Vacant lots make up approximately 12-15% of the land area in post-industrial American cities. While they are typically viewed as urban blight, they are also unique green space embedded in dense urban landscapes. They house rare and threatened species, and also reduce urban heat island effects and stormwater runoff. Vacant lots are not highly managed; some are mowed occasionally and others are not. Human activities in these spaces are also minimal, but remnants of past land use are evident. Investigating the ecology of vacant lots is therefore an interesting opportunity to examine the social and biophysical intersections in cities. Our study sought to disentangle some of these social and biophysical complexities. We conducted 2x2 m vegetation point surveys in 35 vacant lots on in Chicago, IL, USA in the summer of 2015. Our goal was to assess differences in plant diversity and structure. We used AIC variable importance weights to construct a best-fit biophysical and social model for each of four responses; species richness, evenness, vegetation height, and vegetated area. These four characteristics allow us to broadly understand the plant diversity as well as the ecosystem services and wildlife habitat provided by these spaces. We then used a variation partitioning approach to compare the relative strength of social and biophysical hypotheses.
Results/Conclusions
Species richness and vegetated area were both better explained by our social models, while vegetated height and evenness were better explained by biophysical drivers. We saw minimal overlapping explanatory power between the two domains across our responses, and vegetation height was our strongest model (residual 0.47). We identified 109 species of herbaceous plants and seedlings in our 35 vacant lots. Species evenness was quite high, suggesting that there are very few rare species in Chicago’s vacant lots. It is also interesting to note that we documented a fairly strong positive relationship between species richness and evenness in our system. This relationship is likely linked by the amount of turf grass in a lot. Turf maintenance reduces—but does not prohibit—establishment of new species. When these species are allowed to grow tall enough to set seed and reproduce, they do so readily. The amount of trash in a lot was the only variable that was important in all four of our responses. Lots with taller vegetation, higher richness and evenness, and lower vegetated area had a higher amount of trash.