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.