Coral Reef Connectivity Is Essential for Conservation Efforts

When Luisa Fontoura first arrived at Australia’s Macquarie University in 2017 to begin her Ph.D. research, it was just after the Great Barrier Reef suffered massive coral bleaching in the Southern Hemisphere summer of 2016. Working on the hard-hit Lizard Island, she was confronted by the stark contrast between her colleagues’ “amazing” memories of the reef and its current diminished state.

“It was pretty devastating,” she tells Treehugger. 

Six years and a doctorate later, Fontoura is working on better understanding the connections between the world’s coral reefs in order to give them their best shot at survival. At the start of this year, she served as lead author on a study published in Science that shows that 70% of reefs linked by the ecologically vital movement of fish larvae remain unprotected even as world leaders have set a goal of protecting 30% of Earth's land and oceans by 2030.

“This could help to improve the expansion of marine protected areas that are expected by the next decade,” she says.

Sources, Sinks, and Bridges 

A coral reef does not exist in isolation. Instead, the world’s reefs are joined by a network of fish larvae that passes from one reef to another before reaching their final destination. 

The study broke the world’s reefs down into three distinct roles in this network:

1. Sinks: These are reefs that primarily send fish larvae off to other reefs.
2. Sources: These are reefs that primarily receive fish larvae from other reefs.
3. Corridors: These are reefs that largely serve as a bridge between reefs.

Whether a reef is a sink, a source, or a corridor is “basically shaped by ocean currents and the biological traits of fish larvae,” Fontoura says. 

The researchers used these two attributes to model how fish larvae are exchanged between reefs worldwide and created a dashboard of their results. They also discovered important traits of sources, sinks, and corridors that they hope can inform conservation best practices.

“Although technical guidelines and tools for protecting connectivity exist, empirical studies are needed to determine how different connectivity attributes, for instance, whether the reef is a sink, source, or a corridor for larvae, influence the desired outcomes of a conservation area,” co-author Dr. Joseph Maina of Macquarie University says in a press release shared with Treehugger.

For example, sink reefs contain twice as much fish biomass—which is a proxy for fish stocks—as source reefs and are more resilient to human pressures when protected. Corridors, meanwhile, are richer in species biodiversity. However, the vast majority of this network of reefs remains unprotected. 

“Almost like 70% of those reefs that we identify as the most critical sources, sinks, and corridors … are actually outside protected areas,” Fontoura tells Treehugger. “So by mapping those, we can perhaps help. We bring opportunities to better implement connectivity along with other characteristics as well that are important for the reefs, biodiversity maintenance, and local fisheries.”

30 X 30

The study authors say their research can help inform the creation of new marine protected areas (MPAs) and other area-based conservation measures (OECMs) as world leaders move towards the goal of protecting 30% of terrestrial and ocean ecosystems by 2030.

This guidance works on both a global and a local level. Regionally, the study authors point out that only 5% of the corridors in the marine biodiversity hotspot of the Indo-Pacific are protected, according to a graphical abstract shared with Treehugger. Meanwhile, on the African coast of the Western Indian Ocean, where many coastal communities are dependent on subsistence fishing, only 11% of larval sources are protected.

The study authors say local fisheries can be protected by prohibiting fishing of any kind in source reefs. This will allow the females of important fishery species to grow larger and lay more eggs, Fontoura says, leading to richer sinks. At the same time, the sinks still need to be managed sustainably to maintain the fisheries that local communities depend upon.

“Getting the local context right is crucial. A deeper understanding of the interactions between human activities and the local environment is necessary to tailor management and support the continuity of ecosystem services to maximise the contributions of larval sinks to sustainable fisheries,” co-author Dr. Stephane D’agata of the French National Institute of Sustainable Development says in the press release.

In addition to looking at reef connectivity, the study was the first to map that connectivity at this scale and resolution (8 kilometers, or approximately 5 miles) for four different types of fish. Understanding the unique movements of different fish types can also inform conservation decisions. 

“In coral reefs, different types of fish species may contribute to different ecosystem services—for example, whereas large, carnivorous fish with a relatively short spawning season may make a substantial contribution to local fisheries, small reef fishes that reproduce more frequently during the year are responsible for much of the stunning diversity of fish we observe on coral reefs today. Understanding the role of connectivity patterns from different species in sustaining ecosystem services can provide insights into the optimal design of protected areas based on their conservation and sustainability goals,” Fontoura says in the press release.

One thing the research revealed is that corridors are especially important for the smaller reef fish, Fontoura tells Treehugger.

Accounting for Change

As the circumstances of her arrival in Australia demonstrate, all of Fontoura’s research occurs in the shadow of the climate crisis, which the National Oceanic and Atmospheric Administration says is the “greatest global threat to coral reef ecosystems.” Warmer ocean temperatures make coral bleaching events and disease outbreaks more common. In addition, the increased carbon dioxide in the ocean is already causing ocean acidification, which lowers the ocean’s pH and reduces the ability of corals and other reef-building organisms to calcify.

Fontoura says her next step is to incorporate the movement of coral larvae into her model, and then to see how the climate crisis will impact the movement of both coral and fish larvae, and how this will play out in the next half a century depending on whether and how quickly world leaders succeed in reducing greenhouse gas emissions. One concern for Fontoura is that coral and fish larvae might have a hard time adapting to warmer ocean temperatures and that this might “shorten the connectivity” between reefs, she tells Treehugger. 

“The priority now is to understand the influence of climate change on coral reef connectivity to forecast potential impacts on coastal communities worldwide that rely on coral reef ecosystem services," Fontoura adds in the press release.