Biological invasions by non-indigenous species are widely recognized as an important threat to biodiversity. However, the dimension, magnitude and mechanism of the impacts of invasive species remains poorly understood. We assessed the role of invasive plants by comparing vegetation changes that occurred between 1939 and 1999, a snapshot period that coincides with the onset of invasion, in Macabé Reserve in Mauritius. This Reserve was described as biotically homogeneous in 1939. In both surveys all native trees (> 10 cm DBH) were recorded from 10 1,000 m2 random plots. In 1999 the mean richness of plant species in plots was significantly lower: 15 species compared to 28 in 1939. The density and basal area of native species were both 70% lower in 1999. Plots with north and north-west aspects had significantly lower species richness and density than other aspects. We suggest that invasive plants have been the main trigger for the rapid and severe decline of native flora but we also observed that the decline of native species has been more spatially variable in the Reserve than the homogeneous distribution of invasive plants. This indicates that there are other feedback mechanisms driving biodiversity loss.