Global patterns of sea surface climate connectivity for marine species

Published in Earth & Environment
Global patterns of sea surface climate connectivity for marine species
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Climate change is among the phenomena that greatly affect marine biodiversity. Ocean warming and marine heatwaves become more frequent and intense, leading even to completely novel climates. Facing these global changes, marine habitats are becoming unsuitable to local biodiversity and marine species are forced to shift their distributions to reach new areas with suitable climatic conditions. However, the risk of failure in doing so can be substantial to species. Many current habitats may not have their climatic analog in the future, meaning that their unique combination of climatic characteristics may become completely absent at a worldwide scale within the next decades. Even when climatically analogous habitats are expected to exist in the future, reaching them from current habitats may demand long trajectories that can in some cases exceed species abilities to disperse. Along these trajectories, even highly dispersal ability species can be exposed to highly dissimilar climates, jeopardizing their performance and viability. It is thus particularly interesting and crucial to understand, assess and quantify how marine areas will be climatically connected to each other in the future, identify connectivity routes among habitats and highlight climate risk zones. This is where our work comes in, by proposing an innovative, integral framework for assessing global patterns of ocean climate connectivity, based on circuit theory and least-cost path analyses.

The objective of our work is to highlight ocean areas that promote surface climate connectivity among thermal analogous sites, providing conservation targets that could promote species persistence. We evaluate sea surface climate connectivity between thermal conditions of the recent past (i.e., 1950) and their future thermal analogous at the end of the century (i.e., 2100), under various climate change scenarios. We investigate which marines areas will probably have thermal analogs and which may become analogs of other marine areas in the future. By evaluating the connectivity between these thermal analogous areas, we aim to delineate trajectories that minimize exposure to dissimilar climates, reducing the effort required from the species to move between the thermal analogous sites.

Going beyond previous methodological approaches, we do not consider simple linearity between present and future time slots, which might considerably underestimate the potential impact of extreme climatic events, expected to occur during the considered period. This is one of the innovative elements of this work. Previous studies only used climatic data for two distinct time slots, reflecting present and future conditions, and interpolated the intermediary climate with a linear function. This produces climatic trajectories that actually neglect climatic stochasticity and are unsuitable for minimizing climate exposure to possible extreme events. In our analysis, we propose an alternative framework that incorporates user-defined, short intervals (here five-year periods) and assume linearity only during these short intervals, thus producing temporally consecutive thermal analogs that permit the estimation of species cumulative exposure over long time periods.

Under this framework, we show that species in the northern oceans will require longer trajectories towards their thermal analogs in the future, with more exposure to dissimilar climates. On the contrary, species in the southern oceans will require shorter trajectories, often with minimum exposure to non-analogous thermal conditions. We highlight key areas that will facilitate trajectories between thermal analogs, most notably in the northern oceans, as well as areas in North Pacific and North Atlantic regions.

Linking this work to major management and conservation concerns, we evaluate the connectivity patterns between contemporary biodiversity hotspots and their future analogs and highlight that while most of these hotspots are currently located at coastal areas, their future thermal analogous sites will probably be located in the open sea. With this outcome, the persistence of local biodiversity is under question, as these thermal analogs are very distant and might not correspond to suitable habitats for the species. These conclusions are consistent to the connectivity patterns that we obtain when considering the Major Fishing Areas (MFA) according to the Food and Agriculture Organization (FAO) categorization. Apart from some limited exceptions, future analogous MFAs are expected to be distant from current ones, jeopardizing marine species ability to reach them.

Our study provides an integrative assessment of the sea surface climate connectivity at the global scale. The conclusions of this work can be used for efficient conservation, climate smart planning. Many challenging but crucial perspectives remain on the inclusion of depth specific climatic data and the development of 3D approaches for conducting more comprehensive assessments of marine connectivity patterns in the future.

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