The United States created national parks to conserve indigenous species, ecological processes, and cultural resources unimpaired for future generations. Curtailing impacts of exotic species is important to meeting this mission. This synthesis identified 56 studies reported in 60 publications that evaluated effects of exotic plant treatments on National Park Service lands. Studies encompassed 35 parks in 20 states and one U.S. territory and included 157 exotic plant species. Eighty-seven percent of studies reported that at least one treatment reduced focal exotic species. Of 30 studies evaluating response of native vegetation, 53% reported that natives increased, 40% reported neutral responses, and 7% reported that natives decreased. For at least some of the neutral cases, neutrality was consistent with management objectives. In other cases, insufficient time may have elapsed to thoroughly characterize responses, or restoration might be needed. Nonfocal exotic species increased in 44% of the 16 studies evaluating them, but the other 56% of studies reported no increase. Results suggest that: (1) a range of exotic species spanning annual forbs to trees have been effectively treated; (2) developing effective treatments often required extensive experimentation and balancing nontarget impacts; (3) presence of multiple exotic species complicated treatment efforts, highlighting importance of preventing invasions; and (4) placing treatment objectives and outcomes in context, such as pretreatment condition of native vegetation, is important to evaluating effectiveness. Attaining the goal in national parks of conserving native species and ecological processes minimally influenced by exotic species will likely require comprehensive management strategies inclusive of treatment interactions with focal exotic species, other potential invaders, and native species.
Management Implications: National parks in the United States are areas where a management goal is to conserve native species and processes free from influences of exotic species (National Park Service 2006). Meeting this goal in light of the approximately 4,000 exotic plant species (in addition to other types of exotic organisms) already present in parks will be challenging and require effective treatment strategies applied at different scales in comprehensive ecological and social frameworks. A systematic search of published literature uncovered 56 projects conducted on National Park Service lands in which effects of exotic plant treatments were assessed.
On one hand, results were encouraging in that most projects (87%) found at least one treatment reduced focal exotic species. Moreover, treatments negatively affected native vegetation in only two projects. There also were five projects reporting eradication of 1 to 21 species within parks and some examples of effective broad-scale treatments aligned with large-area infestations. On the other hand, < 4% of exotic plant species already present in national parks have had effects of treatments upon them evaluated on National Park Service lands. There also is little to no documented information on overall trends of exotic plant abundance for most parks (Allen et al. 2009). Priorities for increasing effectiveness of management interventions include evaluating treatment effects in a comprehensive ecosystem benefits/tradeoffs approach (Steers and Allen 2010), enhancing knowledge on species trends and impacts to facilitate prioritization of limited treatment resources (Vilà et al. 2011), continued development of single- and multi-species treatments that minimize undesired effects and are cost-effective (