Threats to Birds - Introduced Species

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Between 3,500 - 4,000 species of animals and plants in the U.S. are alien or exotic. In other words, these species did not evolve here. While many of these species are beneficial to people and provide food for human consumption, pets for companionship, plants for landscaping, erosion control, and other uses, at least 400 species are considered problematic for wildlife or the environment. Some species were deliberately introduced; others came accidentally with shipments of goods from other countries or in ballast waters. Lacking predators from their place of origin or other natural controls on their populations, these species have become invasive and directly threaten the ability of our native plants and animals to survive.

The magnitude of the problem is truly immense. It is estimated that invasive plants in all 50 states have filled more than 100 million acres, and are spreading at the alarming rate of 14 million acres/year, and 4,300 acres/day on public lands alone. This “biological pollution” may cause $130 billion worth of damage/year. Invasive species are considered the #1 threat to the nation’s National Wildlife Refuge (NWR) system–a system that often provides the last remaining habitat for rare plants and animals. Invasive species affect at least 540 NWRs and 3,000 waterfowl areas on 95 million acres in the U.S. & territories managed by the U.S. Fish and Wildlife Service (USFWS).

Invasive species are especially problematic for birds. World-wide, invasive plants and animals have entirely or partially caused the majority of all bird extinctions since 1800. More than one third of the birds on American Bird Conservancy’s “Watchlist” are threatened by invasive species. Monocultures formed by exotic plants are particularly unfit for bird habitat. Birds require habitat containing a variety of plant species, variation in horizontal canopy cover, and complexity in vertical structure. In contrast, invasive exotic infestations are comprised of one species, mostly uniform in height and structure, and form a canopy coverage that is too dense.

Invasive plants reduce biodiversity, destroy wetlands and streams, and decrease forage for animals. However, besides being victims, birds are also culprits, helping to spread seeds of invasive weeds. The following plants, animals, and insects are some of the worst invasive species for birds in the U.S.

Vertebrates	Plants	Birds	Insects	Disease