Invasive species: impacts on forests and forestry
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;
- conflict and reconstruction;
- regulatory regimes;
- biological control programmes;
- public health and environmental concerns.
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.
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.
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.
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.
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.
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.
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 unchecked.