Thermal Carrying Capacity for a Thermally-Sensitive Species at the Warmest Edge of Its Range

Abstract

Anthropogenic environmental change is causing unprecedented rates of population extirpation and altering the setting of range limits for many species. Significant population declines may occur however before any reduction in range is observed. Determining and modelling the factors driving population size and trends is consequently critical to predict trajectories of change and future extinction risk. We tracked during 12 years 51 populations of a cold-water fish species (brown trout Salmo trutta) living along a temperature gradient at the warmest thermal edge of its range. We developed a carrying capacity model in which maximum population size is limited by physical habitat conditions and regulated through territoriality. We first tested whether population numbers were driven by carrying capacity dynamics and then targeted on establishing (1) the temperature thresholds beyond which population numbers switch from being physical habitat- to temperature-limited; and (2) the rate at which carrying capacity declines with temperature within limiting thermal ranges. Carrying capacity along with emergent density-dependent responses explained up to 76% of spatio-temporal density variability of juveniles and adults but only 50% of young-of-the-year's. By contrast, young-of-the-year trout were highly sensitive to thermal conditions, their performance declining with temperature at a higher rate than older life stages, and disruptions being triggered at lower temperature thresholds. Results suggest that limiting temperature effects were progressively stronger with increasing anthropogenic disturbance. There was however a critical threshold, matching the incipient thermal limit for survival, beyond which realized density was always below potential numbers irrespective of disturbance intensity. We additionally found a lower threshold, matching the thermal limit for feeding, beyond which even unaltered populations declined. We predict that most of our study populations may become extinct by 2100, depicting the gloomy fate of thermally-sensitive species occurring at thermal range margins under limited potential for adaptation and dispersal.

Global Warming of Salmon and Trout Rivers in the Northwestern U.S.: Road to Ruin or Path Through Purgatory?

“…For resident trout species, our results suggest that river warming in recent decades is shifting thermally suitable habitats, which may account for the upstream distribution shifts observed in some of the Northwest's trout populations (Eby et al 2014;Al-Chokhachy et al 2016). Shifts are expected at warm downstream population boundaries because increasing temperatures will make growth, disease resistance, and survival more difficult during one or more life stages (Hari et al 2006;Ayll on et al 2013). Low densities of individuals may persist near warm boundaries if cold microrefugia are available (Hillyard and Keeley 2012), but refugia will become increasingly 580 isolated in future years as the surrounding matrix of warmer waters continues to grow.…”

“…; Ayllón et al. ). Low densities of individuals may persist near warm boundaries if cold microrefugia are available (Hillyard and Keeley ), but refugia will become increasingly isolated in future years as the surrounding matrix of warmer waters continues to grow.…”

Brown trout thermal niche and climate change: expected changes in the distribution of cold‐water fish in central Spain

“…Tokeshi, 1993). Other environmental variables (hydraulics, temperature) also influence the density of fish populations (Ayllón et al 2013).…”

“…; Ayllón et al . ). According to Haldane's second hypothesis (Haldane ), these losses are more likely to be noticed at the limits of the thermal axis of the species' ecological niche.…”

“…The simulation of climate scenarios can help us assess the magnitude of the loss of suitable habitat ranges, not only in terms of the distribution range but also in terms of the physiological efficiency (i.e. Hari et al 2006;Jonsson and Jonsson 2009;Wenger et al 2011;Ayllón et al 2013). According to Haldane's second hypothesis (Haldane 1956), these losses are more likely to be noticed at the limits of the thermal axis of the species' ecological niche.…”