Abstract
Rising ocean temperatures and extreme temperature events have precipitated declines and local extinctions in many marine species globally, but patterns of loss are often uneven across species ranges for reasons that are poorly understood. Knowledge of the extent of local adaptation and gene flow may explain such patterns and help predict future trajectories under scenarios of climate change. We test the extent to which local differentiation in thermal tolerance is influenced by gene flow and local adaptation using a widely distributed intertidal seaweed (Hormosira banksii) from temperate Australia. Population surveys across ~2,000 km of the species range revealed strong genetic structuring at regional and local scales (global F ST = 0.243) reflecting extremely limited gene flow, while common garden experiments (14‐day exposures to 15, 18, 21°C) revealed strong site differences in early development and mortality in response to elevated temperature. Embryos from many sites spanning a longitudinal thermal gradient showed suppressed development and increased mortality to elevated water temperatures, but populations originating from warmer and more variable thermal environments tended to be less susceptible to warming. Notably, there was significant local‐scale variation in the thermal responses of embryos within regions which was corroborated by the finding of small‐scale genetic differences. We expect the observed genetic and phenotypic differentiation to lead to uneven responses to warming sea surface temperatures in this important marine foundation species. The study highlights the challenges of predicting species responses to thermal stress and the importance of management strategies that incorporate evolutionary potential for “climate‐proofing” marine ecosystems.
Highlights
The speed and magnitude of biotic shifts being triggered by climate change pose a major challenge for marine biodiversity conservation
Such declines appear to be long term or even permanent, highlighting a need to understand the adaptability and vulnerability of marine macrophytes to future change. This is pertinent in areas where marine macrophyte communities are showing signs of acute and chronic climate stress, such as climate change “hot spots” that are prone to extreme temperature events, and that are experiencing sea surface temperature orders of magnitude above the global average (Babcock et al, 2019; Hobday & Pecl, 2014; Wernberg et al, 2011)
This was expected as all gamete mixes were initially prepared under common temperatures (15°C), where fertilization was expected to be largely complete within minutes (Kevekordes & Clayton, 1996), prior to exposure to experimental temperatures
Summary
The speed and magnitude of biotic shifts being triggered by climate change pose a major challenge for marine biodiversity conservation. The resilience of marine macrophyte populations to environmental change will be largely determined by levels of standing adaptive genetic variation and patterns of gene flow among populations.
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