Coral reefs: Just keep swimming

A diversity of fish species with different shapes and swimming skills key to the resilience of coral reefs

Coral reefs are productive and important ecosystems for marine life and people around the world. One important ecosystem service provided by these biologically diverse habitats is to dissipate ocean waves and protect the coast from erosion. This also implies that all reefs contain a broad spectrum of sheltered and wave exposed habitats, which in their turn influence the many species of reef living animals and plants. Herbivorous fishes, essential for maintaining productive and healthy coral reefs and ecosystem services, are no exception.

This aspect of coral reef ecology was recently analysed by Dr. Sonia Bejarano and Dr. Sebastian Ferse from the Leibniz Centre for Tropical Marine Research in Bremen (Germany) as well as Jean-Baptiste Jouffray, PhD candiate at the Stockholm Resilience Centre. Their work, which also included contributions from colleagues from Australia, Germany, Oman, UK and USA was recently published in the journal Functional Ecology.

Their study was conducted in the eastern barrier reef of the Palau archipelago and investigated how differences in wave exposure influence herbivorous fishes, thus having effects on the whole reef’s functioning.

“Shallow wave-exposed reefs are the most productive and a desirable place for herbivorous fishes to feed, but the privilege is not easily accessible to all fishes,” the authors explain.

The results showed that the feeding capacity of certain species was limited under turbulent conditions, but also that others were either unaffected or increased their feeding frequencies under high wave exposure. Streamlined herbivorous fish species, seemed to be better at eating under strong swell compared to laterally-compressed fishes.

Implications for management of biodiversity

A healthy herbivorous fish community is important because it grazes down algae and maintains reef resilience in many different ways, for instance by creating surfaces where new coral larvae can settle.

But the team of researchers also learned three things that are useful beyond the world of coral reefs and fishes.

First, animals that do different jobs in an ecosystem also tend to be different in the way they cope with environmental stress, and this makes them even more complementary than people usually think. Second, animals that do similar jobs in an ecosystem (e.g. grazing or photosynthesis) may also respond in subtly differently ways to environmental stress thus ensuring that the job gets done across a broad range of environmental conditions (something that is called response diversity).

“Now, species are different and these may sound as trivial discoveries, but it is surprising how widespread the notion is that two species that have similar functions can replace each other fully, if one of them disappears,” explains lead author Sonia Bejarano.

Third and last, some species go and others come as environmental stress increases, but because of this response diversity, vital ecosystem functions can be preserved even under harsh conditions.

Next step, the authors write, is to assess whether this is also the case across man-made environmental stress gradients. To what extent can human impacts disrupt the resilience provided by response diversity in coral reefs and other ecosystems? Can we succeed in protecting it with the management tools we have at hand?

Challenging common perceptions

The results of the study challenge the way biodiversity is often perceived and confirm that species presence does not imply their complete functionality, especially under harsh environmental conditions, says Jean-Baptiste Jouffray.

As such, the study offers a cautionary note concerning the complex issues surrounding the management and protection of herbivorous fishes on coral reefs around the world. Without the new knowledge provided by the study there is risk that fish communities comprised by larger numbers of species of the same broad functional group could be erroneously regarded as resilient to overexploitation.

With coral reefs being increasingly stressed by global human-made stresses like climate change and ocean acidification, this kind of knowledge is more important than ever before to inform local and regional management efforts to strengthen reef resilience.

The study was conducted in February and March 2012 and focused on shallow reefs on the seaward side of the eastern barrier reef of the Palau Archipelago. The swimming performance of 37 fish species was quantified based on ten morphological traits related to swimming mode, speed, and manoeuvrability. Feeding frequency was recorded with unmanned video cameras on 12 sites distributed across sheltered, moderate, and turbulent reefs.

Bejarano, S., Jouffray, J.-B., Chollett, I., Allen, R., Roff, G., Marshell, A., Steneck, R., Ferse, S. C. A. and Mumby, P. J. 2017. The shape of success in a turbulent world: wave exposure filtering of coral reef herbivory. Funct Ecol. DOI:10.1111/1365-2435.12828