Direct effects of a non‐native invader erode native plant fitness in the forest understory

Abstract

The direct role of non‐native plant invaders in driving negative population‐ and community‐level processes of native species has been recently questioned. Addressing this controversy requires determining quantitatively if invaders negatively affect native population fitness. Because the invasion of non‐natives often coincides with other anthropogenic stressors, experiments that partition the putative impact of non‐natives from other known stressors and assess their potential synergies are required. While many studies have examined the effects of non‐natives on components of native plant performance, studies that decompose the net fitness effects of non‐natives from other anthropogenic stressors on population growth rate are lacking. We used 6 years of detailed demographic data to parameterize a size‐dependent integral projection model to examine the individual and combined effects of an allelochemical‐producing invader (Alliaria petiolata) and an overabundant ungulate herbivore (Odocoileus virginianus) on the population dynamics of an understory perennial (Trillium erectum). We show that Alliaria consistently and negatively affects the population dynamics of Trillium. Specifically, this invader reduces native population growth rate and alters the size distribution of the population at equilibrium. Alliaria also works in concert with the known negative impacts of overabundant white‐tailed deer, illustrating the additive effects of anthropogenic stressors on native plant dynamics. Synthesis. Alliaria's effects on vital rates differed in magnitude and sign across the native's life cycle, highlighting the importance of detailed demographic analyses. Our study provides novel empirical support for the claim that non‐native invasive species can significantly and directly reduce the fitness of native plants.

Negative effects of an allelopathic invader on AM fungal plant species drive community‐level responses

“…Garlic mustard consistently has negative effects on native mycorrhizal plant species (e.g., Stinson et al 2006, Cipollini et al 2008, Wolfe et al 2008). These negative effects on mycorrhizal plant species are consistent with garlic mustard’s disruption of native mycorrhizal mutualisms, which leads to water stress and carbon stress (Brouwer et al 2015, Hale et al 2016) and decreases in vital rates (Brouwer et al 2015) and population growth rates (Bialic‐Murphy et al 2019). In addition, garlic mustard alters plant communities (Stinson et al 2007).…”

“…Experimental evidence in both controlled and field settings has established that garlic mustard reduces native plant performance and population growth through the disruption of their mycorrhizal associations (e.g., Stinson et al 2006, Callaway et al 2008, Wolfe et al 2008, Barto et al 2011, Hale et al 2011, 2016, Brouwer et al 2015, Bialic‐Murphy et al 2019). Furthermore, garlic mustard is a poor direct competitor (Meekins and McCarthy 1999, Bossdorf et al 2004), and has no known direct phytotoxic effects (Brouwer et al 2015, Hale et al 2016).…”

Deer Indirectly Alter the Reproductive Strategy and Operational Sex Ratio of an Unpalatable Forest Perennial

“…Arisaema is unpalatable and is rarely browsed by deer at our field sites (Heckel et al 2010). Modification in abiotic conditions and direct consumption of leaves, flowers, and fruits by overabundant deer reduce plant fitness of palatable herbaceous understory species (Knight 2004;Kalisz et al 2014;Bialic-Murphy et al 2019). Thus, we may expect that the negative effects of deer on the population dynamics of palatable species would result in competitive release of co-occurring unpalatable herbaceous species (Keane and Crawley 2002;Rooney and Waller 2003).…”

Invasion-induced root-fungal disruptions alter plant water and nitrogen economies

“…Our previous work demonstrated that Alliaria can have cascading negative effects on the performance and population growth rate of AM dependent understory plant species 43 . We also previously found that Alliaria alters the diversity of native plant communities by reducing the abundance of mycorrhizal plant species, with no subsequent changes in non-mycorrhizal plant species 53 .…”