Insects in high‐elevation streams: Life in extreme environments imperiled by climate change

Abstract

High-elevation streams are some of the most extreme ecosystems on Earth, yet they harbor extensive aquatic insect biodiversity and support a high degree of endemism (Hotaling et al., 2017). Highelevation streams occur from >2,000 m (at higher latitudes) to >4,000 m (lower latitudes) and represent nearly 5% of the world's waterways (Figure 1). They are typically fed by multiple meltwater sources, can be covered by snow and ice for most of the year, and are often fragmented and isolated, with cold, turbulent, fast-flowing water, low ionic strength, low oxygen availability, and high levels of UV radiation (when not covered by snow; Jacobsen & Dangles, 2017). High-elevation stream conditions are also highly variable in space and time, depending on source, drainage geology, elevation, aspect, latitude, and time of year. Collectively, however, high-elevation streams are experiencing some of the most rapid climate-driven

Temperature dependence of metabolic rate in tropical and temperate aquatic insects: Support for the Climate Variability Hypothesis in mayflies but not stoneflies

“…Mountain stream temperatures are already increasing worldwide (Isaak & Rieman, 2013;Niedrist & Füreder, 2020). Temperatures of shallow mountain streams, such as those in this study, are also likely to show concomitant increases with air temperature (Birrell et al, 2020;Mohseni & Stefan, 1999;Pilgrim et al, 1998). Projected mean air temperature increases of 2°C-3°C in the Andes and 3°C-4°C in the northern Rocky Mountains by the 2050s (Nogués-Bravo et al, 2007;Rangecroft et al, 2016) The effects of increasing temperatures may also lead to mismatches between dissolved oxygen supply and demand (Pörtner, 2001;Verberk et al, 2011;Verberk, Overgaard, et al, 2016).…”

Flow increases tolerance of heat and hypoxia of an aquatic insect

“…In faster flows, boundary layers are thinner and pose less of a barrier to oxygen transport from the environment to the respiratory surfaces (figure 1). In turn, faster oxygen transport across thinner boundary layers supports higher metabolic demand arising at higher temperatures and can support ongoing activity in deeper levels of environmental hypoxia found at high elevation or in eutrophic conditions [12,45,46]. In the ongoing discussions about the mechanistic causes of upper thermal limits in aquatic systems [12,15], our results support the idea that oxygen is important and suggest that renewed attention should be paid to oxygen transport in relation to physical aspects of flow.…”

“…Thus, long-term shifts in flow regimes will have systemic effects on stream and river ecosystems [62]. In temperate streams, lower seasonal flows and warmer water temperatures have become increasingly frequent [49,63] primarily because the annual snowpack is decreasing and drought severity is increasing in many regions [45,64]. Our results, in particular, suggest that low flows will magnify the climatic risks of oxygen limitation stemming from previously recognized interactions between temperature and oxygen [12,63,65].…”

“…Climate change and anthropogenic disturbances are causing many streams and rivers to warm, to become more hypoxic and to show greater overall variability in flow, resulting in periods of low flow that may coincide with warm conditions during dry summers [48][49][50][51][52]. Although water temperature and dissolved oxygen receive much attention, flows vary more in time and space, with strong effects on aquatic insects [45]. For example, flow directly affects the size and shape of individual macroinvertebrates [53], where they choose to position themselves [54,55], how readily they can find and consume food [56] and whether they can obtain sufficient oxygen [29,30,33,57,58].…”

Rock glaciers and related cold rocky landforms: Overlooked climate refugia for mountain biodiversity

“…The surfaces of CRLs are typically boulder‐strewn and heterogeneous, and include dry, rocky ridges, sediment‐filled depressions and unstable, shifting margins (Figure 2). Paired with the environmental challenges that already stem from high‐elevation habitat in mountain ecosystems (e.g., extreme cold, intense solar radiation, reduced oxygen availability; Birrell et al, 2020; Elser et al, 2020), instability of CRL mantles, routine avalanches, and rockfall make their surfaces particularly harsh environments. For temperature, cold is not the only risk.…”