Stream biodiversity is disproportionately lost to urbanization when flow permanence declines: evidence from southwestern North America

Climate change is predicted to increase the frequency and magnitude of extreme weather events. Such events coupled with demand for fresh water to support rapidly growing cities may reduce flow permanence and threaten stream biodiversity. We examined the role that flow permanence and urbanization play in limiting stream biodiversity in the Austin, Texas, USA, metropolitan area. We contrasted responses of macroinvertebrate communities to impervious cover (IC) among streams spanning a wide range of flow permanence using long-term biomonitoring and discharge data collected across 104 sites. The number of Ephemeroptera, Plecoptera, and Trichoptera taxa significantly and independently declined with increasing IC and decreasing flow permanence. However, the decline in the number of all taxa as flow permanence decreased depended on the level of IC. Higher IC levels were associated with disproportionately greater declines in taxon richness. Threshold Indicator Taxa ANalysis (TITAN) further parsed the community response by identifying 49 taxa that sharply declined in density and frequency of occurrence primarily between 3.7 and 24% IC. However, when the analysis was repeated after splitting the data into 2 groups of high and low flow permanence, the range of IC resulting in sharp density declines was lower for the low-flow permanence group (3-11% IC, greatest change at 9% IC) than the high-flow-permanence group of streams (7.7-28% IC, greatest change at 23% IC). Despite being composed of taxa largely regarded as pollution tolerant, the low-flow-permanence group of streams was characterized by taxa that evolved in habitats with intermittent flow and are integral components of their biological integrity. These results imply that future urban growth may have disproportionately negative effects on streams forecasted to become increasingly intermittent consequent to climate change.