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A Reef Too Far?

Goldtail damselfish. Credit: Mary Bonin

Goldtail damselfish. Credit: Mary Bonin

By Mary Bonin, Glenn Almany & Geoff Jones

Coral reefs are being subjected to more disturbances than ever before, but a new study has surprisingly found that reef fish can benefit from habitat fragmentation.

Corals are pretty amazing animals. These tiny creatures are capable of building huge reef structures like the Great Barrier Reef, which is the world’s largest structure created by animals and can be seen from outer space.

Corals are like the trees in a rainforest, providing habitat for the animals that make reefs their home. They are so important to the coral reef fish community that at least 10% of the fish species on the Great Barrier Reef cannot survive without live corals.

Living corals are especially important for juvenile fish, which shelter in coral branches to avoid predators. The survival of these young fish is critical for the replenishment of adult populations – fewer juveniles mean fewer adults.

Unfortunately, these vital coral habitats are subjected to more frequent and intense disturbances than ever before. Every summer, severe tropical storms leave large expanses of coral habitat battered and broken. Warming ocean temperatures are causing more frequent coral bleaching events, which can kill corals and cause widespread habitat degradation. These habitats are also increasingly threatened by outbreaks of disease, predatory crown-of-thorns starfish, and decreased water quality due to coastal deforestation and development.

As a consequence of these impacts, the coral habitat is already extensively damaged on 20% of the world’s coral reefs, and 50% of the remaining reefs face a similar fate if these disturbances continue.

More than 60% of the fish species in the reef community decline in abundance by least a 50% following disturbances that reduce live coral cover. Many fish species that have no direct dependence on corals for food or shelter also become much less abundant when live coral cover declines, suggesting that live coral is even more important for fish communities than first expected.

The Problem
When disturbances occur they usually have two effects on the habitat: they reduce the total amount of habitat available and they change the configuration of the habitat that remains. Disturbance often creates greater habitat patchiness; what was previously one continuous stretch of habitat may now be broken up into smaller fragments that are separated by uninhabitable space.

Since habitat loss and habitat fragmentation usually happen simultaneously, their individual effects are confounded. This makes it difficult to determine which type of habitat change is responsible for declines in coral reef fishes after a disturbance. Is it the reduction in the total amount of habitat available, the increased habitat patchiness, or a combination of both?

To address this problem we measured the independent effects of habitat loss and habitat fragmentation, as well as their combined effects, on the survival of juvenile coral reef fish. We also examined the effects of these different types of habitat change on the abundance and diversity of new juvenile fish that colonised following disturbance.

Separating the effects of habitat loss and habitat fragmentation is vitally important, not only for coral reef fishes but also for animals in tropical and temperate forests, grasslands and freshwater ecosystems. Indeed, significant habitat degradation is occurring in almost every ecosystem on the planet, and is considered to be the primary cause of the current extinction crisis.

In an era of increasing habitat degradation, effective conservation and management requires knowledge about the underlying cause of species declines. However, to date very few studies in any ecosystem have successfully separated the effects of habitat fragmentation from those of habitat loss.

Building Reef Microcosms
To tease apart the effects of habitat loss and fragmentation, we conducted a patch reef experiment. This involves building a system of identical coral reefs, each of which represents a microcosm of a natural coral reef.

This is a common technique used by coral reef fish ecologists, and it has two important advantages. First, it is possible to manipulate both the fish community and the habitat on these small experimental reefs. This is nearly impossible to do at the larger scale of a natural coral reef. Second, because the experimental reefs are constructed in the natural environment, they experience the same conditions as natural reefs and therefore provide much more realistic insights than laboratory experiments conducted in aquaria.

We built our system of experimental reefs on a large expanse of sandy seafloor along the coastline of Kimbe Bay in Papua New Guinea. This bay is located in the heart of the Coral Triangle, the world’s epicentre for marine biodiversity, and is renowned for its diverse assemblage of fishes and corals. We have a longstanding relationship with the subsistence fishing communities in the area and work closely with a local non-government organisation to conduct conservation-based research.

Building experimental reefs is no easy task! It took three of us three full weeks of hard manual labour to build the 30 reefs in our experimental system.

The first step was to lay down a grid on the sea floor to map out the location of each reef. Reefs in the grid were separated from one another by 15 metres of open sand and were at least 20 metres away from the nearest natural reefs in the area, ensuring that each reef was independent of all other reefs.

We collected live branching corals from nearby natural reefs and carefully transported them to the study site. Corals were arranged on top of rock foundations to provide 1 m2 of live coral habitat on each experimental reef.

Many coral reef fish spend their entire lives on only a few metres of reef. The habitat on the experimental reefs therefore provided fish with a similar home range to what would be used in a natural reef environment.

Experimental Disturbance
Our study species for the experiment was the goldtail damselfish (Chrysiptera parasema). Like many coral reef fish, this species lives exclusively in branching corals during its juvenile phase, so it is vulnerable to coral habitat degradation. We collected juveniles from natural reefs using small aquarium hand nets and a mild anaesthetic, and then released 20 fish onto each experimental reef.

