Responses of non‐native earthworms to experimental eradication of garlic mustard and implications for native vegetation

Abstract

Recent studies in invasion biology suggest that positive feedback among two or more introduced organisms facilitate establishment within a new range and drive changes in native plant communities. Here, we experimentally tested for relationships between native plants and two non‐native organisms invading forest habitats in North America: garlic mustard (Alliaria petiolata, Brassicaceae) and earthworms. In two forested sites, we compared understory vegetation and earthworm biomass in plots where garlic mustard was removed for three years, plots without garlic mustard invasion, and plots invaded by garlic mustard that was not removed. Earthworm biomass was highest in the plots with garlic mustard, and long‐term eradication of garlic mustard reduced earthworm biomass to levels similar to those observed in the uninvaded control plots. Invasion treatment, and the interactions between earthworm biomass and treatment, explained most of the variation in plant community composition and diversity—suggesting that earthworms alone do not necessarily drive forest understory floristic patterns. In contrast to broader geographic patterns indicating earthworms as the main driver of vegetation change in the presence of non‐native plants, we show that garlic mustard solely, or in conjunction with earthworm biomass, drives changes in native plant composition and diversity at the scale of individual forests. From a local management perspective, our data suggest that garlic mustard eradication can directly assist in the conservation of native plant communities and simultaneously reduce earthworm biomass.

Functional shifts of soil microbial communities associated withAlliaria petiolatainvasion

“…Most populations were found on the crest of or along mild slopped hills. In all the sites, we found signs of Lumbricus terrestris invasion, which was expected since the removal of A. petiolata has been shown to reduce L. terrestris densities (Stinson et al 2018).…”

Where Is Garlic Mustard? Understanding the Ecological Context for Invasions of Alliaria petiolata

“…Soil micronutrients (e.g., copper, iron, manganese, zinc, and sodium) have not been associated with garlic mustard success, although they have been measured in few studies (Lankau 2012a , Anderson et al 2019 ). Garlic mustard was positively associated with higher soil moisture at regional scales (Haines et al 2018 ), but other studies showed no effect of garlic mustard on soil moisture up to 8 years after removal (Stinson et al 2018 , Burke et al 2019 ). In summary, the complexity and variability of findings for garlic mustard interactions with soil suggest a need to investigate the scales at which garlic mustard associates with elevated nutrient levels and other soil characteristics.…”

“…Earthworms, as ecosystem engineers, have broad-reaching effects on ecosystems (Le Bayon et al 2017 ), but their role in facilitating the expansion of garlic mustard is unclear. In general, nonnative earthworm biomass is positively associated with invasive plant cover (including garlic mustard) and negatively associated with native plant presence (Nuzzo et al, 2009 , 2015 , Craven et al 2017 ), and removal of garlic mustard decreases nonnative earthworm biomass (Stinson et al 2018 ). However, Hopfensperger and Hamilton ( 2015 ) found a negative relationship between garlic mustard cover and the proportion of immature nonnative earthworms, which they speculated may be associated with allelopathy.…”

“…Although garlic mustard grows at a wide range of soil pH levels both within (Phillips-Mao et al 2014 ) and across sites (Haines et al 2018 ), it is often associated with less acidic soils (Castellano and Gorchov 2012 , Morris et al 2012 , Haines et al 2018 ). Garlic mustard has been found to increase soil pH (Stinson et al 2018 ), which may be driving higher rates of nitrogen mineralization in invaded sites (Morris et al 2012 ), although some studies have also observed no effect of garlic mustard on soil pH (Burke et al 2019 ). Alerding and Hunter ( 2013 ) observed a positive correlation between garlic mustard presence and springtail (detritivore) abundance, and they suggested that increased soil pH in invaded sites may be the mechanism underlying this pattern.…”