Dynamics and mechanisms of secondary invasion following biological control of an invasive plant.

Secondary invasions in which non-target invaders expand following eradication of a target invader commonly occur in habitats with multiple invasive plant species and can prevent recovery of native communities. However, the dynamics and mechanisms of secondary invasion remain unclear. Here, we conducted a common garden experiment to test underlying mechanisms of secondary invasion for 14 non-target invaders after biological control of Ambrosia artemisiifolia in two consecutive years. We found secondary invasion for all tested non-target invaders, but secondary invasiveness (change relative to natives) varied with species and time. Specifically, secondary invasiveness depended most strongly on phylogenetic relatedness between the target and non-target invaders in the first year with closely related non-target invaders being most invasive. In contrast, secondary invasiveness in the second year was mostly driven by functional traits with taller non-target invaders or those with higher SLA, or smaller seeds especially invasive. Our study indicates that secondary invasion is likely to occur wherever other invasive plants co-occur with an invasive species targeted for control. Furthermore, the most problematic invaders will initially be species closely related to the target invader but then species with rapid growth and high reproduction are most likely to be more aggressive secondary invaders.