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.