Exotic plants accumulate generalist fungal taxa that are known to
cause plant disease
Fungal pathogen richness did not differ between native and exotic
plants, with individual plants hosting approximately 8 OTUs (± 0.4 s.e.,
χ2=0.84, p>0.36) identified as pathogenic
on average. Fungal pathogen richness did not change with the proportion
of exotic plants planted in the community or differ in home vs. away
soil (proportion of exotics: χ2=1.05, p=0.30; soil
treatment: χ2=2.67, p=0.10, Table 1). Fungal pathogen
relative abundance did not differ between native and exotic plants at
the individual plant level (χ2=1.31, p=0.25, Table 1),
but pathogen relative abundance in communities increased along the
exotic plant gradient (χ2=5.17, p<0.02,
Table 1, Fig. 1A). The soil treatment had no effect on pathogen
abundance in communities (χ2=0.27, p=0.60) and no
interaction with the proportion of exotics (χ2=3.31,
p=0.07, Table 1). This gain in pathogen relative abundance along the
exotic gradient was not simply due to reductions in other fungi, as
total fungal biomass (using phospholipid fatty acid [PLFA]
biomarkers) did not change along the exotic plant gradient
(χ2=0.16, p=0.69, Table 1).
On average, plants interacted with an increasing proportion of available
pathogens along the exotic plant gradient, as indicated by
plant-normalized degree (χ2=37.97, p<0.001,
Table 1, Fig. 1B). Likewise, more generalist pathogens inhabited the
roots of plants in exotic-, compared with native-dominated communities,
as indicated by the normalized degree of fungal pathogens
(χ2=45.12, p<0.001, Table 1, Fig. 1C).
When growing in 100% native or 100% exotic mesocosms, exotic plants
were more generalist than native plants in their interactions with
pathogens, as indicated by normalized degree
(χ2=21.80, p<0.001, Table 1, Fig. 2A).
Further, exotic plants were more central in the network, as indicated by
mean closeness centrality values for natives vs. exotics
(χ2=36.57, p<0.001, Table 1, Fig. 2B),
suggesting that exotic plants had a greater ability to indirectly affect
others in the community by spreading pathogens, compared with native
plants.