Global warming can disrupt ecological connectivity among marine reserves by shortening potential dispersal pathways through changes in larval physiology. These changes can compromise the effectiveness of marine reserve networks, thus requiring adjusting their design to account for warmer oceans.

To address this challenge, researchers from the Conservation Planning Group are collaborating with scientists, agencies, NGOs, and fishers to develop an approach to planning for connectivity under future ocean warming. The project, coordinated by Comunidad y Biodiversidad, A.C. (COBI), is guiding the design and implementation of a network of marine reserves in the Gulf of California, Mexico.

Using the Midriff Islands Region as a case study, we developed a framework to design marine reserve networks that integrates graph theory and changes in larval connectivity due to ocean warming. Given expected changes in larval connectivity, we show that our graph-theoretical approach based on centrality of habitat patches can help design better connected marine reserve networks for the future with equivalent economic costs. The results of this study, led by Jorge G. Alvarez-Romero, were recently published in Global Change Biology.

Our study area was the Midriff Islands Region in the northern Gulf of California, Mexico, which is a marine area of high conservation significance and an important area for industrial, artisanal, and recreational fisheries. Satellite imagery by ESRI, DigitalGlobe Vivid Mexico, 2013 ©

The project was supported by data from a long-term monitoring program (PANGAS) developed in collaboration with fishers. The photo shows a local fisher monitoring fish in the Gulf of California. Photo by Arturo Hernandez-Velasco ©

Why is this work important and timely?

Worldwide, marine reserves are expanding rapidly to achieve international conservation goals. Yet, implementing new reserves needs to ensure persistence of biodiversity through maintaining ecological processes, such as the movement of organisms across seascapes. These processes are particularly important to enhance recovery processes after disturbances, such as coral reef bleaching and cyclones. Recovery through resettlement depends largely on maintaining the supply of larvae, underpinning the need for well-connected networks of marine reserves.

Consequently, linking marine reserves within networks is pivotal to the long-term persistence of marine populations. However, global warming can disrupt connectivity by shortening potential dispersal pathways through changes in larval physiology. These changes can compromise the performance of marine reserve networks, thus requiring adjusting their design to account for future warmer oceans.

With imminent and ongoing changes in our oceans, our study pioneers the globally-important challenge of maintaining the effectiveness of marine reserves despite changes in ecological connectivity associated with global warming.

What did we do?

We developed a method to design reserve networks that accounts for reduced larval connectivity due to ocean warming. Our approach includes mapping and modelling the distribution of ecosystems and >200 species (using thousands of records from more than a decade of fieldwork in the region), modelling larval dispersal of focal species (under current and future ocean warming scenarios), and estimating the potential costs associated with excluding fishing from marine reserves. We generated alternative systems of marine reserves using the decision-support tool Marxan. We compared the performance of marine reserve networks based on both conventional (designed to represent only species and ecosystems) and graph-theoretical (designed to maximize connectivity under current and ocean-warming scenarios) approaches.

We focused primarily on rocky-reef ecosystems and associated species and habitats, which have outstanding biological and socioeconomic importance in the region. Species associated with rocky reefs, such as the leopard grouper (Mycteroperca rosacea), are particularly important for small-scale fisheries and sustainable local livelihoods.

Rocky reef in the Midriff Islands, Gulf of California and Leopard grouper (Mycteroperca rosacea), one of our focal species used to plan for connectivity and ocean warming. Photos by Octavio Aburto ©

Our analysis included Sargassum forests, mangroves, and seagrass meadows because they provide important habitats for reef-associated species and have local socioeconomic benefits. These ecosystems provide spawning and recruitment habitats of our focal species, thus we considered them as habitat patches that could act as ‘hubs’ (strongly connected both upstream and downstream and potentially important for metapopulation robustness) or ‘stepping stones’ (if lost, would strongly reduce the connectivity of the reserve network).

Mangroves, Sargassum forests, and seagrass meadows are key habitats for reef-associated species in the Gulf of California. (a) Mangroves bordering an island, Photo by Octavio Aburto ©. (b) Sargassum beds around Isla Tiburon, Photo by Thor Morales ©. (c) Satellite photograph of seagrass meadows around Infiernillo Channel, Satellite imagery by ESRI, DigitalGlobe Vivid Mexico 2013. (d) Jewel moray (Muraena lentiginosa) in Sargassum forest, Photo by Octavio Aburto ©

Our approach accounts for the socioeconomic impacts of marine reserves based on the economic loss associated with the exclusion of fishing, namely opportunity cost to fleets operating in the region. We calculated opportunity costs to fisheries using an end-to-end ecosystem model (based on the Atlantis framework) for small-scale, sport fishing, and industrial fisheries.

Small-scale fisheries are very important in the study region in terms of catches and food security. Artisanal fishers commonly use small boats called ‘Pangas’, which are multipurpose boats that can use different fishing gears, hold two to three fishers, and are the main vessels used by small-scale fishers in the northern Gulf of California. Photo provided by COBI ©

What did we find?

Our results indicate that current larval connectivity may be reduced significantly under ocean warming for various species. Across our three focal species, the ecological networks will become less connected because some areas previously connected by larval dispersal were predicted to be unreachable due to estimated reduction in planktonic larval duration (PLD) under warmer ocean conditions. These findings emphasize the need to consider the potential impacts of global warming on connectivity and indicate that important changes in marine reserve network design may be needed to maintain connectivity under future climatic conditions.

Regional patterns of larval connectivity for the Leopard grouper (Mycteroperca rosacea) under current and global warming scenarios based on a dynamic larval dispersal model. Estimated reduced connectivity is due to potential shortening of PLD associated with ocean warming. Satellite imagery by ESRI, DigitalGlobe Vivid Mexico, 2013 ©

We show that designing marine reserve networks using our novel graph-theoretical approach based on centrality of habitat patches produces better-connected marine reserve networks than typical approaches without additional socioeconomic costs. Further, we found notable differences in the structure of marine reserve networks designed with and without consideration of ocean warming. For example, we noted a better distribution of centrality among reserves, which means that network connectedness relied less on few highly central reserves, which can increase the resilience of networks to impacts on individual reserves.

Larval connectivity patterns for the Leopard grouper (Mycteroperca rosacea) for alternative marine reserve networks under the global warming scenario. Disregarding connectivity and changes due to ocean warming results in less connected networks (left) than when planning for connectivity (right). The size of nodes (reserves) corresponds to their importance as stepping stones to maintain the overall connectivity across the region (larger nodes are more important).

In summary, our study shows that maintaining dispersal connectivity incidentally through representation-only reserve design is unlikely, particularly in regions with strong asymmetric patterns of connectivity. We suggest that, given potential reductions in PLD due to ocean warming, future marine reserve networks would require more and/or larger reserves in closer proximity to maintain larval connectivity. However, given possible changes in larval connectivity due to ocean warming these adjustments may not be sufficient to maintain the current levels of connectivity.

Keep an eye on the Midriff Islands Project’s website for updates!

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