Effectiveness of Exotic Plant Treatments on National Park Service Lands in the United States

Abstract

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.

Does removal of invasives restore ecological networks? An experimental approach

“…For example, a recent review of 56 studies of exotic plant removals (157 exotic plant species) across 35 parks in 20 states within the USA found that although 87% of studies reduced density of at least one exotic plant species, the response of native vegetation was highly variable (Abella 2014). Fewer studies have evaluated how ecological networks may respond to control of invasives (Devoto et al 2012).…”

Effects of tree cutting and fire on understory vegetation in mixed conifer forests

“…It should be noted, however, that untreated forest that burns in stand-replacing wildfire can become heavily invaded over time (McGlone and Egan, 2009). These observations suggest that: (1) monitoring non-native plant dynamics is warranted, (2) consideration could be given to proactively treating incipient infestations of priority species as a precautionary approach, and (3) non-native abundance after severe wildfire is likely an appropriate benchmark against which to compare non-native abundance after tree cutting and prescribed fire treatments (Abella, 2014).…”

Status and management of non-native plant invasion in three of the largest national parks in the United States

“…The potential influences of multiple species are numerous, such as: (i) herbicide effectiveness can vary with plant growth form, (ii) treatment timing can be difficult when species' phenologies differ, (iii) required treatment duration can fluctuate among species varying in soil seed bank longevity, (iv) more complicated treatment regimes can increase costs and potential for negatively impacting native species, and (v) chances increase that other non-native species replace a focal treated species (Abella 2014).…”