How do alien pathogens make inroads into native plant networks?

We’re used to alien species causing problems for native wildlife. So, it’s no surprise that alien pathogens will cause problems for native plants. That’s what pathogens do. But how will they attack plants? Jennifer Bufford and colleagues have been looking at the interactions between alien pathogens and native plants as a network in New Zealand. They found that exotic pathogens form different connections compared to the native pathogens.

The relationship between native plants and pathogens could be described as an arms race. Pathogens specialise on the plants they can attack, so networks tend to be modular. Alien pathogens, however, are expected to behave differently.

In order for an alien pathogen to establish itself in a territory, it must find a suitable host. A specialist might be lucky and do this, but a generalist is far more likely to find a viable host. That means that a successful alien pathogen is more likely to be a generalist than a specialist.

To find out if this idea was correct Bufford and colleagues created a database of plant-fungal associations in New Zealand, covering over a century and a half of botany in the country. They compared the ranges of native and alien pathogens. They also examined the networks the pathogens made.

They found that alien pathogens were indeed found on more native plant species than native pathogens. This confirmed their suspicion that alien pathogens would be more generalist than native pathogens. It wasn’t simply a species count. Alien pathogens also attacked plants over a broader phylogenetic range than than native plants, showing they were more adaptable to different plant families.

This ability to go to more hosts also led the alien pathogens to create more links between host plants. The alien network led to generalists interacting with specialist pathogens, while this didn’t happen much in the native networks.

Bufford and colleagues see serious ecological implications from their results. They write: “Links in a plant-pathogen network can reveal which pathogens are mostly likely to spillover and module structure can identify hosts that are most at risk. Where specialization is lower, as in the network that included alien pathogens, there is more opportunity for spillover because there is a greater diversity of interacting pairs… Measuring host range and spillover can highlight biosecurity risks and the potential impact of alien and emerging pathogen… Alien plant hosts could also increase spillover, for example by functioning as disease reservoirs.”

The authors conclude that it’s not only the presence of a pathogen that matters, but also how infectious it is to native plants as a network that will describe the effect of an alien pathogen.