Imprints of racism: Cities create challenging environments for many nonhuman species, and the presence of nonhumans in cities influences the health and well-being of the humans with which they share the environment. Distinct urban conditions are created by landscape modification, but the history of this transformation is not equal across urban environments. Schell et al. review how systematic racist practices such as residential segregation, enacted in part through redlining, have led to an unequal distribution of “nature” within cities. These inequities continue to play out in both the ecological processes of cities and the welfare of their residents. Science , this issue p. eaay4497

Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO₂) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO₂. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32‐fold to 66‐fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO₂ emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting‐drying cycles on respiration and CO₂ emissions in stream networks.

Intermittent streams lose surface flow during part of the year but can provide important habitat for imperiled fishes in residual pools. However, extended intermittency can drive high mortality as pool contraction decreases pool quality, and some pools dry completely. We evaluated the influence of a suite of abiotic habitat characteristics on the over‐summer survival of two imperiled salmonid fishes (coho salmon Oncorhynchus kisutch; steelhead trout Oncorhynchus mykiss) at four study sites on two tributaries of Salmon Creek (Sonoma County, California, USA) from 2012 to 2014, during deepening drought conditions. Study sites spanned an intermittency gradient from continuous flow to near‐dry conditions, and included alluvial and bedrock stream reaches. We estimated over‐summer survival at the pool scale from fish presence–absence data based on paired early‐late summer snorkel surveys. We measured pool dimensions and water quality parameters monthly (more frequently during summer dry down) and, in 2013 and 2014, recorded water quality with continuous loggers in selected pools. We performed: (1) logistic regression in a generalized linear modeling framework to identify factors limiting over‐summer survival and (2) classification trees using the random forests ensemble learning method to identify abiotic thresholds for sustaining salmonids. Results suggested that different factors governed mortality of the two species. Coho salmon, which tended to survive in large, deep pools, were limited by minimum dissolved oxygen (DO) concentrations. In contrast, steelhead trout, which tended to survive in pools with large surface area, were sensitive to pool geometry and temperature. Both species persisted for weeks in large pools with low DO levels, including in pools where at least part of the water column reached sublethal or lethal levels. Our results suggest that shallow, lateral hyporheic flow may be important for maintaining DO and temperatures suitable for sustaining salmonids in isolated pools, whereas groundwater discharge originating from deeper flow paths may generate low‐DO conditions that inhibit salmonid persistence. Geomorphically complex watersheds with a variety of pool geometries and high rates of lateral hyporheic exchange are those most likely to serve as “sanctuary reaches” for imperiled Pacific salmonid populations in semi‐arid regions in the context of a changing climate.

Non-perennial rivers, watercourses that cease to flow at some point in time and space, are widespread globally but often lack effective protections. Although it is thought that these ecosystems are undervalued by society, empirical studies exploring people’s perceptions of non-perennial rivers are uncommon. We carried out an image-based survey at three U.S. universities to measure students’ perception of riverscapes according to seven characteristics: aesthetics, naturalness, habitat for biodiversity, habitat for fish, need of human intervention, importance for human well-being, and conservation value. Our results showed that non-perennial rivers are generally considered less valuable and worthy of conservation than their perennial counterparts. Furthermore, several factors influenced peoples’ perception of non-perennial rivers, including where they live, their educational history, how often they visit rivers, their leisure activities, and whether they live close to a river. Our findings suggested the need to improve people’s perceptions of non-perennial rivers as a step toward increased protection for these ecosystems. This current challenge demands combined actions by researchers from diverse disciplines and professionals working from different perspectives, including policymakers and educators.