Discussion
Quantifying the habitable temperature range of ectotherms is critical to predicting how global warming will impact different species (Huey & Kingsolver, 2019) and understanding the influence of temperature on the minimum energy demands. Payne and Smith (2017) suggested that species with narrower temperature ranges may simply reflect the acute effects of temperature on biological rates of organisms living in warmer environments. Water temperatures in shallow mountain streams can dynamically change with changes in air temperature (Birrell et al., 2020; Mohseni & Stefan, 1999; Pilgrim et al., 1998), and water temperatures in mountain streams are rapidly increasing worldwide with air temperatures (e.g., Isaak & Rieman, 2013). For example, in the Pacific Northwest, stream water temperatures have increased by 0.22°C per decade in the last decades (Isaak et al., 2012). We found the invertebrate taxa across various elevations from 0 to 3000 m, with less tolerances of warmer stream temperatures. We specifically tested the thermal tolerance differences among functional feeding guilds and voltinism. Our use of linear models to predict thermal breadth and habitat temperature tolerances among diverse stream macroinvertebrates can be used to better understand how elevation among temperate streams constrains species traits (Shah et al., 2017, 2020). Although our model results provide further inference towards predicting thermal responses of stream communities and ecosystems under temperature increases, holistic integration of dynamic taxon-specific variation to changing temperatures is needed (Nelson et al., 2020b; Shah et al., 2021).
change effects on the fitness and habitats of stream invertebrates, especially higher elevation stream invertebrate communities, are predictable. We found that thermal tolerance was strongly correlated with voltinism among stream taxa. For example, multi-voltinism such as bivoltine and trivoltine taxa were found in streams with higher maximum temperatures. Semi- and univoltine species have thermal tolerance for colder water temperatures. Voltinism was explained by the combined predictions of thermal adaptation (Kong et al., 2019), but typically, semivoltine species generally inhabit in colder waters (Braune et al., 2008; Danks, 2007; Huryn, 1990). Our synthesis, across a large geographic range of temperate streams, suggests that there are tradeoffs between higher thermal tolerance and energy allocation towards growth that are generalizable across taxa with different life history development strategies. In this study, we found the unexpected result that longer-voltinism species may have thermal adaptation for warmer temperatures. Therefore, our results provide further evidence that thermal adaptation for warmer temperatures is a habitable trait of longer-voltinism species.
Thermal ranges varied among stream functional feeding guilds. We found higher maximum temperatures for collector-gatherer taxa than other feeding guilds. Collector-gatherers can feed on various types of food sources, including animal and detrital plant matter (Merritt et al., 2017), and we have identified that they can tolerate a larger temperature range than other feeding guilds. Collectively, this may extend their habitats more than other species, such as shredders and scrapers. Therefore, the apparently higher thermal tolerance of collector-gatherers may be due to their ability to occupy a broader range of habitats with more diverse food resources than other macroinvertebrates. Among the functional feeding guilds, differences in thermal sensitivity, e.g., tolerance and breath, have been observed previously (Gilman et al., 2010; Grigaltchik et al., 2012; Kordas et al., 2011; Pincebourde & Casas, 2019; Shah, 2020; Vucic-Pestic et al., 2011), but have not summarized. Our large-scale synthesis illustrates a phenomenon of thermal tolerance differences among functional feeding guilds, especially for collector-gatherers. These results could have functional consequences for how increasing temperatures interact with stream communities to alter organic matter processing (Ferreira & Canhoto, 2014; Ylla et al., 2014). Further studies needs to consider the effects of thermal tolerance differences among functional feeding guilds on the stream community and ecosystem function with climate change (Pyne & Poff, 2017).
Temperature variability in aquatic ecosystems is changing worldwide with increased air and land surface temperatures and shifts in precipitation, as well as human-driven flow alterations. Although minimum stream temperatures are increasing in winter months in North American streams, regulated streams and rivers can have lower stream water temperatures in summer months compared to unregulated streams and rivers (Carlisle et al., 2016). As temperature is a primary factor influencing metabolic rates (Nelson et al., 2020b; Shah et al., 2021), organic matter and biogeochemical processes (Ferreira & Canhoto, 2014; Ylla et al., 2014) and stream community composition (Carlisle et al., 2016; Nelson et al., 2017a, 2017b, 2020a), understanding the structural and functional constraints of increasing temperatures on stream ecosystems is critical and likely to be highly variable among taxa (Nelson et al., 2020b; Shah et al., 2021).
Although our synthesis focused on water temperature variation from a large public database (Vieira et al., 2006), we should note that the most extreme biological responses are often triggered by the synchronous occurrence of multiple environmental stressors, e.g., water quality, water flow, and UV radiation (Denny et al., 2011; Jackson et al., 2016). Therefore, the consequences of multiple interacting environmental changes on stream communities and their ecosystem functions is an imperative to understanding climate-induced changes to streams. For example, a study in a mountainous headwater stream in Portugal found complex food web interactions from experimental warming, whereby the presence of a dominant shredder increased fungal biomass and influenced fungal composition on decomposing litter only in the reach with elevated stream temperature (Domingos et al., 2015). Changes in stream temperature from climate and land-use changes have complex effects on stream community composition, biological interactions, and ecosystem functioning. Shifts in food web relationships regardless of changes in temperature or macroinvertebrate community composition can still impact rates of organic matter processing in streams (Demi et al., 2019; Domingos et al., 2015; Kominoski et al., 2011; Kominoski et al., 2013; Rosemond et al., 2015). Understanding how variation in thermal tolerance among stream taxa interacts with other global environmental changes is critical for holistically assessing stream ecosystem integrity. Further, studies that explicitly test for interactive effects of temperature and other environmental changes are needed to elucidate the specific physiological, genetic or environmental drivers behind functional changes in stream ecosystems in a changing world.