We assume that models of variation of exotic plant richness along main ecological gradients provide predictors of invasiveness at large spatial scales. We asked whether we find similar patterns of variation along altitudinal gradients in contrasted geographic islands. Using respectively field data and bibliographic data, we studied the richness variation of exotic plants in two oceanic islands, Reunion Island (Indian Ocean, 0-3069 metres above sea level (masl.) and New Caledonia (Pacific Ocean, 0-1629 masl). We elaborated models using 120 species in Reunion Island and 44 species in New Caledonia. We found very different patterns of richness variation between the two islands. In Reunion Island, altitudinal variation of richness is hump-shaped, with a maximum value in mid-altitude. Conversely, we found a monotonic decrease of richness in New Caledonia, showing a linear pattern of variation between 0 and 900 m (r = 0.957, P < 0.0001). A similar monotonic decrease is observed above 1400m in Reunion Island (r = 0.934, P < 0.001), in the lower part of the gradient. In Reunion Island, 93.2% of the altitudinal gradient is colonized by exotic plants, versus only 52.1 % in New Caledonia. While the Reunion Island's pattern is consistent with other studies on richness variation of plant communities, the New Caledonia's pattern seems original. An important prediction of our study is that (i) the linear model of variation of species richness with altitude and (ii) the lack of exotic species in the upper part of the altitudinal gradient is an indication that the general plant invasion process is only starting in New Caledonia. That may be an opportunity to set up monitoring systems so that invasions can be detected in their earliest phases.