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
Recent experiments support the idea that upper thermal limits of aquatic insects arise, at least in part, from a lack of sufficient oxygen: rising temperatures typically stimulate metabolic demand for oxygen more than they increase rates of oxygen supply from the environment. Consequently, factors influencing oxygen supply, like water flow, should also affect thermal and hypoxia tolerance. We tested this hypothesis by measuring the effects of experimentally manipulated flows on the heat and hypoxia tolerance of aquatic nymphs of the giant salmonfly (Plecoptera: Pteronarcys californica ), a common stonefly in western North America. As predicted, stoneflies in flowing water (10 cm s −1 ) tolerated water that was approximately 4°C warmer and that contained approximately 15% less oxygen than did those in standing water. Our results imply that the impacts of climate change on streamflow, such as changes in patterns of precipitation and decreased snowpack, will magnify the threats to aquatic insects from warmer water temperatures and lower oxygen levels.
1. Vulnerability to warming is often assessed using short-term metrics such as the critical thermal maximum (CT MAX ), which represents an organism's ability to survive extreme heat. However, the long-term effects of sub-lethal warming are an essential link to fitness in the wild, and these effects are not adequately captured by metrics like CT MAX .2. The meltwater stonefly, Lednia tumana, is endemic to high-elevation streams of Glacier National Park, MT, USA, and has long been considered acutely vulnerable to climate-change-associated stream warming. As a result, in 2019, it was listed as Threatened under the U.S. Endangered Species Act. This presumed vulnerability to warming was challenged by a recent study showing that nymphs can withstand short-term exposure to temperatures as high as ~27°C. But whether they also tolerate exposure to chronic, long-term warming remained unclear.3. By measuring fitness-related traits at several ecologically relevant temperatures over several weeks, we show that L. tumana cannot complete its life-cycle at temperatures only a few degrees above what some populations currently experience.4. The temperature at which growth rate was maximized appears to have a detrimental impact on other key traits (survival, emergence success and wing development), thus extending our understanding of L. tumana's vulnerability to climate change. 5. Our results call into question the use of CT MAX as a sole metric of thermal sensitivity for a species, while highlighting the power and complexity of multi-trait approaches to assessing vulnerability.
Species vulnerability to global warming is often assessed using short-term metrics such as the critical thermal maximum (CTmax), which represents an organism's ability to survive extreme heat. However, an understanding of the long-term effects of sub-lethal warming is an essential link to fitness in the wild, and these effects are not adequately captured by metrics like CTmax. The meltwater stonefly, Lednia tumana, is endemic to high-elevation streams of Glacier National Park, MT, USA, and has long been considered acutely vulnerable to climate change-associated stream warming. In 2019, it was listed as Threatened under the U.S. Endangered Species Act. This presumed vulnerability to warming was challenged by a recent study showing that nymphs can withstand short-term exposure to temperatures as high as ~27 °C. But how this short-term tolerance relates to chronic, long-term warming has remained unclear. By measuring fitness-related traits at several ecologically relevant temperatures over several weeks, we show that L. tumana cannot complete its life-cycle at temperatures well below the CTmax values measured for its nymphs. Although warmer temperatures maximized growth rates, they appear to have a detrimental impact on other key traits (survival, emergence success, and wing development), thus extending our understanding of L. tumana's vulnerability to climate change. Our results call into question the use of CTmax as a measure of thermal sensitivity, while highlighting the power and complexity of multi-trait approaches to assessing climate vulnerability.
Recent increases in the frequency and size of desert wildfires bring into question the impacts of fire on desert invertebrate communities. Furthermore, consumer communities can strongly impact invertebrates through predation and top‐down effects on plant community assembly. We experimentally applied burn and rodent exclusion treatments in a full factorial design at sites in both the Mojave and Great Basin deserts to examine the impact that fire and rodent consumers have on invertebrate communities. Pitfall traps were used to survey invertebrates from April through September 2016 to determine changes in abundance, richness, and diversity of invertebrate communities in response to fire and rodent treatments. Generally speaking, rodent exclusion had very little effect on invertebrate abundance or ant abundance, richness or diversity. The one exception was ant abundance, which was higher in rodent access plots than in rodent exclusion plots in June 2016, but only at the Great Basin site. Fire had little effect on the abundances of invertebrate groups at either desert site, with the exception of a negative effect on flying‐forager abundance at our Great Basin site. However, fire reduced ant species richness and Shannon's diversity at both desert sites. Fire did appear to indirectly affect ant community composition by altering plant community composition. Structural equation models suggest that fire increased invasive plant cover, which negatively impacted ant species richness and Shannon's diversity, a pattern that was consistent at both desert sites. These results suggest that invertebrate communities demonstrate some resilience to fire and invasions but increasing fire and spread of invasive due to invasive grass fire cycles may put increasing pressure on the stability of invertebrate communities.
For insects, aquatic life is challenging because oxygen supply is typically low compared to air. Although many insects rely on stream flows to augment oxygen supply, oxygen limitation may occur when oxygen levels or flows are low or when warm temperatures stimulate metabolic demand for oxygen. Behavior may allow insects to mitigate oxygen shortages – by moving to cooler, more oxygenated, or faster flowing microhabitats. However, whether stream insects can make meaningful choices depends on: i) how much temperature, oxygen, and flow vary at microspatial scales in streams and ii) the ability of insects to exploit that variation. We measured microspatial variation in temperature, oxygen saturation, and flow velocity within riffles of two streams in Montana, USA. Additionally, we examined the preferences of nymphs of the stonefly Pteronarcys californica to gradients of temperature, oxygen, and flow in lab choice experiments. Temperature and oxygen level varied modestly within stream riffles (~ 1.8 °C, ~ 8.0% of air saturation, respectively). By contrast, flow velocity was highly heterogeneous, often varying by more than 125 cm s−1 within riffles and 44 cm s−1 around individual cobbles. Exploiting micro-variation in flow may thus be the most reliable option for altering rates of oxygen transport. In alignment with this prediction, P. californica nymphs showed relatively little ability to exploit laboratory gradients in temperature and oxygen. By contrast, they readily exploited micro-variation in flow – consistently choosing higher flows when conditions were warm or hypoxic. These behaviors may help stream insects mitigate low-oxygen stress from climate change and other anthropogenic disturbances.
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