What makes a species invasive
Not every non-native species is a problem. Many introduced plants and animals stay in gardens and farms or settle into the landscape without causing noticeable harm. A species is generally called invasive when it is not native to the area, spreads on its own, and causes ecological or economic damage. Some native species also behave aggressively after a disturbance, but the term invasive is normally reserved for introduced ones.
Invaders can include plants, insects, other animals, fungi and microbes. Forests are affected by all of these.
How invaders arrive and spread
Most arrive through human activity, whether deliberately or by accident. Common pathways include:
- Ornamental and garden plants that escape cultivation and spread into nearby woods.
- Shipping and trade. Insects can travel in wood packaging, crates and pallets, in cut firewood, on nursery stock and with imported timber. Seeds and soil on equipment or in ballast also move around.
- Firewood moved by campers and homeowners, which can carry insects and fungi to new regions.
- Vehicles, boots and machinery that carry seeds and soil along trails, roads and logging tracks.
- Deliberate introductions of animals for food, fur, hunting or pest control that later cause unintended problems.
Once established, roads, trails, streams, birds and other animals that eat fruit and spread seeds, and disturbed ground can help invaders expand.
Why invaders can be so damaging
A newly arrived species is often separated from the predators, parasites and diseases that kept it in check at home. Native species, meanwhile, may have no history with the newcomer, so they have little or no defense against it. This mismatch helps explain why introduced pests and diseases can be much more destructive than similar native ones, a contrast with the outbreaks of native insects described in the article on insect pests. How trees defend themselves is partly the product of long shared history with specific enemies, and that history is missing when a pathogen arrives from elsewhere.
Invasive plants
Non-native plants are among the most visible invaders in forests, especially shrubs, vines and herbs.
- Shading and crowding. Dense shrub layers or vines can shade out native wildflowers and tree seedlings, reducing the next generation of trees. Over time, a stand may age without being replaced.
- Smothering. Some vines climb into and overload tree crowns or cover the ground.
- Altered soil and chemistry. A few invaders change soil nutrient cycling, leaf litter quality or the fungal partners of tree roots. Because many trees rely on mycorrhizal fungi, plants that disrupt those relationships may impair native tree establishment, although research findings vary by species and site.
- Changed fire patterns. Certain invasive grasses can increase fuel and carry fire into forests that rarely burned, as has been documented in some regions. In such cases invasion and fire reinforce each other.
- Edge effects. Invaders often start along roads, trails and clearings, then work inward. Forests that are fragmented tend to have more edge, so they are generally more vulnerable. This is one of the ways forest fragmentation affects ecosystems.
Invasive animals
Introduced mammals can browse seedlings, strip bark or root up soil. Non-native earthworms have changed the leaf-litter layer in some northern forests where no earthworms had lived since the last glaciation, with consequences for the understory plants that depend on a thick litter layer. Introduced predators can reduce native birds and small animals, which is a particular concern on islands. Nonnative insects range from borers that kill trees to sap-feeding species that weaken them.
Invasive diseases
Some of the most dramatic forest losses on record involved introduced fungi and other pathogens. Well-known examples include chestnut blight in North America, Dutch elm disease affecting elms across several continents, and sudden oak death, caused by an introduced water mold. In each, a widely distributed native tree species with little resistance was reduced across large areas. The common pattern is a pathogen arriving from another region, finding susceptible hosts and spreading faster than natural defenses or management could respond. The aftermath can reshape habitat, food supplies for wildlife and the structure of the whole forest.
Ecological and human consequences
The effects of invaders vary, but frequently reported outcomes include:
- loss of native plant and animal diversity, contributing to wider forest biodiversity loss
- loss of important tree species, with knock-on effects for the wildlife that depends on them
- changes in soil erosion, water flow and nutrient cycling
- lower timber value, higher management costs and reduced recreational use
- loss of non-timber products and cultural resources tied to particular native plants
Climate change may favor some invaders by altering the range of conditions where they can survive, and stressed forests can be easier to invade. Not every invasion is permanent or catastrophic, though; some species spread widely and then decline, and effects can differ greatly from one forest to the next.
Control options
Management usually follows a rough order of effectiveness: prevention, early detection and rapid response, then long-term control.
Prevention. It is almost always cheaper to keep an invader out than to remove it. Measures include inspecting and regulating imports and wood packaging, discouraging movement of untreated firewood, cleaning boots and equipment, and avoiding planting known invasive ornamentals.
Early detection and rapid response. Small, new infestations can often be eliminated if found quickly. Trained staff, landowners and volunteers all contribute, and citizen science programs help with reporting.
Physical control. Hand-pulling, cutting, mowing, girdling and repeated removal can work for small infestations, though many plants resprout or regrow from seed banks, so follow-up for several years is common. Controlling invasive plants on small properties is a particular task for private owners.
Chemical control. Herbicides are used in many programs, sometimes applied directly to cut stumps or leaves to limit effects on other plants. Use should follow label directions and local regulations, and should consider water and non-target species.
Biological control. Releasing a natural enemy from the invader’s home range can reduce some pests over time. Because introducing another species carries its own risks, programs usually involve extensive testing and regulatory approval.
Restoring native cover. Removing an invader without replanting or encouraging native vegetation often leaves room for it, or another invader, to return. Follow-up planting or natural regeneration of native species is typically part of a lasting solution.
Breeding and resistance. For introduced diseases, efforts to breed or identify resistant trees, sometimes through crossing with related species, have been pursued in a few cases. Results take many years and are not guaranteed.
Realistic expectations
Eradication is possible in some cases, particularly when a population is small and caught early, but many established invaders can only be contained or reduced. Managers often set priorities according to which invaders pose the greatest threat and which sites are most valuable. For anyone working with a particular forest, local agencies, extension services and native plant groups are the best source of species-specific advice.