Trait-dependent declines of threatened endemic trees following plant invasion on a tropical oceanic island

Biological invasions are a leading cause of the ongoing biodiversity crisis, and particularly so on islands. However, the role of invasive alien plants (IAPs) as a driver of native plant declines and extinctions remains unclear. The inherently slow and gradual nature of plant extinctions, especially that of long-lived woody species, could be a reason. Here, we examined temporal trends in subpopulations of 28 threatened endemic tree (TET) taxa. We questioned the frequency with which they decline in association with IAPs on Reunion Island (South-West Indian Ocean), and asked whether the most susceptible TET taxa exhibit characteristics that could reveal the underlying ecological mechanisms. We resurveyed 182 historically described subpopulations and tested whether observed trends of juvenile and adult TETs correlate with the abundance in IAPs using path analyses, while distinguishing TET taxa with respect to their leaf–height–seed characteristics and extinction risk. The trend of adult TETs was not affected by IAPs but that of juvenile TETs was negatively correlated with the abundance of IAPs in the understory layer. This was particularly the case for TET taxa with conservative resource-use strategies (low specific leaf area or low maximum height), whose juveniles might be particularly susceptible to competition with IAPs, but not for TET taxa with large seeds, whose metabolic reserves make seedlings more likely to attain a critical size. These findings suggest that IAPs can significantly contribute to the extinction dynamics of trees, primarily by limiting regeneration through competitive exclusion, and that their impacts vary depending on the traits of the affected taxa. • Historical populations of threatened trees were resurveyed to estimate their trends. • Population trends were tested for correlation with abundance in invasive plants. • Populations of juvenile trees decreased with increasing understory invasions. • Susceptibility of juvenile trees was trait-dependent. • Invasive plants can push trees towards extinction by disrupting regeneration.