The genus Psidium in the Galapagos archipelago is represented by an endemic species, P. galapageium, which coexists with the introduced P.guajava on Santa Cruz, San Cristobal and Isabela islands. This study, restricted to San Cristóbal island, aims to stablish ecological differences between these congeneric species of different origin (continental – insular).
The main questions addressed by this research were: (1) does plant species composition change according the dominant Psidium species? and (2) how different are the morphology and phenology of these species?
Vegetation sampling was carried out during 2014 and 2015 using the standard approach of comparing 14 plots (20 x 20m) with P. guajava (7) and P. galapageium (7). All P. guajava and P. galapageium individuals in each plot were counted and measured (height and d.b.h). Plymouth Routines in Multivariate Ecological Research version six (PRIMER 6.0) was used to identify diversity, dominance, and similarity between plots with different Psidum species. Morphology and phenology studies were based on collections and observations of both species made during 2015 and 2016. In order to differentiate the species, 19 morphological characters were observed and phenology focused on flowering, leaf growth and fruiting phenology of both species.
Results/Conclusions
This study clearly established a difference in species composition between plots with P. guajava and P. galapageium. Species diversity (represented by the Shannon-Wiener diversity and evenness index) was significantly different, being lower in plots with P. guajava. Community-level characterization showed different plant community structure. While plots with P. guajavawere dominated by herbaceous species, shrubby species were dominant in the P. galapageium areas.
Regarding morphology there is a clear difference between the species studied. Flowers and their parts, buds, fruits and leaves are on average smaller in P. galapageium var. howellii than in P. guajava. Phenology on both species displayed similar phenological flowering and fruiting patterns. A complex assortment of floral buds, open flowers, and fruits of varying sizes were found on both species during similar periods.
The potential risk of hybridization between P. galapageium and P. guajava is relatively high. First, both Psidium spp., at least on some sites, share similar areas. Second, both species are capable of flowering during the same time of the year. Third, both Psidium populations are visited by the large Galapagos carpenter bee.