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Interview: Sustaining the Pulse of Coral Reefs

Coral reefs are home to more than 800,000 species and provide invaluable services to humans. But what keeps them running? In a new synthesis, Simon Brandl and colleagues have identified four pairs of ecological processes that form the pulse of the reef. We asked him: What are these processes, and can restoration strengthen a reef’s heartbeat?


Lots of corals, lots of fish: A presumably healthy reef in the waters of Raja Ampat, Indonesia (The Ocean Agency/ XL Catlin Seaview Survey)


SECORE: When we think about healthy, flourishing coral reefs most people conjure up images of colorful fish and critters swimming in underwater cities built by countless hard and soft corals. But do the number of reef fish or coral species growing in a reef really tell us whether a reef is healthy or not?

Dr. Simon Brandl: People have different definitions of what healthy reefs are. But I think we would all come to the agreement that we like pretty much the same things: lots of corals, lots of fish – big fish, if possible – and lots of colors. They have become variables of interest for us and that is why scientists, reef managers, and volunteers across the globe run surveys to determine what percentage of the seafloor is covered with corals and how many fish there are. But the reality is that both coral cover and fish biomass are results of reefs performing reasonably well rather than causes, at least so we think. What is actually underlying the productivity of reefs is a completely different story. And missing that can severely hamper our efforts to protect and restore coral reefs because once all these desirable features are gone, we have no idea how they may come back. 


What are the causes or internal drivers that really underpin reefs and the way they work?

Like everything in nature, reefs are incredibly dynamic ecosystems. In our paper, we partition coral reef ecosystem functioning into four pairs of ecological processes that reflect this dynamic nature. Together, they form the pulse of coral reefs and offer a gauge by which we can measure reef ecosystem functioning. Those key processes are: (1) calcium carbonate production by corals and other calcifying organisms and bioerosion through coral-eating or eroding creatures such as parrotfish, sponges or Christmas tree worms, (2) primary production by algae and herbivory by grazers such as sea urchins or herbivorous fishes, (3) secondary production and predation, and (4) nutrient uptake and release. These complementary processes result in the gain and loss of primary habitat and the constant movement of energy and nutrients through coral reef ecosystems – or in other words: they form the constantly-moving cogwheels in the complex engines of coral reefs that ultimately produce, or reduce, the variables we currently measure, like coral cover, reef complexity, or fish biomass...


Marine biologist Dr. Simon Brandl and colleagues defined four pairs of ecological core processes on coral reefs and their linkages (illustration: Brandl et al/photo: private archive)


... just as breathing, digesting or cell divisions keep the human body functioning?

Yes, it is in many ways similar to the human body, where we have a lot of dynamic processes occurring too. For example, our kidneys are constantly engaging in the process of excreting waste from our system; our blood is constantly transporting oxygen at a given rate over time. These processes make sure that we are healthy. But the key for this analogy is that, if we show symptoms of not being healthy, we go to the doctor to get a diagnosis of which processes are not functioning well. The doctor may, for instance, take blood samples to understand the performance of different organs or internal processes and find out what the causes for our poor condition could be.


Depending on these causes, physicians prescribe drugs or therapies as remedies. How can this new understanding of a reef’s pulse help us to conserve or restore those increasingly threatened ecosystems?

I think the start for all of this is to increase our efforts to actually understand those processes and what drives them. When we look at a reef and measure its calcification rate over time and then link that to the coral species that are there, it will reveal really valuable information that we can use to guide our conservation and restoration efforts. Coral restoration, for example, could be a good way of addressing the loss of primary habitat on reefs. However, as far as I understand, many coral restoration efforts rely on very few, fast-growing coral species. If we find that calcification rates are driven by a broad consortium of species more so than the few high-performing species we usually outplant for restoration, then we might want to focus on getting a balanced, diverse assemblage of corals instead that can also withstand some of the future disturbances better than simply outplanting the same one or two species. It could be a very valuable avenue for scientists and managers to monitor not just how much coral cover we can achieve but to get into the details of how a restored assemblage performs in terms of producing calcium carbonate skeletons over time: if we plant an assemblage of five coral species rather than just one, do we get a higher and more stable rate of calcification over time than with one or two fast-growing branching corals?



Examples that highlight the “pulse” of energy, material, and nutrient fluxes on coral reefs. Calcification-Bioerosion (top): Reef-dwelling Christmas tree worms build calcareous tubes as dwellings in skeletons of various coral species and might harm their host coral (P. Selvaggio); Primary production – herbivory (bottom, left): A parrotfish grazes on macroalgae growing on a reef (P. Selvaggio); Secondary production – predation (bottom, middle): Small reef fish such as this red lipped blenny are known to fuel biomass production through rapid growth and mortality (Cristina Mittermeier/Coral Reef Image Bank); Nutrient release – nutrient uptake (bottom, right): Travelling school of fish import nutrients to coral reefs from nearby ecosystems (P. Selvaggio)


So, thinking in processes rather than patterns could help us to understand the underlying mechanisms of restoration success or failure even better?

