Residential yards make up a large portion of land in urban areas, and can contribute to biodiversity conservation as well as ecosystem services such as urban heat island mitigation, stormwater regulation, provisioning of food from gardens, and human-environment interactions. People choose how to design their yards based on their aesthetic tastes, level of concern for the environment, and neighborhood norms, among other factors. Some studies have shown evidence of neighbor mimicry in yard design within neighborhoods, where nearby yards have similar plant species composition and/or structure. In this study, we investigate the variability in front yard vegetation in sixteen neighborhoods in Chicago, selected to span gradients in income and race. We address the following questions: Can we explain and predict variability in residential yards? and What effect does variability have on large-scale urban biodiversity? We used measures of species turnover, or beta diversity, (e.g. Sorenson’s index) and spatial autocorrelation of species richness, or alpha diversity, (e.g. Moran’s I) to detect patterns of neighbor mimicry at different scales. We also used linear regression to determine whether neighborhood demographics (e.g. age, income, race, education level) predicted alpha or beta diversity patterns.
Results/Conclusions
We documented 462 plant species across the sixteen neighborhoods, about 400 of which were intentional (non-weedy) species. Of the predictors we examined, the percent of residents with a bachelor’s degree in a neighborhood was the best predictor of the mean number of intentional plant species per yard (R2 = 0.70), while weed species richness was not predicted by any demographic variables. When data from all neighborhoods were considered together, there was a significant positive spatial correlation in species richness of intentional plants between yards that were 2 yards apart and about 10 yards apart, suggesting that mimicry may occur both between close neighbors and within the broader neighborhood. Weeds showed spatial patterns within a distance of three yards, likely reflecting short dispersal distances of many yard weeds. We also found that neighborhoods with older residents had more similar yards (i.e. lower beta diversity across yards within a neighborhood; R2 = 0.28), perhaps reflecting more long-standing residents and convergence in yard vegetation over time. Finally, using the Chao2 estimator to scale up species richness from the subset of sampled neighborhoods to the entire city, we estimate that there may be over 700 intentional plant species in the city of Chicago.