Vital clues to coral reef recovery have been identified in a remarkable research project in which three scientists laboured to hand-build thirty coral reefs from hundreds of tonnes of rock and gravel.
Inhabitants of the experimental reefs. Like many other coral reef fish, these damselfish require delicate branching corals as habitat during their juvenile life-stage. These critical coral habitats are increasingly being degraded and destroyed by impacts such as coral bleaching and severe tropical storms.
Working in a shallow, sandy area of Kimbe Bay, Papua New Guinea, the team constructed their artificial reefs over several weeks, with just a boat and their bare hands, to find out whether it is possible to rescue a damaged coral reef from obliteration and restore its richness.
'When a reef suffers a heavy impact— such as a storm or outbreak of coral bleaching—there are two different effects on the coral habitat. First, a lot of the habitat that was once available to fish is totally lost. And second, the habitat that still remains is often fragmented, or broken up into smaller patches,' explains Dr Mary Bonin of the ARC Centre of Excellence for Coral Reef Studies and James Cook University.
'We wanted to figure out if the increased habitat patchiness is actually a problem for coral reef fishes, or whether it is really the loss of habitat that causes fish to decline following an impact.'
With colleagues Professor Geoff Jones and Dr Glenn Almany, she set out to construct 30 real-life coral reefs in an open sandy area where there were no natural reefs nearby. Each ‘reef’ was constructed from boulders and coral debris, carted manually from the shore in a 6m boat, crowned with a square metre of living bottlebrush coral and stocked with twenty small blue and yellow damselfish. After a time it acquired richer diversity, as baby fish came in on the current and settled.
The scientists then simulated the effects of habitat loss – the sort of colossal damage that reefs suffer during a bad bleaching event or cyclone – habitat fragmentation, where the existing reef is broken into smaller fragments, and a combination of the two.
'As you’d expect, the effects of a loss of 75 per cent of the habitat were awful. With most of their home gone, the fish just disappeared.
'But when we divided some of the reefs into three separate parts we found that fish survival and diversity actually improved for a time,' she says.
'We think this is because a fragmented or patchy habitat reduces the competition between fishes, creating more room for the weaker ones or for newcomers to settle. When the habitat is just a single patch, the tough guys will tend to dominate the whole area and drive the others away.'
The researchers’ finding challenges a widely-held view that habitat fragmentation always leads to a dramatic loss of fish numbers and diversity.
'Habitat fragmentation has had a bad wrap,' she says. 'Our findings suggest that it is actually habitat loss that is the major problem for coral reef fishes following an impact,' Dr Bonin says.
'It is certainly the case that in the small areas we created and studied, you get a positive effect on fish numbers and diversity from habitat fragmentation - whereas habitat loss of 75 per cent or more is a disaster,' Dr Bonin says.
Because fragmentation can bring greater diversity, there may be things which reef managers can do to restore badly-damaged coral communities by mimicking its effects.
'The fact that habitat patchiness can have a positive effect on fish diversity is really exciting because it means that even if it isn’t possible for managers to restore an entire coral reef, it will still be highly beneficial to restore small patches of habitat' says Dr Bonin.
'We now want to investigate what happens over much larger areas, and examine how the degree of isolation of the remaining habitat fragments influences species’ responses to a disturbance.'
Even in Australia’s well-managed Great Barrier Reef zone most of the coastal fringing corals have been lost – so a tantalising question is whether they could gradually be restored over time, providing human impacts from the land are also reduced.
With coral reefs facing growing losses from global warming, ocean acidification (caused by humanity’s carbon emissions), storms and other human impacts like pesticides, sediment and nutrients, understanding how to bring them back is becoming critically important to the management of treasures such as Australia’s Great Barrier Reef, the team says.
'Whether the patch-scale patterns documented apply to the larger reef landscape is currently unknown, but should be emphasized in future research, given the increasing degradation of coral reef habitats worldwide,' they say.
Their paper, Contrasting effects of habitat loss and fragmentation on coral-associated reef fishes; by Mary Bonin, Glenn Almany and Geoffrey Jones, appears in Ecology, the journal of the Ecological Society of America.
ARC Centre of Excellence Coral Reef Studies website