Plant species diversity and abundance in the Río Piedras watershed: Associations with physical and socioeconomic factors

Background/Question/Methods:    The potential contribution of residential landscapes to overall urban biodiversity has been at the forefront of urban ecology studies in recent years.  Most studies evaluating vegetation complexity in residential yards have shown that plant diversity may depend on the combined action of biophysical, socioeconomic, institutional and cultural factors.  In this, study we evaluated the relative role of physical vs. socioeconomic factors on variation in plant abundance and diversity across an urban-rural gradient within the Rio Piedras Watershed in the San Juan Metro Area, Puerto Rico.  We randomly sampled 165 households at three sites within the watershed where we conducted a social survey and a vegetation survey of its front yard between January of 2011 and May 2011.  Social variables per household included:  age, sex and marital status of respondent, house ownership (owned/rented), income, education level, and number of residents.  Vegetation variables included species richness and abundance of woody plants.  For each household we also obtained the total yard green area and the estimated the distance of the yard from the nearest stream.  We used multiple regression analysis to evaluate the variation of plant species diversity and abundance as a function of the remaining variables obtained for each household.  

Results/Conclusions:  The area of front yards that was green had a positive and significant contribution to the variation in species diversity and abundance of woody species across the watershed.   Household income also had a positive influence on plant abundance variation but not in plant species diversity.  This result contrasts with other studies where family income had a positive influence on species diversity.  None of the other socioeconomic factors analyzed had a major influence on plant diversity and abundance.  Given these results, urban design strategies that integrate biodiversity management at RPWS would be best served with data on factors that influence homeowner decisions over how yard area is kept green or else converted to gray infrastructure.   On the other hand, reduced regression models that included yard green area and also household income explained less than 29% of the variation in plant abundance and 24% of the variation in species diversity.  This suggests that factors not included in this analysis would also need to be explored to understand the drivers of plant biodiversity in this tropical city. Results presented here are part of the San Juan ULTRA-ex project which is part of the Urban Long Term Research Areas NSF-USDA Initiative.