
• 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.
• 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).
• Invasive grasses that are particularly fire-prone may lead to a permanent loss of forests (Mooney and Hofgaard, 1999).
• 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).
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.
• 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.
• 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).
• 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.
While these estimates do not take all components into account, they nonetheless illustrate the enormity of the costs of alien invasive species.