Flow regime, temperature, and biotic interactions drive differential declines of trout species under climate change

Abstract

Broad-scale studies of climate change effects on freshwater species have focused mainly on temperature, ignoring critical drivers such as flow regime and biotic interactions. We use downscaled outputs from general circulation models coupled with a hydrologic model to forecast the effects of altered flows and increased temperatures on four interacting species of trout across the interior western United States (1.01 million km 2 ), based on empirical statistical models built from fish surveys at 9,890 sites. Projections under the 2080s A1B emissions scenario forecast a mean 47% decline in total suitable habitat for all trout, a group of fishes of major socioeconomic and ecological significance. We project that native cutthroat trout Oncorhynchus clarkii, already excluded from much of its potential range by nonnative species, will lose a further 58% of habitat due to an increase in temperatures beyond the species' physiological optima and continued negative biotic interactions. Habitat for nonnative brook trout Salvelinus fontinalis and brown trout Salmo trutta is predicted to decline by 77% and 48%, respectively, driven by increases in temperature and winter flood frequency caused by warmer, rainier winters. Habitat for rainbow trout, Oncorhynchus mykiss, is projected to decline the least (35%) because negative temperature effects are partly offset by flow regime shifts that benefit the species. These results illustrate how drivers other than temperature influence species response to climate change. Despite some uncertainty, large declines in trout habitat are likely, but our findings point to opportunities for strategic targeting of mitigation efforts to appropriate stressors and locations.global change | hydrology | invasive species | niche model | distribution modeling N early all broad-scale analyses of climate effects on freshwater species have focused on temperature shifts, to the exclusion of other climate-driven drivers. Although temperature is a critical determinant of metabolic and physical processes (1), important ecosystem effects on streams and rivers may also be mediated by flow regime and biotic interactions. Flow regime has been described as a "master variable" (2) that controls or influences many aspects of the physical aquatic environment, as well as the timing of reproduction and migration of many organisms (3). Biotic interactions are increasingly recognized as important components of climate-species relationships (4, 5), but are rarely included in projections of species distributions under future climates, with some notable exceptions (6). Competitive interactions in particular are not commonly modeled (but see ref. 7), despite interest in invasive-native species interaction under climate change (8). It is likely that all three factors-temperature, flow regime, and biotic interactions-will play important roles in future aquatic species distributional shifts (8-11).Trout serve as excellent model organisms for examining how these mechanisms could alter population dynamics and species distributio...

Slow climate velocities of mountain streams portend their role as refugia for cold-water biodiversity

“…To characterize the thermal environments associated with cold-water species, we referenced site occurrence locations from published biological databases (7,19) with S1 NorWeST temperature predictions and X-HS velocity estimates from the 1-km stream reaches that encompassed individual sites. We considered seven species: (i) three native species of conservation concern (bull trout = 1,100 sites; cutthroat trout = 927 sites; Rocky Mountain tailed frogs = 953 sites); (ii) three trout species that have been broadly introduced in the region Table 2.…”

“…Mountains also host a suite of endemic species, many of which are perceived to be condemned to extinction as their habitats contract or disappear as a result of climate change-related temperature increases, environmental stochasticity, and nonnative species invasions (3)(4)(5). A substantial literature, to which we have contributed, has developed in previous decades suggesting a similar fate for cold-water fishes and other aquatic taxa in montane environments (6)(7)(8), but it rests largely on predictions about temperature increases and untested assumptions about the relationship between air temperature and water temperature. In particular, previous studies have failed to recognize that the highest and coldest streams are relatively insensitive to air temperature fluctuations (9,10), and that the morphologies of many mountain ranges and their stream networks may mediate climate warming such that shifts in thermal habitat are small (2,11).…”

Observed groundwater temperature response to recent climate change

“…Numerous studies on the impact of recent or projected climate change on the thermal regimes of surface water bodies and the associated impact for cold-water fish habitats have already been conducted (e.g., Kaushal et al, 2010;van Vliet et al, 2011van Vliet et al, , 2013Wenger et al, 2011;Isaak et al, 2012;Wu et al, 2012;Jones et al, 2014), but the thermal sensitivity of shallow aquifers to climate change is a relatively unstudied phenomenon (e.g., Brielmann et al, 2009Brielmann et al, , 2011Taylor and Stefan, 2009;Kurylyk et al, , 2014a. The thermal response of GWT to climate change is of particular interest to river temperature analysts, as the thermal regimes of base-flow-dominated streams or rivers and hydraulically connected aquifers are inextricable linked (Hayashi and Rosenberry, 2002;Tague et al, 2007;Risley et al, 2010).…”

Waning habitats due to climate change: the effects of changes in streamflow and temperature at the rear edge of the distribution of a cold-water fish

“…Beer and Anderson, 2013;Eby et al, 2014); however, the combined effects of rising stream temperatures and reductions in streamflow remain relatively unexamined, with some exceptions (e.g. Wenger et al, 2011;Muñoz-Mas et al, 2016). Jonsson and Jonsson (2009) predicted that the expected effects of climate change on water temperatures and streamflow will have implications for the migration, ontogeny, growth and life-history traits of Atlantic salmon, Salmo salar Linnaeus, 1758, and brown trout, Salmo trutta Linnaeus, 1758.…”