Higher plant colonisation and lower resident diversity in grasslands more recently abandoned from agriculture

1. Rates of species colonisation and extirpation are increasing in plant communities worldwide. Colonisation could potentially help compensate for, or compound, resident diversity loss that results from global environmental change. 2. We use a multifactorial seed addition grassland experiment to examine relationships between plant colonisation, resident species diversity and key community assembly factors over three years. By manipulating colonist seed rate, imposing disturbance and examining abundance and diversity impacts of 14 formerly absent sown colonists in communities that varied in successional stage and time since agricultural abandonment, we were able to disentangle effects of global change factors (species introduction, novel disturbance and land use change) that are usually confounded. 3. Evidence suggested that cover abundance of sown colonists was most strongly influenced by successional stage of recipient communities, though number of growing seasons was also important for the group of seven colonists with resource conservative "slow" life history traits. Colonist type, seed rate and disturbance had weaker relationships with colonist cover. 4. Factors affecting sown colonist cover were highly conditional. A negative relationship between plot-level disturbance and colonist cover in early successional communities meant that, despite a positive relationship in late succession, colonisation was negatively related to disturbance overall, defying theoretical expectations. 5. Non-sown resident diversity was negatively related to colonist cover and positively related to successional stage. Resource acquisitive colonists with "fast" life history traits appeared to limit cover of "slow colonists" when the two groups were sown together, likely reflecting niche pre-emption. 6. Communities at earlier stages of succession had lower resident diversity and experienced higher levels of colonisation than communities at later stages of succession. Elevated colonisation and lower resident diversity both appeared to be symptoms of human-induced land use change. However, results suggested that resource competition from plant colonists may also limit resident diversity in grasslands abandoned from agriculture more recently. Synthesis: Our findings point to the importance of resource availability and competition on plant colonisation and colonist impacts on residents. Although colonisation is potentially a source of biodiversity in the short-term, our results suggest that plant colonists that reach high abundance may be a further threat to resident plant diversity in secondary grasslands recovering from a recent history of agriculture.