Context dependence
Plant traits have been assessed for usefulness to predict invasibility
of plants in introduced ranges
(Lambdon & Hulme
2006; Pyšek & Richardson 2008; van Kleunen et al. 2010);
however, conclusions are equivocal.
Thompson & Davis (2011)
argue that invasive plants do not possess traits significantly different
from native species, therefore, traits are not useful to predict
invasibility of plants. In contrast,
(van Kleunen et
al. 2010; Van Kleunen et al. 2010)) oppose this argument.
Recent studies concluded that it is challenging to use traits to predict
invasibility of plants, in part, because it is context dependent;
interaction between a trait and environmental factors most likely
determines invasibility of a given non-native species
(Alpert et al.2000; Leffler et al. 2014). Pyšek et al.
(2012) concluded that traits
of an invasive plant can be useful to assess their impacts on
ecosystems, but this is also strongly context dependent. This likely
also applies to traits determining pathogen susceptibility. For
instance, a non-native plant may have a higher chance to be infected by
pathogens if the introduced range has phylogenetically closely related
species (Agrawal &
Kotanen 2003; Parker & Gilbert 2007; Parker et al. 2015). We
emphasize that the importance of plant traits may also be context
dependent for predicting susceptibility of plants to pathogens.
Many invasive species tend to form near mono-specific patches, which is
less common for native species
(Burdon et
al. 1989; Levine et al. 2003; Hejda et al. 2009).
Studies have shown that density of host plant populations was positively
correlated with disease levels in agricultural
(Burdon & Chilvers 1982)
and natural systems
(Cobb et
al. 1982; Jennersten et al. 1983; Alexander 1984; Burdonet al. 1995). While this tendency of invasive species may in
part be due to escape from enemies in the introduced range, it may also
make them more susceptible to population decline if a compatible
pathogen is encountered
(Clay et al.2008; Mordecai 2011). This density-dependent pathogen severity suggests
that invasive plants with high density may decline due to pathogens over
the time.
Evolution of both non-native host plants
(Maron et al. 2004)
and pathogens (Parker &
Gilbert 2004) is another factor that may regulate susceptibility of
plants to pathogens. Recent studies indicate that some invasive plants
have the ability to rapidly adapt to new environments modifying their
traits such as total biomass, SLA, or flowering time
(Flory et al.2011b; Colautti & Barrett 2013). Furthermore, levels of phenotypic
plasticity and ontogenetic change in traits is another factor to
consider in determining disease susceptibility
(Boege et al. 2007).
There is unlikely a single predictor of pathogen accumulation and there
will be context dependence complicating observed patterns. This context
dependence may be related to different types of pathogens, species
interactions, and/or environmental conditions. For example, it has been
suggested that responses of pathogens to plant N status may depend on
the types of pathogens
(Hoffland et al.1999, 2000). High tissue N content is predicted to increase disease
severity by obligate or biotrophic pathogens but reduce severity of
facultative or necrotrophic pathogens
(Hoffland et
al. 2000; Dordas 2008; Veresoglou et al. 2013). Similarly,
plant senescence is also related to different types of pathogens. Early
senescence reduces impacts of biotrophic pathogens while delayed
senescence acts as defense against necrotrophic pathogens
(Häffner et al.2015). Furthermore, pathogen dispersal mode may influence relationships
with plant traits. For example, taller plants may be more susceptible to
wind dispersed pathogens, whereas plants with foliage low to the ground
may be more affected by pathogens from the soil surface
(Marquis et
al. 2001; Robert et al. 2018; Vidal et al. 2018),
suggesting that traits conferring tolerance, resistance, and escape may
be pathogen dependent.