Decadal declines in bird abundance and diversity in urban riparian zones

Abstract

Urbanization is frequently cited as a major driver of species losses worldwide; however, most studies in urban areas use a space-for-time substitution approach to document effects of urbanization through time. Ultimately, understanding the effects of urbanization on biodiversity requires long-term datasets. We examined long-term changes in bird assemblages at 12 riparian sites in the greater Phoenix metropolitan area and nearby Sonoran Desert region, featuring a range of human modifications and levels of water flow. Riparian areas in arid cities represent a key habitat type that is sensitive to human modification and supports high levels of species diversity. We used long-term data to: (1) explore variation in bird communities as a function of water permanence and degree of human-modification; (2) identify which environmental variables best describe differences found across riparian site types; and (3) assess how riparian bird communities, abundance, and species richness have changed through time. Engineered riparian sites supported more broadly distributed generalists; whereas, natural riparian sites supported more specialists. Sites with perennial flows had more vegetation and water compared to ephemeral sites and engineered sites had more impervious surface compared to natural sites. In nearly all comparisons, bird species richness, diversity, and abundance declined across riparian types during the period of study, even for common species. Bird communities in natural settings have changed more than communities at engineered sites. Overall, the riparian bird community is shifting toward urban dwelling, resident species that are characteristic of riparian sites with less water and more impervious surface.

Long‐term effects of land‐use change on bird communities depend on spatial scale and land‐use type

“…The more rapid declines in riparian bird communities were consistent with our expectations, as this trend has been found previously in urban riparian areas in this region (Banville et al 2017). For species richness, we found interaction between time and land use in both seasons.…”

“…These changes are consistent with observations from previous studies in Phoenix that the bird species composition of urban riparian sites becoming more similar to that of upland and urban sites (Banville et al 2017). At the beginning of the time series, urban riparian sites contributed the most to beta diversity in both seasons.…”

“…Finally, previous analyses of CAP LTER bird community data have shown that winter and spring bird communities are distinct from each other (Banville et al 2017). Because the identity of observers did not remain consistent throughout the study period, it would be difficult to use observer identity as a covariate in our analyses since it would be confounded with the time period with which they were making observations.…”

“…Phoenix, Arizona, United States, is situated within one of the dryland climates that make up 41% of the terrestrial biosphere (Reynolds et al 2007), where two primary ecosystem types exist: uplands that typically lack trees and where vegetation is dominated by shrubs, succulents, or drought-resistant grasses; and lowland riparian and floodplain areas that border perennial, intermittent, and ephemeral streams and rivers which often harbor pioneer tree species. Declines in bird communities in the southwestern United States have been reported in the desert in response to climate change (Iknayan and Beissinger 2018) and in riparian areas in response to recent large-scale biocontrol efforts to defoliate nonnative riparian vegetation (Darrah and van Riper 2018) and to urbanization (Banville et al 2017). Declines in bird communities in the southwestern United States have been reported in the desert in response to climate change (Iknayan and Beissinger 2018) and in riparian areas in response to recent large-scale biocontrol efforts to defoliate nonnative riparian vegetation (Darrah and van Riper 2018) and to urbanization (Banville et al 2017).…”

The more things change: species losses detected in Phoenix despite stability in bird–socioeconomic relationships

“…Levels of species losses similar to those we report here were detected in a 12‐yr study conducted in riparian areas in Phoenix, Arizona, USA (Banville et al. ). However, our findings are in sharp contrast to several other long‐term analyses of bird and other animal community changes in urban settings.…”