1. Outcompeting native species
Many invasive species contribute to the decline of native biodiversity by changing the natural trophic structure of the invaded ecosystem. By outcompeting native species for limited resources, invasive species deplete the available resources for all consumers. This effect is particularly harmful when it occurs at or near the base of food webs. Invasive quagga mussels (Dreissena rostriformis bugensis) deplete plankton populations, a critical food source for a broad range of Great Lakes species, destroying the trophic balance throughout the lakes.
2. Threatening the safety of park employees and visitors
Several invasive species have biological characteristics that pose a danger to the safety of park employees and visitors. Often the danger presented by invasive species is unexpected by park employees and visitors and improving awareness of these dangers is critical to reduce further harm. For example, the Lionfish (Pterois volitans), which have invaded many of the southeastern ocean and coastal parks, disrupt food webs in parks by consuming native fish and invertebrates. They also have venomous spines that can cause painful stings to swimmers, snorkelers, divers or anglers. The National Park Service adopted a Service-wide Lionfish Response Plan in 2012 to mitigate impacts to marine resources and communicate risks to visitors and employees in parks.
3. Changing or degrading the experience of park visitors
Visitors travel to National Parks to experience and observe the natural scenery and biodiversity, and to experience the nation's natural and cultural heritage. Reduction of native animal and plant populations, extensive habitat alteration by invasive species, and efforts to maintain habitat integrity by eradicating invasive species may detract from visitors' appreciation and experience of National Parks.
4. Requiring intensified maintenance and monitoring
Park staffs must increase monitoring of park habitats and plant and animal communities for evidence of disturbance and invasion. This puts pressure on limited budgets and park employees to maintain the unique natural beauty of each park. Physical removal and control of invasive species are intensive activities that require long-term and diverse management techniques.
5. Altering natural ecological processes
Some invasive species physically alter the natural structure of park habitats and landscapes. Baby's breath (Gypsophila paniculata) is an invasive dune-dwelling plant in many coastal and Great Lakes parks that prevents the natural movement of sand dunes, critical habitat for many native plants.