Forest health

Forest invasive species

An integral part of sustainable forest management is 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 - invasive species. Indeed invasive species is considered to be "an alien species whose introduction and/or spread threaten biological diversity” (Source: CBD Glossary). The increasing global movement of people and products is also facilitating the movement of alien species around the 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 can prosper in new environments and spread at the expense of native species, affecting entire ecosystems.

Forest invasive species database Invasive species occur in all major taxonomic groups from micro-organisms to mammals. This database provides a sampling of invasive species that negatively affect forests and the forest sector. Dendroctonus micans. ©Flickr/Gilles San Martin

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. 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.

There is an overall lack of information on 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 invasive alien species. To know more on strategies to combat invasive species, click on the dedicated section Pest management strateg

Factors contributing to the introduction and spread of 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 invasive species include: land use changes including forest sector activities (see Forest sector impacts on invasive species); economics and trade; climate change and changes in atmospheric composition; tourism; conflict and reconstruction; regulatory regimes; biological control programmes; public health and environmental concerns.
a. 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. 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. 
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 invasive species. 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. Unprocessed wood, wood products and nursery stock are also a major source of forest pests and diseases.
In addition to the possibility of the commodities themselves carrying invasive species, the containers and vehicles in which they are transported can also facilitate invasions. Wood packaging materials 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. Containerized cargo can shelter alien species from microorganisms to reptiles and mammals and since inspecting such freight is very difficult and costly, many invasive species may enter a country undetected. 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.
b. Climate change and changes in atmospheric concentration
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. 
Climate change may produce more favourable conditions for invasive species. Once dominant species in native areas are no longer adapted to the environmental conditions of their habitat, it is likely that alien species will displace them thus drastically changing successional patterns, ecosystem function and resource distribution. 
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 invasive species establish more easily in habitats disturbed by human and other factors, such changes can provide more opportunities for them to invade. Climate change also affects the frequency and intensity of extreme climatic events which disturbs ecosystems and thus provides increased opportunities for dispersal and growth of invasive species. 
In addition to the effects of climate change, increasing concentrations of greenhouse gases can have significant effects on the success of invasive species as well. Higher concentrations of carbon dioxide increase photosynthetic rates and water use efficiency of plants and ecosystems and the resulting increased soil moisture has the potential to provide habitat for late-season annuals which may be invasive. Substantial changes in the species composition and dynamics of terrestrial ecosystems would be expected since not all plant species are affected by increased levels of CO₂ in the same way. Changes in the deposition of nitrates from the atmosphere through the burning of fossil fuels, has resulted in large changes in vegetation which may favour the growth of some invasive species.
c. Conflict
Conflict and civil unrest can contribute to the introduction and spread of invasive species as a result of:  the breakdown of phytosanitary and animal health controls and the loss of supply lines for materials; the displacement of large numbers of people and their belongings can be a dispersal mechanism for, or the source of, invasive species; the lack of inspections and border controls and the increased unregulated movement of military personnel and refugees; increased smuggling; border areas may be difficult to survey because of landmines and other hazards; military transport, equipment and supplies, often covered with dirt or mud from the field, can introduce invasive species into new environments; foreign food aid which may be contaminated with pests and diseases; emergency relief, reconstruction efforts, and humanitarian assistance after wars and disasters.
d. Regulatory regimes
A country's lack of regulatory regimes, including resources for prevention and enforcement measures as well as attitudes and views regarding risks, make it more vulnerable to invasions.
e. Tourism
With millions of tourists crossing international borders every year, the opportunities for the introduction and spread of invasive species is profound and increasing. 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. Travellers themselves can also be the vectors of parasites and diseases between countries.
f. Biological control programmes
Another source of 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.
g. Public health and environmental concerns
Concerns about the effects of pesticides on the environment and human health can also promote the spread of invasive species by allowing such species to spread unche

