Increasing tree invasion on Isla Victoria: 10 years after the original “gringos en el bosque” study

Knowing which species will become invasive has been the holy grail of invasion biology. A survey of woody plants was conducted in 2001 on an island (Isla Victoria) where 135 non-native woody plant species had been introduced 80 years previously. The survey showed that nearly 10% of introduced species had spread further than 100 m from the original sites. For some introduced species on this island longer time lags between introduction and invasion (or further spread) could be at play. To test if new invasions have begun since the original survey or if ongoing invasions have spread further, we repeated the survey a decade later. The proportion of introduced species that became invasive did not increase between surveys, suggesting that longer time lags may not reveal major invasions from new species on this island. However, we found that the relative frequency of taller individuals (above 2 m) has increased, suggesting a change in age structure of these invasive populations. Further, we found that woody invader densities and maximum heights have increased since the original survey, suggesting that woody plant invasion is progressing, and for some species even accelerating. These results highlight the importance of research on how long invasions of long-lived woody plants can take, providing key data to guide long-term monitoring of sites with multiple non-native plant introductions.