Disturbance, invasion, and re-invasion: Managing the weed-shaped hole in disturbed ecosystems

Disturbance is widely considered to play a role in the spread and subsequent dominance of invasive plants. Ironically, control measures used to manage these populations often involve applying additional disturbance to the community. We develop a simple model to explore how disturbance and propagule pressure successfully drive invasion. Disturbance is often thought to favour invaders as it allows recruitment, however the effects of disturbance are more complicated when it also results in mortality of the invader. When disturbance rates in invader occupied and unoccupied sites are the same, these effects are balanced and successful invasion is then independent of the disturbance regime. However, differences in disturbance rates between invader occupied and unoccupied sites can occur through targeted management and when the presence of the invader alters the disturbance regime. Under these conditions we find a novel mechanism for the generation of an Allee effect, which occurs if the invader promotes disturbance in those sites it already occupies. When Allee effects occur, one-off large scale disturbances can result in permanent, dramatic shifts in invader abundance and conversely reduction of the invasive population below a critical threshold can lead to extinction. We also found that invasion is easier in disturbed systems where the invader suppresses disturbance in the sites it occupies.