Impacts of the forest sector on the introduction and spread of invasive species
Activities of the forest sector can contribute to the introduction and spread of invasive species through forest utilization practices and the intentional introduction of species for commercial forestry, agroforestry and other purposes.
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.
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 exotic 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.
Forest sector activities can promote the emergence or re-emergence of infectious diseases which degrade human health and that of other species. Clear-cutting and road building may increase exposure of workers to infectious diseases such as human immunodeficiency virus (HIV), Ebola hemorrhagic fever, and Marburg hemorrhagic fever, yellow fever, leishmaniasis, malaria and Ross River virus disease. 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.
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 Kyasanur forest disease. 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). Deforestation and desertification may also be accompanied by changes in the distribution of vectors such as ticks, blackflies, tsetse flies and Anopheles mosquitoes.
Reforestation activities can also affect the population dynamics of vectors and reservoirs which can promote the emergence of infectious diseases.
The forest sector itself is a major source of 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. Alien 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.
    A. Positive impacts of introduced species on forests and forestry
The forest sector often depends on alien 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.
With growing concerns about the degradation and loss of natural forests, planted forests and trees outside forests, composed most often of alien species, are becoming increasingly more important sources of products such as timber, fibre and fuelwood. They also provide non-wood 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 and medicine as well as income from their sale or by providing employment.
Alien tree species planted in forest plantations and other areas help provide many vital ecosystem services such as: combating desertification; protecting soil and water; rehabilitating lands exhausted from other land uses; diversifying the rural landscape; maintaining biodiversity; enhancing carbon sequestration; 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 helps 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 valuesattached to forests, particularly as natural forests decrease in size through deforestation or are designated for conservation or other purposes. The forest sector also employs introduced species in biological control programmes to help combat pest problems.
Positive impacts may be best achieved by careful management of introduced species in order to prevent them from becoming invasive.
    B. Negative impacts of invasive species on forests and forestry
Invasive species negatively impact the forest sector in economic, ecological, environmental, social and health terms.
Economic
The most direct economic impact of invasive species on the forest sector is related to the loss or reduced efficiency of production. 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. The introduction and spread of 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. In addition, the associated control costs, including the costs of inspections, monitoring, prevention and response, of even just a few species can be enormous. Invasive species can also generate substantial costs to the forest sector in lost conservation values and ecosystem services.
Ecological and environmental
The ecological and environmental impacts of 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 species could interbreed with native species resulting in changes to the genetic makeup of either species which could result in a 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.
Species. Invasive species can influence species diversity, richness, composition and abundance. At the species level, direct effects of 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.
Habitats. Through their impacts on species and ecosystem processes, 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.
Ecosystems. The impacts of 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 regime of an ecosystem.
Social and health
As a result of the negative impacts of 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, 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 invasive alien species such as the reservoirs and hosts of many emerging infectious diseases such as Lyme disease, human immunodeficiency virus (HIV), Ebola and Marburg hemorrhagic fevers, malaria, yellow fever, leishmaniasis, trypanosomiasis and Kyasanur forest disease. People living in and around invaded forest areas may also suffer allergic or other negative reactions to the invasive species themselves or to the measures used to control them such as pesticides and biological controls.

While many alien 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.
-- Pinus and Eucalyptus species are the most important introduced species used in commercial forestry enterprises worldwide and most particularly in the tropics and subtropics. Many other introduced species such as rubber (Hevea brasiliensis), coconut (Cocos nucifera) and oil palm (Elaeis guineensi) are becoming important sources of wood and fibre. 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 shifts in life-form dominance, reduced structural diversity, increased biomass, disruption of existing vegetation dynamics and altered nutrient cycling.
-- Many species of Australian Acaciahave been introduced into the Cape Floristic Region of South Africa for timber, fuelwood and building materials ( mearnsii); for tannins which are used by leather industries (A. saligna; A. mearnsii); and for sand stabilization (A. cyclops; A. saligna). 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. They have also decreased water supplies for nearby communities and increased fire hazards. 
-- Leucaena leucocephalahas been widely introduced as a source of timber, fuelwood, fodder and shade and is also used to restore degraded lands, improve soils and stabilize sand. Leucaena is a fast-growing, nitrogen-fixing tree that is tolerant of arid conditions and saline soils and as such is highly regarded in arid regions in Asia and Africa. 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. Also, the toxicity of its seeds and foliage decrease its value as a source of fodder.
-- Prosopis juliflora, introduced 70 years ago in the Thar Desert of India, has very 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. Such benefits however are being overshadowed by the negative impacts of this species.  julifloradisplaces native flora resulting in reduced biodiversity and reduced diversity of products available to rural communities. Its dense impenetrable thickets also render invaded lands useless for agricultural purposes.
-- Salt cedar (Tamarixspp.), introduced from central Asia to the southwest 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 and are having severe impacts on hydrological systems. 
-- Australian brush-tailed possums (Trichosurus vulpecula), introduced into New Zealand for a successful fur industry, have caused considerable damage to native forests, changing forest composition and structure through the defoliation and eradication of preferred food plants.
-- The flatworm Platydemus manokwarihas been introduced into many areas where it successfully controls populations of another invasive species, the giant African snail, Achatina fulica. Although successful as a biological control agent,  manokwari is now considered a significant threat to native gastropod species, including rare and endemic species, in the areas where it was introduced.