Vector transmission within and between native and introduced grass populations: Patterns and consequences

When a species is introduced into a novel region, it may experience enemy release as a result of the loss of specialist pathogens and other enemies from its native range.  Generalist enemies are more likely than specialists to be co-introduced to the same region as their host, and introduced populations can acquire generalist enemies from native host populations.  This raises the question of how durable enemy release is.  To examine this, I compiled estimated dates of introduction for over 100 plant species introduced to North America from Europe.  Plant species introduced longer ago were reported to currently be host to more fungal and viral pathogen species.  This suggests that the duration of enemy release for introduced plant populations is roughly on the order of centuries.  As introduced host populations acquire enemies over time, they will come to share increasing numbers of generalist pathogens with native species.  This creates the potential for pathogen spillover and apparent competition between native and introduced species.  Depending on the resistance and tolerance of native and introduced hosts, this could provide either a negative or positive feedback to growth of introduced populations.  Field experiments in North Carolina with one of “the world's ten worst weeds,” johnsongrass (Sorghum halepense), show that it maintains high population densities despite heavy pathogen loads.  Many of its pathogens are shared with native species, including aphid-transmitted viruses and a beetle-transmitted bacterium.  Results to date are consistent with the idea that the invasiveness of johnsongrass may be reinforced by pathogen-mediated apparent competition.