North America is home to many animals and plants that have been introduced since early European settlements. Many of the most damaging invasive animal species were originally introduced either for sport, as pets, or as livestock and pack animals. Invasive plants were introduced in a variety of ways, for example as crops, pasture and garden plants and to prevent erosion. Without their natural enemies, some non-native plants became invasive, reducing the diversity and quantity of native plants. Millions of acres of once healthy, productive rangelands, forestlands and riparian areas have been overrun by noxious or invasive weeds. They are invading recreation areas, public lands, National Parks, State Parks, roadsides, stream banks, federal, state, and private lands. Invasive weeds destroy wildlife habitat, reduce opportunities for hunting, fishing, camping and other recreational activities, displace many threatened and endangered species, reduce plant and animal diversity because of weed monocultures- single plant species that over run all others in an area, disrupt waterfowl and neo-tropical migratory bird flight patterns and nesting habitats, cost millions of dollars in treatment and loss of productivity to private land owners.
Invasive species also include disease-causing organisms such as fungi and viruses. These organisms can be a threat to a wide variety of native plants and animals. Some of these have become invasive - they have spread and multiplied to the point where they damage the environment, threaten the continued existence of native plants and animals, or create significant problems for agriculture.
What can we do about invasive species?
It would be desirable to rid USA of the worst invasive species, but this is not achievable in most cases. Thus, management of invasive species conventional or biological focuses on reducing their impacts as cost effectively as possible. Management may involve eradication of the pest particular area, repeated reductions of pest numbers for periods of time, lasting reductions of pest numbers, removal of the most destructive individuals or exclusion of the pest species from an area. This approach means that control can be targeted - for example, to protect a threatened native species. Interactions between native species and invasive species are often hard to measure and can complicate decisions about controlling the invaders.
Control methods for invasive plant species include use of herbicides, manual removal, controlled burn and ploughing in. Problems that may arise from the use of herbicides include the pollution of waterways and the killing of native insects and small invertebrates.
Conventional techniques for control of invasive animals include fencing, trapping, poisoning and shooting. There has been some community concern for the welfare of invasive animals and it is now generally accepted that any pest control program must be humane and must have minimal impact on non- target species.
Biological methods to control pests include the use natural enemies such as predators, parasites and disease-carrying bacteria or viruses. Biological controls are most effective if used in combination with conventional methods. In California, the Biological Control Program is an integral component of the Plant Health and Pest Prevention Service's (PHPPS) Pest Prevention Program. The program helps to minimize the economic and environmental impact of noxious weed and insect species through the implementation of biological control programs throughout the state.