Coral reef resilience enhanced by adaptation qualities of fish

jewel damsel fish

The Jewel Damsel is conventionally classed as a territorial herbivore but switched to planktivorous feeding on the reef at Wakatobi

A study into coral reef resilience and the ability of specialist reef species to adapt has found that fish can change their feeding behaviour depending on habitat quality within the Wakatobi Marine Park, Indonesia.

Coral reefs are degrading worldwide, and the reefs within the Wakatobi Marine Park are no exception. Ecological monitoring data of these reefs over a 10 year period, between 2002 and 2011, showed a marked decline in habitat quality together with declining fish abundance.

Importance of fish

Reef fish play important ecosystem functional roles, and the importance of one such role, herbivory, is well documented as essential in the removal of algae to allow coral to recruit and grow. The assessment of reef health, vulnerability and resilience is mostly achieved by estimates of the total biomass held within specific functional guilds and the number of species that contribute to this biomass.

The relationship between species richness and functional biomass is crucial in assessing the levels of redundancy common in reef systems. Redundancy levels refer to the number of species performing similar roles within the system; high levels of redundancy equates to a more resilient system. It is particularly important to understand how changing environments influence levels of redundancy and to identify ecological tipping points for the systems’ vulnerability to large scale ecological change.

Niche expansion for this species seemed to be driven by resource availability

Accurately assessing functional biomass depends largely on our ability to define, with known degrees of certainty, the fundamental niche of a species. Importantly, predicting functionality also heavily depends on an understanding of how the fundamental and realised niche of key species varies across environments, termed as niche plasticity.

Fish behaviour

This study collected feeding behaviour data on two reef fish species. The aim was to enhance our understanding of the levels of variability that exist within the realised niche of fish species present on reefs of varying habitat quality. The species studied were specialised in their feeding guild, and were therefore thought to lack the capability to successfully adapt to changes in resource availability and reef quality.

Specialists can become generalists

The Jewel Damsel (Plectroglyphidodon lacrymatus) is conventionally classed as a territorial herbivore and the Eastern Triangular Butterfly fish (Chaetodon baronessa) as a specialised corallivore. It was hypothesised that the degree to which the two species depend on herbivory and corallivory, respectively, varies across habitat quality gradients.

Diving in IndonesiaResearch was conducted on three reef sites of varying quality within the Wakatobi Marine Park. The results were surprising: the Jewel Damsel switched to planktivorous feeding at two of the reefs. The expansion in the realised niche occurred on the same two reefs where: higher frequencies and durations of aggressive behaviours were recorded; territory volume was significantly smaller; and the abundance of competitors was higher. The results suggest that competition was the driver for niche expansion.

The behavioural and isotope data for the Eastern Triangular Butterfly fish found the fish fed on a broad range of coral genera on sites where its preferred coral, Acropora sp., was less abundant. The species was also found to feed on algae at the site with the lowest coral cover. In conclusion niche expansion for this species seemed to be driven by resource availability.

The ability of these two specialist reef species to adapt to reef quality and perform various functional roles will add to the reef resilience that previously has not been taken into account. Determining all fish species’ responses to habitat change, and the relative importance of resource availability or competition across species, is the key to understanding reef resilience and the future successful conservation management of these systems.

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