Restoring community assembly: Insights from excluding dominant native and exotic plants

Predicting and restoring the structure of ecological communities is challenging because community assembly can be highly divergent. Specifically, stochastic forces can cause divergence in community structure among localities, even under the same regional species pool and environmental conditions. However, the degree to which community assembly is actually divergent has long been a subject of debate. This is largely because most studies on natural community assembly have used point-in-time observations rather than experimental manipulations of species abundances over time. In addition, human-altered communities often contain exotic species, but how their role differs from that of natives in community divergence is poorly understood. In an experiment conducted in a New Zealand riparian system, we continually prevented establishment of one, both or neither of two dominant shrub species, a nitrogen-fixing native and a fast-growing exotic, following a catastrophic flood. Meanwhile, the abundance of all plant species was measured annually for the first four years following the disturbance. The experimental results suggest that native and exotic species can play differential roles in driving divergence in taxonomic and functional community structure. We discuss implications of these differential roles for ecological restoration.