When the economic engine stalls: An examination of plant communities in post-recession urban landscapes

Background/Question/Methods

It is commonly accepted that urban design and management govern vegetation in cities through a variety of mechanisms including planting ornamentals, selectively spraying unwanted species, and pruning woody perennials. All of these mechanisms alter ecological processes like dispersal, competition, and resource partitioning. Human activities also mitigate the effects of natural disturbances through supplemental watering, and the use of pesticides. But when the housing bubble burst, followed by the Great Recession of 2008, the urban landscape was transformed. Foreclosures and abandoned development sites dotted the urban landscape, and unemployment rates spiked. This led us to ask: Did the Great Recession affect urban vegetation? If so, how?

            The Phoenix metropolitan area houses nearly five million people and is among the fastest growing metropolitan areas in the U.S. We examined how Central Arizona-Phoenix Long-Term Ecological Research (CAP LTER) vegetation composition and diversity varied after the Great Recession by land-use type at different scales. Specifically, we looked at (1) the regional-level where policy-makers manage for resources (ecosystem services), and (2) the residential-level where a resident manages an individual yard. We used species richness, indicator species analysis, and multivariate ordination methods to assess plant species composition and vegetation change before and after the Great Recession.

Results/Conclusions

The Great Recession had important and rapid impacts on urban plant communities. We show how vegetation composition and species diversity shifted between pre- and post-recession surveys, with weedy annuals as a leading contributor on residential sites. After the recession, there was an increase in species richness at the regional-level on urban and agricultural sites, but not at Sonoran Desert sites. Species richness also increased relative to pre-recession levels in residential yards, across all three common landscaping types. Indicator species analysis revealed higher compositional similarity among residential land-use types than between urban Phoenix and Sonoran Desert landscapes. Urban biotic homogenization – increased species similarity among diverse metropolitan areas – may explain the unexpected dissimilarity between composition of xeriscaped yards and native desert.

Previous work has shown that a shift occurs from bottom-up to top-down controls on plant diversity along a desert-to-urban gradient. But we show that an economic disturbance is tightly linked to a decrease in management activities and accounts for the re-emergence of bottom-up controls on vegetation. It is well-understood how ecological processes shape plant communities in ‘natural’ systems, but this research is unique by demonstrating how an economic disturbance like the Great Recession can shape urban landscapes and plant community dynamics.