Community‐level effects of herbicide‐based restoration treatments: structural benefits but at what cost?

Abstract

Invasive species alter ecosystem structure, impact biodiversity, and have significant economic costs. In Oregon's Willamette Valley, invasive grasses Arrhenatherum elatius and Schedonorus arundinaceus alter the dynamics of the phenologically paired interaction between an endangered butterfly, Icaricia icarioides fenderi (Fender's blue), and its larval host plant, Lupinus oreganus (Kincaid's lupine). To test methods to restore this interaction, we established a 3-year experiment where a post-emergent grass-specific herbicide, fluazifop-p-butyl, was applied to Fender's blue habitat. Plant community data were recorded throughout the growing season at eight paired plots for 1 year prior to treatment and 3 years during treatment. We asked whether annual application of herbicide could reduce the height of invasive grasses to levels at or beneath the height of Kincaid's lupine racemes throughout the Fender's blue flight season. We hypothesized that native forb species, which are critical nectar sources for Fender's blue, would increase in cover and frequency following the release from competitive dominance of invasive grasses. Grass-specific herbicide reduced grass height during the flight season of Fender's blue, but with several costs. We found no change in nectar and a suppression of lupine growth in plots in response to experimental herbicide treatment. Each study site had multiple secondary invaders; the long-term impact of these new invaders is unknown. We suggest that herbicide application results in a net negative effect in the context of Fender's blue habitat restoration. That is, the costs to primary resources for Fender's blue and the influx of secondary invaders may be as problematic as the primary invasion by non-native grasses. Implications for Practice• Herbicide-assisted management tools for restoration should consider potential costs and benefits relative to desired restoration outcomes.• Grass-specific herbicide application reduces the height and "thickness" of the plant community, which is a clear benefit for access by pollinators.• Grass-specific herbicide application allows rapid expansion of invasive forbs.• Practitioners should carefully consider net potential benefits of grass-specific herbicide application with potential costs of invasive forb invasion when developing restoration plans.

Demographic costs and benefits of herbicide‐based restoration to enhance habitat for an endangered butterfly and a threatened plant

“…We did not see a systematic increase in plant growth. Neither did we see expansion of lupine cover across the plot (see Bennion et al for assessment of changes in lupine cover). We attribute this influence on seed‐set to reduced height and thickness of the vegetation and increased access to plants by pollinators (see Bennion et al ).…”

“…However, this increase in population growth did not persist. Secondary invasion by non‐native forb species increased over time in these plots (Bennion et al ) limiting the long‐term efficacy of the approach as a tool to augment the endangered butterfly population. Thus, we conclude that grass‐specific herbicide application can be included as a restoration tool in Fender's blue butterfly habitat, but only if it results in reaching restoration goals unrelated to butterfly population enhancement.…”

“…We describe butterfly and lupine responses to treatments below. We include vegetation community and structure responses to treatments in our companion paper (Bennion et al ).…”

“…The a priori assumptions of managers are that (1) annual application of herbicide will ultimately reduce/eliminate problematic grasses; (2) grass‐specific herbicides have minimal consequences on nontarget forbs and nontarget insects; 3) effects of annual herbicide application on larval survivorship would be comparable from year to year; and 4) butterfly fecundity in sprayed areas would increase over time as habitat quality increases. For overall responses of the plant community to restoration treatments, we refer the reader to our companion paper (Bennion et al ). A central conclusion from Bennion et al is that annual treatment with grass‐specific herbicide reduces grass height below the height of flowering Kincaid's lupine throughout the butterfly flight season.…”

Can Nature-Based Solutions Deliver a Win-Win for Biodiversity and Climate Change Adaptation?

“…Population trends in one taxonomic group are often not indicative of similar trends across other taxonomic groups (Carignan and Villard 2002;Prendergast et al 1993); so positive outcomes for plants, for example, does not imply that there are necessarily increases in animal species (Cristescu et al 2013;Marshall et al 2020Marshall et al , 2021. For example, metrics may indicate a high quality plant community but the successful establishment of associated faunal communities depends on other factors such as the ability of fauna to disperse into the area from elsewhere (Baur 2014;Gilbert-Norton et al 2010), and the impact of vegetation management techniques (such as thinning, mowing, burning or spraying) on fauna (Bennion et al 2020;Humbert, Ghazoul, and Walter 2009;Prowse et al 2017). There was a similarly strong bias towards taxa that reside above ground, with 72% of interventions only assessing effects above ground, rather than also including below ground and aquatic habitats.…”