Gentrification drives patterns of alpha and beta diversity in cities

Abstract

While there is increasing recognition that social processes in cities like gentrification have ecological consequences, we lack nuanced understanding of the ways gentrification affects urban biodiversity. We analyzed a large camera trap dataset of mammals (>500 g) to evaluate how gentrification impacts species richness and community composition across 23 US cities. After controlling for the negative effect of impervious cover, gentrified parts of cities had the highest mammal species richness. Change in community composition was associated with gentrification in a few cities, which were mostly located along the West Coast. At the species level, roughly half (11 of 21 mammals) had higher occupancy in gentrified parts of a city, especially when impervious cover was low. Our results indicate that the impacts of gentrification extend to nonhuman animals, which provides further evidence that some aspects of nature in cities, such as wildlife, are chronically inaccessible to marginalized human populations.

Keywords: alpha diversity; beta diversity; camera trap; gentrification; mammals.

Fig. 1.

Four theoretical ways that mammal richness (i.e., alpha diversity) and differences in community composition (i.e., beta diversity) could change in response to gentrification, where filled-in shapes in subfigures (A–D) represent the presence of different species. (A) Alpha diversity could remain constant but species assemblages could completely differ between gentrified and nongentrified areas, resulting in no difference in alpha diversity but the highest beta diversity. (B) More species could be gained than lost in gentrified areas, resulting in a large increase in alpha diversity and a smaller increase in beta diversity relative to fully distinct communities. (C) Communities in nongentrified areas could be a nested subset of those in gentrified areas, which could result in large increases in alpha diversity but low beta diversity. (D) The null prediction: no difference; gentrification is not associated with differences in alpha and beta diversity. (E) The difference in alpha diversity (x axis) as well as beta diversity, where a value of 0 indicates identical communities at a pair of sites and 1 indicates completely different communities at a pair of sites (y axis).

Fig. 2.

Locations of the 23 cities used to assess differences in mammal communities among gentrified and nongentrified parts of a city. Cities are represented by dots. The dot color illustrates the relative effect of gentrification on alpha and beta diversity at average sites in each city that vary in their gentrification status. Gentrification had a more pronounced effect on alpha diversity overall. However, gentrification in more western cities had a stronger effect on beta diversity, central US cities had a mixture, and East Coast cities had a stronger alpha diversity effect. See SI Appendix, Fig. S1 for this map with city names included.

Fig. 3.

Impervious cover had a stronger effect than gentrification on alpha and beta diversity. Alpha diversity, however, was likely greatest in gentrified areas with lower impervious cover because of the negative interaction term. Vertical black lines represent among-city estimates, the gray-filled rectangles are 90% credible intervals, and blue dots are city-specific estimates for each model parameter. Alpha diversity model parameters are on the log scale, while beta diversity model parameters are on the clog scale (i.e., −log(1 − x)). As a result, the beta diversity model parameters are constrained to be greater than 0.

Fig. 4.

Western cities (more negative longitude) had smaller differences in alpha diversity but greater beta diversity, while the remaining cities had greater differences in alpha diversity and lower beta diversity. Dots represent the expected beta diversity and difference in alpha diversity between an average gentrified and nongentrified site in each of the 23 cities. The x axis represents the change in alpha diversity as a function of gentrification, with positive values indicating greater species richness at gentrified sites. The y axis is Sørensen’s dissimilarity index, where 0 and 1, respectively, represent completely identical and distinct communities between sites.

Fig. 5.

The average effect of gentrification on the occupancy of mammals in this study. Overall, 10 species were more common in gentrified areas when impervious land cover was low, as evidenced by the strongly negative gentrification X impervious slope terms for those species. Three species were more common in gentrified parts of a city overall, and there were no species that negatively covaried with gentrification. Dots represent median estimates for each species, and horizontal lines are 90% credible intervals.