Local exclusion and regional decline of an endemic Galápagos tree species (Psidium galapageium) by an invasive relative (P. guajava)

Abstract Invasive species can interact with native relatives in a variety of ways which may jeopardize their long-term coexistence. Here we show that interactions with an invasive species of guava ( Psidium guajava ) appear to be driving the local exclusion and regional decline of guayabillo ( Psidium galapageium ), a tree species endemic to the Galápagos archipelago. We find evidence consistent with recent historic exclusion of guayabillo from the highlands of San Cristóbal Island, signatures of ongoing demographic decline in sympatric populations at lower elevations, and evidence suggesting that the four coinhabited islands represent points along a time series of regional decline, with the extent of guayabillo decline depending on the date that guava was introduced to each island. Based on these results, we then use the percentage of guava cover surrounding guayabillo populations to target populations that are at imminent risk of exclusion to aid in prioritizing management targets.