Plant diversity of Residential yards across the Río Piedras watershed

Background/Question/Methods

This research is part of the San Juan ULTRA-ex initiative (http://sanjuanultra.org/), which studies the vulnerabilities of the Rio Piedras Watershed (RPWS) socio-ecological system. The San Juan metropolitan area is the largest urban area of Puerto Rico and home to 65% of its population. Rapid urbanization has resulted in the rapid loss of forest cover and has most likely changed local biotas.  Here we studied the diversity and abundance of vegetation of residential areas to plant biodiversity at RPWS and addressed how this tropical city compares in terms of species diversity and rates of exotics with other sites. To accomplish this we randomly sampled houses at three circular buffer zones (2km diameter) hereafter Chiclana, Avenida Central, Puerto Nuevo distributed along an urban-rural gradient. A minimum of 67 houses per zone was surveyed from randomly selected streets. At each house we conducted a vegetation survey of front yards for all woody species (trees, shrubs, palms and tree ferns) and estimated each yards’ species richness, plant abundance per species and frequency of exotics and ornamentals. Species area curves were used to evaluate differences in total species richness among zones and the rate of exotic specie at RPWS.  

Results/Conclusions

A total of 219 fronts yard were surveyed within an estimated area of 13895.78 m2. We found a 223 woody species at the three sites with shrubs being the most abundant plant habit. Species area curves were accelerating suggesting that additional surveys should uncover more rare species. Residential front yards had a disproportionate amount of exotic woody species in comparison with the native ones (73% exotic vs. 27% native) and these are the species increase in frequency as more households were sampled. Regarding their potential uses, shrubs species were mostly ornamentals, whereas tree species included species with both food as well as ornamental utility. Expected diversity at the household level is not homogeneous across RPWS sites. The rate of exotic species is high and on the high end of variation for urban sites when compared to other metropolitan areas. High levels of exotics is surprising of given that the commercialization of exotics in PR began in the last century where as in other cities its ahs been a longer-term process. This study provides important information that can be used in the study of sustainability and vulnerability of natural systems and in the improvement of urban biodiversity design within the RPWS.