Introduction
Ocean warming is having profound effects on benthic marine ecosystems across the globe (Hoegh-Guldberg & Bruno 2010). Foundation species such as seaweeds, seagrasses and corals can be particularly susceptible to warming and have undergone extensive thermal stress (Hughes et al. 2018), mortality (Marba & Duarte 2010), and range contraction (Wernberg et al. 2016) over the past two decades. In conjunction with direct physiological impacts, ocean warming is leading to changes in species distributions resulting in new assemblages of species coming together and competing for space and resources within an ever-changing physical environment (Lenoir et al. 2020). This combination of factors creates a diverse and dynamic array of potential outcomes of climate change on the structure and function of marine ecosystems.
Temperature effects on populations across a species geographical range are not homogeneous and depend on the population’s thermal range position and a species evolutionary history and adaptive capacity (Angilletta 2009; Somero 2010). For example, if individuals from different populations display similar optimal and upper thermal limits, then cool-edge populations will have lower sensitivity to warming than warm-edge populations where ambient temperatures are already close to upper thermal limits (Bennett et al. 2019). This is an inherent assumption of many species distribution models that rely on the realised distribution of species to predict the likelihood of extirpation or range shifts under climate change (Araújo & Peterson 2012). If, on the other hand, thermal performance differs between populations– through local adaptation or acclimatization – then the thermal safety margin between ambient conditions and an individual’s upper thermal limit may remain relatively constant between populations across a species range (Bennett et al. 2015). This is an inherent assumption used to estimate the thermal sensitivity of reef building corals, for which the magnitude and the duration of local thermal anomalies are strong predictors of thermal bleaching (McClanahan et al. 2019). Determining whether thermal performance varies between populations and how this relates to local climatology or a species distribution is critically important for our capacity to anticipate and manage climate change impacts. For the vast majority of species, within-species patterns of thermal sensitivity remain unknown, highlighting a fundamental knowledge gap in climate change ecology.
An additional challenge for predicting the impacts of climate change on marine ecosystems is the influence of different ecological processes such as biotic interactions, on the outcomes of warming (Araújo & Luoto 2007). As species redistribute around the world, novel interactions are emerging that may either accelerate or buffer ecosystems from change under different environmental contexts (Gilman et al. 2010; Lurgiet al. 2012). Incorporating this complexity into models is challenging based on current approaches (Pagès et al. 2018). For example, thermal performance studies are routinely conducted in controlled laboratory environments which are invaluable for understanding complex physiological and evolutionary processes, but often miss key ecological processes and trade-offs that may determine an organism’s success in the wild (Buñuel et al. 2020). While the ecological realism of experimental systems is improving (Ullah et al. 2018), calibrating laboratory-based experiments with real world multi-species ecological interactions remains a challenge. At the other end of the spectrum, direct impacts of thermal stress on natural ecosystems from marine heatwaves and long-term warming offer invaluable insights into the whole-of-ecosystem response to warming (Smale et al. 2019). However, such events are irregularly distributed and historically uncommon (Oliver et al. 2018). In addition, the ecological consequences of marine heatwaves can be catastrophic, undermining the health of the ecosystem for which the information is needed (Wernberg et al. 2016).
In this study we took an intermediate approach and conducted a 12-month translocation experiment of the seagrass Posidonia oceanicaacross its geographical range. Translocation experiments have the benefit of directly comparing the performance of geographically distant populations under common, relatively natural ecological settings. We conducted our study across a steep 5ºC gradient in average annual temperatures from the western to eastern Mediterranean Sea. The Mediterranean has experienced rapid warming between 0.25-0.65 ºC dec-1 over the past three decades, 2-3 times faster than the average global ocean (Marbà et al. 2015). Contemporary climate change has impacted central populations of P. oceanica in recent decades (Marba & Duarte 2010) and previous studies have predicted that the species could face functional extinction by 2050 (Jordà et al. 2012; Chefaoui et al. 2018). High thermal sensitivity in central populations raises questions about the sensitivity of warm-edge populations, which regularly face summer temperatures that are similar to heatwave conditions attributed to losses in the western basin. The aim of this experiment was to compare the eco-physiological performance of P. oceanica across its thermal distribution to quantify the thermal sensitivity of different populations to climate warming. We test the hypothesis that thermal performance reflects the thermal geography of a species either through local adaptation or thermal niche conservatism across its distribution.