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.