These relocated fish were monitored closely over the first week, and there were very few disappearances, suggesting that most had adjusted quite well to their new home. This was expected because the reefs provided vital shelter from predators and the juvenile fish were unlikely to travel over an expanse of featureless sand in search of other habitat.

After the fish had settled in, we experimentally disturbed the habitat. Each of the 30 reefs was randomly assigned to experience one of five different disturbance treatments:

1. Habitat Loss – removal of 75% of the live coral from the reef;

2. Habitat Fragmentation – dividing the reef into three fragments but no loss of habitat;

3. Habitat Loss and Fragmentation – removal of 75% of the live coral and then dividing up the remaining habitat into three fragments;

4. Disturbance Control – disturbance and then reassembly of coral habitat to confirm that disruption by divers was not responsible for the trends observed for the habitat loss and habitat fragmentation reefs; and

5. Control – reefs were left completely undisturbed to provide a baseline to which we compared survival and colonisation trends observed for the other treatments.

We monitored the reefs once per week for the first 6 weeks after the experimental disturbance, and then again 4 months after the disturbance. Each time we visited the reefs we counted the number of juveniles that remained and also identified and counted all the new juvenile coral reef fish that had colonised the reefs. We expected that habitat loss, fragmentation or both would decrease the survival of C. parasema and also the abundance and diversity of new juveniles that colonised the reefs.

Surprising Results
We found that habitat loss alone had a strong negative effect on the survival of juvenile coral reef fish after 4 months. Of the 20 juvenile C. parasema initially present on these reefs, only one or two survived after 4 months. This was an eightfold decrease in survival compared with undisturbed control reefs.

However, contrary to our expectations, habitat fragmentation actually had a positive effect on juvenile fish survival. C. parasema survived equally well on the fragmented reefs and control reefs, suggesting that habitat patchiness is not problematic for juvenile coral reef fish.

In fact, fragmentation was actually beneficial to survival when habitat was in short supply. On reefs where habitat had been lost, fragmenting the remaining habitat actually allowed three times as many C. parasema to persist compared with reefs with habitat loss alone.

Similarly, the abundance and diversity of new colonisers was positively affected by habitat fragmentation and negatively affected by habitat loss. On average, new colonisers were at least twice as abundant and diverse on fragmented reefs compared with control reefs. In contrast, reefs with 75% habitat loss had the lowest abundance and diversity of colonisers of any reefs in the study.

Does one positive effect and one negative effect mean that the effects of habitat loss and fragmentation cancel one another out? Unfortunately not, because the strength of these effects differs. Although the positive effects of increased habitat patchiness were highly beneficial to the survival and colonisation of juvenile fish in the 6 weeks following the disturbance, after 4 months the negative effects of habitat loss far outweighed the benefits of fragmentation. Consequently, when both habitat loss and fragmentation occurred together the result was an overall net decline in fish survival, abundance and diversity 4 months after the disturbance.

While the negative impacts of habitat loss were expected, the positive effects of habitat fragmentation were quite surprising. The term “habitat fragmentation” usually has negative connotations, but our results suggest that this is probably because it is usually linked to disturbances that also cause habitat loss. Our experiment revealed that when a reef suffers a disturbance it is actually the reduction in live coral habitat, not increased habitat patchiness, that causes declines in reef fish abundance and diversity.

Why would habitat fragmentation be beneficial? We believe that fragmentation reduces competition between fish for habitat space. When the habitat is a single continuous patch, strong competitors will tend to dominate the whole area and drive the weaker ones away. A patchier habitat allows greater spatial separation between competitors, allowing weaker ones and new colonisers to avoid bullying by dominant individuals.

Conservation Implications
These findings can be used to set priorities for effective management of coral reef fish communities and also inform habitat restoration programs.

Management strategies may differ depending on whether habitat loss or fragmentation is the primary threat. If habitat fragmentation is the primary threat, the most important task for managers would be to facilitate the movement of fish between isolated habitat patches. But if habitat loss poses the greatest risk, then limiting the extent of this loss would be the management priority.

Our study clearly shows that preventing habitat loss should be the main priority for managers working to conserve coral reef fish communities. One way to address this management objective is by protecting larger areas of habitat from the human impacts that contribute to live coral degradation.

If coral habitat does suffer damage despite the best efforts of protection, active restoration can help to boost recovery. Restoration programs involve transplanting corals from healthy reefs to those that are badly damaged. Our results suggest that patchy placement of these coral transplants would increase the abundance and diversity of fish that colonise this new habitat.

The discovery that habitat patchiness can actually benefit coral reef fish communities offers hope for the conservation and management of these extraordinary ecosystems. Coral reefs are undoubtedly under an enormous amount of pressure, and combating their various threats may seem an impossible task. However our study suggests that even if it’s not possible for managers to restore habitat on an entire reef, restoring small patches of habitat could still be highly beneficial to the recovery of the reef fish community.

Mary Bonin is a postdoctoral researcher in the ARC Centre of Excellence for Coral Reef Studies and the School of Marine and Tropical Biology at James Cook University. Glenn Almany is a Future Fellow in the ARC Centre of Excellence for Coral Reef Studies at JCU. Geoff Jones is a Professor in the School of Marine and Tropical Biology at JCU and a Chief Investigator in the Centre.