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.