Menu Close

Crown of Thorns is a symptom of reef decline: let’s address the cause

Killing starfish one by one is no long-term solution. Paul Cizek

A recent report on coral loss from the Great Barrier Reef has pointed the finger at cyclones and Crown of Thorns starfish. The real culprit is human activity, and until we reduce port activity and pollution, coral will be unable to bounce back.

Three recent studies, published in 2004, 2007 and this week, have shown that at least 50% of the corals on the Great Barrier Reef have disappeared in recent decades.

Last year, another report claimed the declines were more modest and the result of a natural cycle. But the latest report, from the Australian Institute of Marine Science, confirms earlier studies – the Great Barrier Reef is in trouble.

Corals are the backbone of the reef, providing habitat for many other species. Measuring coral cover on a reef is the simplest way to monitor its condition. But other metrics - like counts of sharks, dugongs and turtles - also show alarming downward trajectories. The decline in coral cover highlights UNESCO’s concerns about the dwindling Universal Heritage Values of the Barrier Reef.

The key question now is, what are we going to do about these losses?

Storms do affect coral, but cyclone activity has been reduced in the last 100 years. NASA

First, we need to consider why coral cover changes. The amount of coral goes down when they reproduce less, grow more slowly or die more frequently. Even under ideal conditions, about one-quarter to one-third of a coral population dies each year from background mortality. They can die from old age, disease, predation, competition with a neighbour, erosion of their skeleton, smothering by sediment, severe coral bleaching, and from storms.

On a healthy reef, loss of cover is balanced by new recruitment of young corals and by new growth. It’s just like a human population – we measure births, deaths and net migration to track demographic changes. Measuring mortality alone won’t help us to plan for schools or new roads.

Next consider where the loss of coral cover is greatest. The 50% decline in coral cover is averaged over the whole Great Barrier Reef (GBR). However, there has been no net loss of coral cover in the remote north beyond Cooktown or on reefs far from shore. Consequently, most reefs that are close to the coast (and to people) have lost far more than 50% of their cover.

Coastal reefs have been obliterated by runoff of sediment, dredging and pollution. Once-thriving corals have been replaced by mud and seaweed (see Figure 1).

Figure 1. Dramatic loss of coral cover on Queensland’s coastal reefs. Both photographs are from the same site, indicated by the hilly backdrop. Modern photo taken by David Wachenfeld

The latest study attributed 100% of the loss of coral cover solely to higher mortality, due to just three causes – cyclones (48%), crown-of-thorns starfish (42%) and coral bleaching due to climate change (10%). However, reefs have coped with cyclones for millions of years, and - despite some claims to the contrary - the number of cyclones per decade has actually dropped slightly in the past 100 years. Too many starfish is a symptom of the decline of the Great Barrier Reef, not the direct cause.

In reality, we are responsible for the loss of corals, not storms and starfish. Before people, corals recovered from routine shocks like recurrent cyclones, and now they don’t (except in the most remote places).

The rush by many reef scientists to focus solely on climate change research has distracted attention from other ongoing threats to the reef that, so far at least, have been much more destructive. Four outbreaks of crown-of-thorns starfish have occurred on the Great Barrier Reef since the 1960s, and widespread damage from the first two of them led to the initiation of formal monitoring of corals in the 1980s.

There are two plausible but unproven theories about the causes of outbreaks of crown-of-thorns starfish. One suggests that dredging and runoff of nutrient pollution from land promotes blooms of phytoplankton which speeds up the development of starfish larvae, contributing to outbreaks. The other surmises that the changes we have made to the structure of foodwebs have resulted in fewer juvenile starfish being eaten.

Planned marine parks could help reduce damage to coral. Matt Kieffer

The best way to restore foodwebs and rebuild fish stocks is to create a network of no-take fishing reserves. The success of the GBR green zones in rebuilding depleted fish stocks bolsters the Commonwealth’s plan for a national system of marine reserves.

There is no shortage of crackpot solutions being proposed to fix the problems of the Great Barrier Reef – like covering corals with shade cloth to prevent bleaching, moving corals out of harm’s way, or killing millions of starfish one at a time with a syringe. There is a new outbreak of crown-of-thorns underway, the fourth in 50 years, and it is far too late to stop it. Direct intervention to kill starfish is expensive and time consuming. At best, it just might help to control numbers adjacent to a tourist pontoon, but it won’t change the trajectory of the current outbreak.

To increase coral cover, we need to improve the conditions that help them reproduce, survive and grow. The capacity for coral recovery is impaired on a reef that is muddy, polluted or overfished. The ongoing decline of corals demonstrates that the Great Barrier Reef is very poorly positioned to recover from future bouts of coral bleaching. Governments need to focus on controlling pollution and dredging, reducing carbon emissions, and placing a ban on new coal ports.

Want to write?

Write an article and join a growing community of more than 179,400 academics and researchers from 4,902 institutions.

Register now