Introduction
Anthropogenic climate change is having pronounced and accelerated
biological impacts on organisms and ecosystems (Pacifici et al., 2015;
Sinclair et al., 2016). Biologists are trying to understand and predict
these impacts. Inevitably, most of these impacts are mediated by the
behavioral and physiological responses of organisms to changing
biological variables (Doi et al., 2008; Radchuk et al., 2019). The
concept of species habitability with temperature applies community-level
understanding of species thermal tolerance using assembly rules that
estimate the presence or absence of species as well as their abundance
(Menezes et al., 2010). Yet, the need to move beyond descriptive to a
predictive understanding of species thermal tolerances is critical given
climate-driven increases in temperature (e.g., Pacifici et al., 2015;
Sinclair et al., 2016).
Most aquatic species, such as invertebrates, are ecothermic and
therefore have body temperatures that reflect their environment to
varying degrees (Angilletta, 2009; Sinclair et al., 2016). Extremely
high or low temperatures are lethal, and temperature determines all
cellular and physiological functions, including metabolism, development,
growth, migration, and reproduction and indirectly throughout changes in
prey community and environments (Grigaltchik et al., 2012; Ylla et al.,
2014; Nelson et al., 2020b; Schofield & Kline, 2018; Shah et al.,
2021). Changes in water temperature have serious implications at
population, community, and ecological levels (e.g., Grigaltchik et al.,
2012; Sinclair et al., 2016), especially given the uncertainty in how
stream temperature regimes may change with different climate and
land-use changes (Kominoski & Rosemond, 2012).
To date, attempts to measure vulnerability to climate change have
largely assessed species responses to temperature challenges, such as
lethal and critical thermal limits, i.e., habitable temperature (Deutsch
et al., 2008; Pinsky et al., 2019). Metabolic rates of ecotherms can
influence individual fitness, and consequently geographic distributions
and abundances (Angilletta, 2009; Shah et al., 2020; Terblanche &
Chown, 2007; Vannote & Sweeney, 1980), and related to their body
temperatures. For example, in aquatic ecosystems increased metabolic
demand with higher temperature can outpace the supply of oxygen from the
environment causing decreased performance and lowered tolerance to heat
stress (Pörtner et al., 2017). Therefore, understanding habitable
temperature across different environments would help us to predict
geographic variation in fitness, as well as species abundance and
distributions in response to global warming (Dillon et al., 2010).
Addressing the effects of climate change through the lens of exothermic
biology, especially species habitable temperature, will expand
understanding of how temperature affects the majority of species on
Earth.
Recent experimental temperature manipulations have quantified the
habitable temperature ranges of stream invertebrates (Nelson et al.,
2017a, 2017b, 2020a, 2020b; Shah et al., 2020). These studies suggested
that habitable temperature varied among the geographical location of the
habitats, such as elevation and latitude, and species traits, such as
feeding mode (Shah et al., 2017, 2020; Sunday et al., 2011). Shah et al.
(2017) showed the elevation effects of thermal breadth of many stream
macroinvertebrates in the temperate and trophic streams. Also, Nelson et
al. (2020a) reported the effect of thermal breadth diversity on energy
flux through a stream food web. A global analysis of thermal tolerance
among terrestrial and marine ecotherms found thermal tolerance breadths
increased with latitude (Sunday et al., 2017). Yet, how habitable
temperatures and thermal tolerances range across diverse stream
ecotherms from broad geographic regions and along elevation gradients is
needed to better predict the effects of changing temperatures on species
functional traits of inland waters (Shah et al., 2020).
In this study, we compared how geographic location and species traits
vary across North America. First, we hypothesized that geographic
location of stream ecosystems, such as elevation and latitude, influence
the habitable water temperature of lotic (stream) invertebrates, such
that habitats directly influence species’ life cycles and consequently
their fitness and evolution (Kearney et al., 2020; Shah et al., 2020;
Sunday et al., 2011). Second, we hypothesized that species traits (e.g.,
voltinism and feeding behavior) are influenced by habitable temperature,
as these traits are directly related with species adaptations to
temperature (Sgrò et al., 2016). Also, thermal performance in the
different phylogeny (e.g., taxonomic category) is different, as these
traits are directly related with species adaptations to temperature.
Numerous tests of the two hypotheses have been performed (e.g., Kong et
al., 2019; Nelson et al., 2020b; Shah et al., 2021), but such tests have
only been conducted using temperature manipulations in the field and
laboratory for a limited number of species. Here, we tested our
hypotheses with variable taxa and geographic regions using a dataset of
stream invertebrate traits in North America (Vieira et al., 2006). The
dataset we used includes thermal preferences, i.e., maximum and minimum
water temperature surviving in, and the habitat conditions, such as
elevation and latitude of the collected points as well as various
species traits, including voltinism and feeding behavior for overn = 2,200 species. The dataset contained data from various taxa,
including bivalve, amphipods, and insects in streams across North
America, mostly in the United States. Use of a database summarized
various related data which allowed us to compare these factors to the
habitable water temperature of stream macroinvertebrate taxa across
broad geographic ranges in latitude and elevation.