One of the main conservation problems on the Galápagos Islands are introduced species. Approximately 30 introduced plant species have become a serious threat to endemic species, and among those species is Guava (Psidium guajava). Despite its widespread occurrence and colonization in Galápagos, little is know about the impact of this species on native plant communities. Additionally, the genus Psidium in the Galapagos islands is represented by an endemic species, P. galapageium, which coexists with P. guajava on Santa Cruz, San Cristobal and Isabela. Our study, restricted to San Cristóbal island, seeks to determine if there are ecological differences between these congeneric species of different origin (continental – insular), which will help to determine management priorities for P. guajavain the archipelago. The main questions addressed by this study were: (1) Does plant species composition change according to the dominant Psidium species? and (2) Are different functional and ecological groups, especially locally rare and endemic plant species, more affected by the P. guajavainvasion? Vegetation sampling was carried out during 2014 and 2015 using the standard approach of comparing 14 20 x 20m plots in total, seven per each species. Vegetation measurements were carried out along transects, and percentage vegetation cover was estimated for each species by the line-intercept method. Plymouth Routines in Multivariate Ecological Research (PRIMER v. 6.0) was used to calculate diversity, dominance, and similarity between plots with different Psidum species.
Results/Conclusions
A total of 104 species were found across the 14 plots. Plots with different coverage, either of P. guajava or P. galapageium presented similar absolute numbers as well as similar proportions of native, endemic and introduced species. In contrast, species diversity (represented by the Shannon-Wiener diversity and evenness index) was significantly different, being lower in plots with P. guajava. However, in areas where P. guajava has been for longer times, our results showed that the number of plant species and their percentage cover decreased significantly. Moreover, endemic and locally rare species were proportionally more negatively affected by this species. Regarding community-level characterization, our results showed different plant community structure among plots with Psidium species. While plots with P. guajava were dominated by herbaceous species, shrubby species were dominant in the P. galapageium areas. Future studies of these two species, especially of P. guajava, should measure microclimatic impacts in areas dominated by the invasive P. guajava, in order to understand if this introduced tree facilitates invasion by other exotic species.