Abstract

Species inhabiting warm-edge populations of their distribution are suggested to be at the forefront of global warming due to reduced fitness, limited gene flow and living close to their physiological thermal limits. Determining the scale that governs thermal niche and the functional responses of habitat-forming species to environmental stressors is critical for successful conservation efforts, particularly as coastal ecosystems are impacted by global change. Here, we examine the susceptibility of warm-edge populations to warming, in the habitat-forming macroalga, Hormosira banksii, from south-eastern Australia. We use a quantitative breeding design to quantify intraspecific variation in thermal performance (growth, ontogenic development and photosynthetic efficiency) of different genotypes sourced from sites at the equatorward distributional edge (warm-edge) and those towards the center of its distribution (non-edge). The genetic diversity and structure of H. banksii was also examined using microsatellite markers amongst the same sites. Our results found variable responses in thermal performance for growth and development. Warm-edge germlings grew optimally in lower temperatures tested and had narrower thermal breadth compared to non-edge germlings which grew in higher and more broader temperatures. Warm-edge germlings however, showed greater plasticity to tolerate high light indicated by a greater proportion of energy being dissipated as regulated nonphotochemical quenching (Y(NPQ)) than nonregulated nonphotochemical quenching (Y(NO)). Overall genetic diversity was lower at the warm-edge location with evidence of increased structuring and reduced gene flow in comparison to the non-edge location. Evidence of genetic structuring was not found locally between high and low shore within sites. Together, these data suggest that non-edge populations may be “thermally buffered” from increased temperatures associated with ocean warming. Warm-edge populations of H. banksii, however, may be vulnerable to warming, due to narrower thermal breadth and sensitivity to higher temperatures, with genetic impoverishment through loss of individuals likely to further reduce population viability.

Highlights

  • Anthropogenic mediated climate change is already having profound impacts on the physiology and distribution of many species worldwide (Pecl et al, 2017; IPCC, 2018)

  • Mean maximum monthly air temperatures recorded at local weather stations were similar at both locations, minimum temperatures were lower at the non-edge location

  • Populations inhabiting the warm range edge of distributions are suggested to be at the forefront of climate change

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Summary

Introduction

Anthropogenic mediated climate change is already having profound impacts on the physiology and distribution of many species worldwide (Pecl et al, 2017; IPCC, 2018). Understanding how global warming and future extreme climate events will impact species requires knowledge of species’ thermal niche and underlying genetic diversity across its distribution. Thermal niche is developed through acclimation and adaptation to temperatures experienced throughout a species life history which can vary with space and time (for a review, see Bennett et al, 2015). Thermal breadth is influenced by the range of temperatures experienced throughout a species life history (Sunday et al, 2012). As thermal limits often govern species range boundaries, and individuals are pushed beyond their physiological limits, the fitness of individuals diminish toward distributional limits (Sagarin and Gaines, 2002; Thomas et al, 2004; Hampe and Petit, 2005; Pearson et al, 2009)

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