Guadalupe Island: Lost paradise recovered? Overgrazing impact on extinction in a remote oceanic island as estimated through accumulation functions

Guadalupe Island, an oceanic island in the northwest of Mexico, is an outlier of the California Floristic Province that has been disturbed by introduced goats for more than a century, with dramatic effects of goats on plant communities and local species extinctions. In 2004 the island went through a successful eradication program. Since then, six previously unrecorded species have been discovered and four supposed extinct species have been found again. Quantifying the true species richness of the island at the time of eradication, to set a benchmark for the future monitoring of this large-scale natural experiment, is both a challenge and a necessity. For this purpose, we estimated (a) current and (b) accumulated historical plant species richness of the island through accumulation functions. Estimation of current species richness was based on the geographical accumulation process of species richness (80 species) obtained from sampling 110 (50 m × 2 m) transects distributed along the island in year 2004. Historical species richness was estimated through the temporal accumulation of species richness (119 species) from botanical records (1,960 specimens reviewed) between 1875 and 2000. The predicted value of historical richness (213 species) is similar to known historical records (218 species), but estimation of current richness (203 species) is significantly higher than accepted extant plant richness (187 species). Our results suggest that currently there may be more plant species living in the island than estimated through recent botanical exploration. Future monitoring of the island as it recovers will clarify this hypothesis.