Introduction
Non-native invasive species (hereafter invasive species) are considered one of the most important threats to biodiversity today (Vitouseket al. 1997; Pimentel et al. 2005; Simberloff & Rejmanek 2011; Vilà et al. 2011; Doherty et al. 2016; Mollotet al. 2017; Howard et al. 2020) . A leading hypothesis to explain why certain introduced species become invasive is the Enemy Release hypothesis, stating that invasive plants are released from their specialized enemies from their native ranges (Keane & Crawley 2002; Mitchell & Power 2003; Colautti et al. 2004; Torchin & Mitchell 2004), but see (van Kleunen & Fischer 2009). However, over the invader’s residence time in the introduced range, such ‘enemy release’ is expected to decrease as a factor driving invasibility (Diezet al. 2010; Mitchell et al. 2010; Gruntman et al.2017). From an evolutionary perspective, native pathogens may adapt to use invaders as a resource over time (Parker & Gilbert 2004; Mitchell et al. 2010). Furthermore, as an invader spreads in the introduced range, it encounters a greater number of novel pathogens and has a greater chance of encountering pathogens for which it is a competent host (Mitchellet al. 2010; Flory & Clay 2013).
Evidence shows invasive plants can undergo rapid evolution in the introduced range (Prentiset al. 2008). For example, the Evolution of Increased Competitive Ability (EICA) hypothesis suggests that reduced enemy pressure in the introduced range leads to a change in allocation from defense to growth/competitive traits (Blossey & Notzold 1995; Joshi & Vrieling 2005). This reduction in allocation to defense could predispose invaders to enemy attack as novel enemies are encountered and adapt to invader presence in the introduced range. The Enemy Release and EICA hypotheses have been more thoroughly investigated for herbivores than for pathogens; however, evidence suggests that pathogens have the potential to regulate the long-term dynamics of plant invasions (Handley et al. 2008). Nevertheless, it remains unclear how common pathogen-mediated population decline is among invasive plants and how these interactions change over time (Flory & Clay 2013).
As these novel interactions occur, the question arises; can we make predictions about which invasive plants are most susceptible or resistant to pathogen related declines? Plant functional traits are likely to be a major driver of the variation in susceptibility to pathogens. Plant traits covary along specific axes related to growth rate, reproductive strategy, and defense due to necessary trade-offs in allocation making up the ‘fast-slow’ plant economic spectrum (Wright et al. 2004; Reich 2014; Salguero-Gómez et al. 2016). At one end of the spectrum, plants with an acquisitive strategy have rapid growth rates and photosynthesis rates, and high tissue nutrient content and specific leaf area, but short leaf lifespan and plant longevity, while at the other end conservative plants have the opposite traits (Cronin et al.2014; Welsh et al. 2016). It is also expected that plants at the acquisitive end of this spectrum will expend less on defense against enemies compared to conservative plants (Endara & Coley 2011; Parker & Gilbert 2018). Within Grime’s C-S-R model, fast growing competitive species adapted to high nutrient conditions were found to support more pathogens compared to stress tolerators and these acquisitive species benefited more from enemy release in the introduced range (Blumenthal et al. 2009). Specifically, acquisitive plant species produce less costly tissues with high turnover rates and lower investment in defense, thereby allowing for greater enemy attack with lower fitness cost (Coley et al.1985; Endara & Coley 2011). Cronin, Rúa, and Mitchell (2014) showed that plant acquisition strategy was connected to resistance of grass species to viral infection. Therefore, we propose that invasive plant traits can aid in understanding the long-term dynamics of invasions due to associations of plant traits with susceptibility to novel pathogen attack. In this review, we identify which plant traits confer the ability to either resist, tolerate, or escape pathogens, and explore how such knowledge can help us make quantitative predictions about which invaders are most likely to be susceptible.