Yes, and it could also help us to conserve reefs more successfully. We live in a world where both resources and political will to implement conservation efforts can be very limited and conflict-ridden. Throughout much of Southeast Asia, the South Pacific, and Eastern Africa, for example, people’s livelihoods depend on fishing. So, protecting coral reef fishes from harvesting can collide with social necessities and stakeholder needs. This new approach of thinking about processes rather than patterns might help us to find some more differentiated approaches. Instead of saying “we have expansive stocks of reef fishes in this area that we need to protect, nobody can fish in here,” we can make some more informed decisions if we know what the major drivers of reef fish biomass production are. For example, many coral reef nations fish heavily for parrotfishes. If some parrotfish species contribute relatively little to grazing while others are responsible for the bulk of grazing activity, then allocating fishing effort toward select species may be a better strategy than allowing all fishing or closing down parrotfish fisheries entirely. It just allows for a little bit more nuance in how we impose policies that are our gold standard for achieving conservation but that can lack a bit of subtlety and ecological detail at their core.  


A young fisherman shows the catch he has made on a reef in the Dominican Republic. (P. Selvaggio)


But this also means that reef monitoring would get even more complicated because one would need so much more data to get an idea of what is really going on on the reef?

That is correct. But the good news is that for a lot of these processes, we actually have some really good methods in place – they just haven’t really been applied in the context that would reveal the necessary information. For example, scientists have used macroalgal assays to examine herbivory rates for a long time, where a strand of algae is placed on the reef and we measure how much is removed over a certain time frame. But actually linking that to the entire community of herbivores that is present and doing so across many reefs and regions will be the key to obtaining comparative data on how a reef should function. Imagine having a reef with 20 coral species, but only one of them actually produces meaningful amounts of calcium carbonate over time. Then our focus needs to be protecting and restoring that one species. So, knowing what part of the community affects a process and therefore the pulse of coral reefs is going to be absolutely critical if we want to be efficient in conserving coral reefs. I think it requires a shift to focusing on how the species and communities that are present influence the processes that ultimately yield the patterns we observe and care about.


If you would lead a coral restoration project in a reef area that is not doing so well, how would you try to incorporate this new way of looking at coral reef ecosystem functioning in your restoration work?

What I would really want to do is diversify those restoration efforts to include more species to understand how the composition of outplanted corals affects the rate at which they produce calcium carbonate and growth. That could be monitored by simply measuring coral growth. Quantifying the growth rates of outplanted assemblages with different numbers of species will provide some very useful information. At the same time, one could easily examine the effects of those corals on the production of reef fish biomass by monitoring the fish communities associated with the different coral assemblages. If more resources and scientific expertise is present, one could approximate the amount of nutrients that are being exchanged with the water column. Coral reefs are generally assumed to be somewhat nutrient-limited so the retention and recycling of nutrients is an important process for which we don’t have good monitoring data in place and that may provide valuable information about how well a reef does.

Why don’t we know such basics of reef ecosystem functioning already?

We are now looking at roughly 50 years of research on coral reefs, much of which has been spent describing the most basic natural history of reefs. That is really a blink of an eye if you compare it to the research on terrestrial ecosystems, so we really haven’t had that much time to figure out how coral reefs work. What is even worse is that with coral reefs being so diverse and so ecologically complex, we might go to one reef and things might work in a certain way, and then we go to a different one and realize: “Oh dear, this is completely different”. I think putting everything within the framework of the eight core processes as the underlying pulse, it will help us overcome some of these issues. In essence, we are just trying to standardize the metrics we use to compare coral reefs and hopefully find some more general answers to the questions of how does X affect Y, and Y affect Z, and how can we protect the variables we are most interested in.

These answers and data would also help to evaluate different methods of reef restoration. 

Precisely, and facilitating the protection and recovery of reefs is the ultimate goal. We can essentially start creating databases where people archive rates of these processes that we can then compare. The way I see it, a lot of coral reefs are going to shift to states that don’t quite resemble what we think of as a tropical coral reef and therefore, I have little doubt that a substantial proportion of reefs will function quite differently in the future. Identifying these reefs by means of quantifying the eight core processes and comparing them to what we know as ‘healthy’ reefs will be really useful because in some cases we might just come to terms with the fact that it is a different system now that functions completely differently. Such a future reef may, for instance, have a negative budget between bioerosion and calcium carbonate production. We are not going to get this system back to where it was before, even if we try really hard. Knowing this will help us embrace that future and say, ‘well let’s not waste our time trying to get things back the way they were if we can’t. Let’s focus on understanding how this system works and getting the most out of that.’ It is a sobering way to think, but in some ways, I’m afraid it is the reality for some reef systems on our planet – and again, knowing what the processes on a healthy reef should be, what their pulse is supposed to be when all is well, is going to be absolutely critical. 

Thanks a lot for this chat, Simon.


Simon Brandl is a marine biologist and postdoctoral research fellow working at Simon Fraser University in Burnaby, British Columbia, Canada.

The study mentioned above was published by the Ecological Society of America in its scientific journal Frontiers in Ecology and the Environment. The paper’s original citation is:

Simon J. Brandl, Douglas B. Rasher, Isabelle M. Côté, Jordan M. Casey, Emily S. Darling,  Jonathan S. Lefcheck & J. Emmett Duffy, 2019. Coral reef ecosystem functioning: eight core processes and the role of biodiversity, Frontiers in Ecology and the Environment. DOI: 10.1002/fee.2088

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