Invasive Species
EcologyA non-native organism whose introduction causes or is likely to cause ecological or economic harm, outcompeting native species and disrupting ecosystems.
An invasive species is a non-native organism that, when introduced to a new environment, spreads aggressively and causes significant ecological, economic, or human health impacts. In the context of tree health, the term applies both to invasive trees that displace native vegetation and to invasive insects and pathogens that attack native trees.
Invasive trees like tree of heaven (Ailanthus altissima), Norway maple, and various species of buckthorn can dominate forest understories, prevent native tree regeneration, and alter soil chemistry. Meanwhile, invasive insects and diseases represent some of the greatest threats to American forests. The emerald ash borer has killed hundreds of millions of ash trees, Dutch elm disease decimated American elms, and chestnut blight virtually eliminated the American chestnut from its native range.
Prevention is the most cost-effective management approach for invasive species. Key prevention strategies include not moving firewood (which can carry invasive insects), purchasing nursery stock from reputable sources, reporting suspected new invasive organisms to state agriculture agencies, and avoiding planting known invasive species. For established invasive tree populations, management typically requires persistent removal efforts combined with native species replanting to restore ecosystem function.