The Great Barrier Reef has a record coral cover. It might not matter

In what appears to be excellent news, coral cover in parts of the Great Barrier Reef is at an all-time high, according to new data from the Australian Institute of Marine Science. But this does not necessarily mean that our beloved reef is in good health.

In the north of the reef, coral cover usually fluctuates between 20% and 30%. It currently stands at 36%, the highest level since monitoring began more than three decades ago.

This level of coral cover is warmly coming off the back of a disturbing decade in which the reef has endured six massive coral bleaching events, four severe tropical cyclones, active crown-of-thorn starfish outbreaks and impacts on water quality after flooding. So what’s going on?

High coral cover findings can be deceptive as they can be the result of just a few dominant species growing rapidly after disturbance (such as massive bleaching). However, these same corals are very sensitive to disturbance and are likely to become extinct within a few years.

The data is robust

The Great Barrier Reef spans 2,300 kilometers and includes more than 3,000 individual reefs. It is an exceptionally diverse ecosystem with more than 12,000 animal species, plus many thousands more species of plankton and marine flora.

The reef is teetering on the brink of receiving an “at risk” list from the World Heritage Committee. And recently it was described in the State of the Environment Report as in a bad and deteriorating condition.

To protect the Great Barrier Reef, we need to monitor and report on its condition on a regular basis. The Australian Institute of Marine Science’s long-term monitoring program has been collecting and providing this information since 1985.

The approach involves examining a selection of reefs representing different habitat types (inshore, midshelf, offshore) and management zones. The latest report provides a robust and valuable summary of how coral cover has changed over the past 36 years across 87 reefs across three sectors (north, central and south).

The results

Overall, the long-term monitoring team has found that coral cover has increased on most reefs. The level of coral cover on reefs at Cape Grenville and Princess Charlotte Bay in the northern sector has recovered from bleaching, with two reefs exceeding 75% coverage.

In the central sector, where coral cover has historically been lower than in the north and south, coral cover is now high across the region at 33%.

The southern sector has a dynamic record of coral cover. In the late 1980s, coral cover surpassed 40%, before falling to a region-wide low of 12% in 2011 after Cyclone Hamish.

The region is currently experiencing outbreaks of crown of thorns. And yet the coral cover in this area is still relatively high at 34%.

Based on this robust dataset, which shows an increase in coral cover indicative of recovery across the region, the Great Barrier Reef should be doing well – right?

Juvenile branching Acropora colonizing bare space after a bleaching event. [Photo: Zoe Richards]

Are we being ambushed by coral cover?

In the report from the Australian Institute of Marine Science, reef restoration only refers to an increase in coral cover, so let’s unpack this term.

Coral cover is a broad proxy metric indicating habitat condition. It is relatively easy data to collect and report on, and it is the most commonly used metric for measuring coral reefs.

Finding high coral cover can mean a reef in good condition, and an increase in coral cover after disturbance can mean a recovering reef.

But in this case, the reef is more likely to be dominated by only a few species, as the report states that branching and plating Acropora species have stimulated coral cover recovery.

Acropora corals are known for their “boom and bust” life cycle. After disturbances such as a cyclone, Acropora species act as pioneers. They quickly recruit and colonize the bare space, and the laterally growing plate-like species can quickly cover large areas.

Fast-growing Acropora corals tend to dominate during the early phase of recovery after disturbances such as the recent series of massive bleaching events. However, these same corals are often susceptible to wave damage, disease or coral bleaching and tend to break down within a few years.

Deducing that a reef has been restored by a person being towed behind a boat to get a quick visual estimate of coral cover is like flying in a helicopter and saying a forest fire-affected has recovered because the canopy has grown back .

It does not provide information about diversity, or the abundance and health of other animals and plants that live in and among the trees, or coral.

Cautious optimism

My study, published last year, examined 44 years of coral distribution records around Jiigurru, Lizard Island, on the north side of the Great Barrier Reef.

It suggested that 28 of the 368 hard coral species recorded at that location have not been seen for at least a decade and are in danger of extinction.

Lizard Island is a location where coral cover has increased rapidly since the devastating 2016-17 bleaching event. Even so, there is still a real risk that local extinctions of coral species have occurred.

While there is no data to prove or disprove it, extinction or local decline of coral-bound marine life, such as coral-eating fish, crustaceans and mollusks, is also likely to have occurred.

Without more information at the level of individual species, it is impossible to understand how much of the Great Barrier Reef has been lost or recovered since the last mass bleaching event.

The conversation

Based on the coral cover data, it’s tempting to be optimistic. But as more frequent and severe heatwaves and cyclones are predicted in the future, it’s prudent to be cautious about the reef’s perceived recovery or resilience.

Zoe Richards is a Senior Research Fellow at Curtin University. This article is republished from The Conversation under a Creative Commons license. Read the original article.


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