Renato Morais, a Ph.D. student at James Cook University in Australia, gives us a ‘behind-the-scenes’ view of his research on the energetic effects of coral mortality, related to his recent publication in Functional Ecology entitled “Severe coral loss shifts energetic dynamics on a coral reef”.

About the paper

What’s your paper about?

This paper is about the consequences that severe coral mortality can bring to the energetic functioning of coral reef fish assemblages. Specifically, we evaluated how four key metrics of energy flow and storage that underscore a critical ecosystem function, the production of consumer biomass, changed on a coral reef on the Great Barrier Reef after the loss of up to 86% of its coral cover.

What is the background behind your paper?

Field work for this study was done at Lizard Island, on the Great Barrier Reef, Australia, where the authors were lucky to have access to great infrastructure and exceptional autonomy. Photo by Victor Huertas.
Field work for this study was done at Lizard Island, on the Great Barrier Reef, Australia, where the authors were lucky to have access to great infrastructure and exceptional autonomy. Photo by Victor Huertas.

While a large body of work has reported the responses of single fish species or guilds to severe coral mortality, the cumulative consequences of these responses to the energetic functioning of coral reefs has remained far less explored. The Great Barrier Reef, widely recognized as a coral reef oasis largely unaffected by human impacts, has recently suffered large-scale damage from a series of major cyclones and mass coral bleaching events – when corals expel their symbiotic algae in response to heat stress. The reef we studied at Lizard Island was directly hit by two major cyclones (categories 5 and 4) and two bleaching events from 2014 to 2017. Our group has been researching this reef in particular for at least 17 years and, thus, these catastrophic events provided the opportunity to evaluate the energetic responses of reef fish assemblages to cumulative coral loss.

What are the key messages of your article?

That coral loss can change the energetic balance of reef fish assemblages in complex, perhaps even unexpected, ways. In our study, disproportionate increases in herbivorous fishes as a response to increases in their favourite food source – algal turfs – led to more productive fish assemblages. This could be initially interpreted as good news. However, because this extra productivity was due to larger and presumably older fishes, there was a decrease in biomass ‘recycling’ (when old biomass is lost, for example through mortality, and is replaced by new fish growth). This indicates that this extra productivity may not be stable in the long run, and we should therefore be cautious not to assume that reef fish assemblages are operating well on degraded reefs simply because we detect an increase in overall biomass.

How is your paper new or different from other work in this area?

Herbivorous fishes have led the observed changes in the energetic functioning at this reef. The paper hypothesises that this is due to increases in food resources for this varied group of medium to large-sized fishes. Photo by Victor Huertas.
Herbivorous fishes have led the observed changes in the energetic functioning at this reef. The paper hypothesises that this is due to increases in food resources for this varied group of medium to large-sized fishes. Photo by Victor Huertas.

Most studies evaluate the responses of a set of reef fish species or trophic guilds to coral mortality. This is normally done by comparing the abundance or biomass of these species or guild before and after the coral die-off. While these are very relevant and informative metrics, they can be viewed as static ecosystem measures. The novelty of our study stems from combining survey data with a set of physiological and ecological processes to estimate dynamic, flow-based metrics such as productivity, consumed biomass and turnover.

About the research

What is the broader impact of your paper?

The studied reef has lost most of its coral cover due to two cyclones and two coral bleaching events. Despite some hard and soft coral colonies, most of the reef surface is covered in algal turfs, the favourite food of herbivorous reef fishes. Photo by Renato Morais.
The studied reef has lost most of its coral cover due to two cyclones and two coral bleaching events. Despite some hard and soft coral colonies, most of the reef surface is covered in algal turfs, the favourite food of herbivorous reef fishes. Photo by Renato Morais.

Our paper provides a detailed account of the energetic responses of consumers to catastrophic  degradation events that deeply change the resource basis and the physical structure of an ecosystem. This is a unifying theme in ecology and, thus, could have parallels in different ecosystems. We showed that increased resource availability led to herbivore increases and to enhanced consumer productivity, but reduced turnover. We interpreted this reduced turnover as evidence that the built-up biomass was the result of somatic growth of individuals previously present and not enhanced population dynamics. Storage effects such as these could also happen due to habitat degradation in other types of landscapes, for example in riverine networks and forest fragments.

What does your work contribute to the field?

It contributes a new, process-focused way of looking at the effects of coral loss and habitat degradation on consumer communities. The function we quantify, the production of consumer biomass, is a critical one, both from an ecosystem and human perspective. Furthermore, the methods we use to quantify this function, underscored by four metrics of energy flow and storage, are reproducible and could be also employed by other studies.

What would you like to do next?

Surveys involved not only counting fishes, but also using anaesthetics to collect tiny cryptobenthic reef fishes (literally, inconspicuous fishes associated with the bottom), which are also known to be important components of the energetic functioning of coral reefs due to their fast growth and short lives.
Surveys involved not only counting fishes, but also using anaesthetics to collect tiny cryptobenthic reef fishes (literally, inconspicuous fishes associated with the bottom), which are also known to be important components of the energetic functioning of coral reefs due to their fast growth and short lives.

First, we want to know if similar results occur in other areas. We believe our findings may be applicable to other regions, but we need to gather more data. Second, we need to keep monitoring this reef fish assemblage to find out if new energetic shifts happen and, if so, assess their implications . Obviously, it will depend on what happens with the reef as well, that is, if corals recover or if the new low-coral state will be maintained. Then, what will happen to the large and old herbivorous fishes driving the energetic changes observed, will they be replaced by young ones? Only a continued effort to track these fish populations can answer these questions.

About the Author

What are you currently working on?

I’m currently working on developing and streamlining methods to quantify consumer productivity in high-diversity ecosystems. My focus is on coral reef fishes, which are an excellent study system, but the principles developed should be applicable to a wider range of situations.

What’s your current position?

I have just finished my PhD and started a post-doctoral position with my PhD supervisor, Prof. David Bellwood, at James Cook University in Australia. In this new position, I will keep pursuing the same line of research, understanding consumer productivity on coral reefs, but expanding it to accommodate the influence of nutrient and energy connectivity at the seascape level.

What project/article are you most proud of?

The project that started my current line of research aimed to understand the relationship between consumer productivity and consumer biomass on coral reefs, and if/how this relationship was affected by human exploitation. This project combined large-scale empirical data and modelling simulations, finding that size-selective human exploitation generates decoupled declines of biomass and productivity and, in reality, may shape the relationship between these two metrics. This project demanded a lot of work and required much persistence, so I was very happy to see it published earlier this year in Global Change Biology (Morais et al. 2020 Human exploitation shapes productivity-biomass relationships on coral reefs. Global Change Biology, 26, 1295-1305, doi: 10.1111/gcb.14941).

You can read Renato’s recent publication in Functional Ecology here.

For more on coral reefs, read our Special Feature: Coral Reef Functional Ecology in the Anthropocene.