How harnessing ecological processes may boost restoration success
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Photo: A school of Squirrel fish in Raja Ampat. (Yen-Yi Lee)
Around the globe environmentalists have been trying to restore important ecosystems such as mangroves, seagrass meadows or swamps for decades. What can reef restoration practitioners learn from those experts?
Mark Ladd: Restoration practitioners have long recognized the importance of community ecology and ecological processes in promoting ecosystem recovery. Benefits from the presence or behavior of other organisms and ecological processes, like herbivory and succession, are often incorporated into restoration approaches in many terrestrial, aquatic, and marine systems. For example, positive interactions play a big role in the restoration of systems like marshes and grasslands. By manipulating the number of plants transplanted in an area, restoration practitioners can target optimal plant densities to decrease stressful conditions, increase seed retention, and promote growth. Coral restoration can promote positive interactions as well. Outplanting corals in optimal densities can promote positive density-dependent processes that boost growth and survivorship while reducing negative impacts such as disease transmission and competition.
Coral restoration can also take advantage of existing processes to facilitate restoration. On coral reefs, schooling fishes like grunts can create nutrient hotspots in localized areas where they shelter. In these areas, outplanted corals can use the nutrients excreted by fish to grow faster. Further, herbivores such as parrot fish prefer to graze on the nutrient-rich algae in these areas, algae that otherwise would compete with coral outplants and slow coral growth. Thus, careful selection of places within a reef to outplant corals can improve the chances of coral growth and survival. Such an approach is similar to the establishment of recovery nodes in reforestation, designed to attract important animals that will shelter in these areas and provide services like concentrating nutrients, dispersing seeds, and facilitating succession.
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Photos: left) SECORE Research Director and co-author of the study: Dr. Margaret Miller (SECORE International); mid) First author Mark Ladd from the Department of Ecology, Evolution, and Marine Biology at the University of California in Santa Barbara (private); right) Participants of a SECORE workshop on coral restoration in Curaçao are outplanting coral recruits onto a reef in the waters of Curaçao. (Photo: Benjamin Mueller)
You have already mentioned a few of the main players in the reef. Which other animals or plants offer a helping hand to corals, which are dangerous for outplanted coral recruits?
Margaret Miller: All organisms have natural enemies, such as predators and pathogens, and friends, such as probiotic microbes or ‘enemies of my enemies’. Coral reefs are extremely complex ecological communities such that identifying these net effects is sometimes difficult. Simple predators of corals such as fireworms or crown of thorns sea stars are very visible and obvious, but coral disease and the pathogens involved are poorly understood. It appears that some microbes are benign under favorable environmental conditions but become enemies only under environmental stress.
The most commonly cited coral ‘friends’ are grazers. Corals are generally slow-growing while many of their neighbors growing on the reef, such as seaweeds or sponges, have much higher growth rates and can overgrow or compete with existing corals and make it difficult for new corals to recruit. Grazing fishes and urchins are the important groups that benefit corals in this regard.
Corals enjoy more intimate partnership with tiny plants called zooxanthella that live inside the coral tissues and share the nutritional energy they derive from the sun. This important partnership is of great concern in SECORE’s efforts raising baby corals because it is established after settlement. Ecological studies also suggest that different strains or individuals of the zooxanthellae partners may provide greater benefits to the corals than others. Shaping this partnership toward the most beneficial pairings may be a future strategy to improve the resilience of restored corals in the future.
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Photo: One of the coral predators - a Crown of Thorns sea star, Acanthaster planci. (jon hanson on Flickr. CC-BY-SA 2.0)
How should reef restoration practitioners incorporate all this knowledge in their restoration planning to increase the success?
Mark Ladd: Coral reef scientists have accrued an amazing amount of knowledge on how coral reefs function. We know that herbivores play a critical role in keeping fast-growing seaweeds at bay and allowing corals to establish and grow, especially after a disturbance. Therefore, restoration efforts should aim to promote herbivory in areas where corals are being outplanted. This can be done in different ways. Restoration practitioners can take advantage of existing herbivory by selecting reefs with abundant herbivorous fishes or selecting areas on a reef with urchins, which can provide a source of intense, localized herbivory. Alternatively, coral restoration can be designed to enhance herbivory. Reducing the area available for grazing by outplanting corals may concentrate existing herbivory on the remaining space where seaweeds exist. Further, coupling coral outplanting with the re-stocking of grazers such as urchins could help restored corals grow and survive.
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Photos: left) A starfish on a reef in the waters of Guam (Paul Selvaggio); mid) Algae-eating parrotfish, like other herbivorous reef fish, play an important role in coral reef ecosystems by removing the algae that compete with corals. (The Ocean Agency); right) Many damselfishes are territorial algal gardeners that could promote or hinder coral restoration efforts, depending on geographic location and species-specific behavior. (The Ocean Agency)
Does nature offer any help when it comes to predation as well?
Mark Ladd: Predation on corals is a major source of partial and full mortality that can hinder or nullify restoration efforts. Fireworms and coral-eating snails are two of the major predators impacting restored corals in the Caribbean. However, these coral predators have their own enemies on the reef, such as fishes feeding on invertebrates and lobsters, which are not distributed evenly among reefs or even within a single reef. Thus, choosing to outplant corals to reefs, or locations within a reef, with populations of organisms that consume or deter coral predators could help boost the chances of restored corals doing well.
As coral cover declines, predation pressure is focused and intensified on the remaining coral colonies. Therefore, outplanting corals to extremely coral depauperate reefs may actually work against restoration goals, as coral predation may be too intense to allow corals to survive. Instead, outplanting corals to reefs with some existing coral cover may reduce damage to restored corals from coral predators.
In your paper you even suggest to protect one coral species from predation by planting it next to a species, which is less palatable. Which corals do you have in mind here?
Mark Ladd: Coral predators often demonstrate preferences for specific species of coral. Short coral snails (Coralliophila abbreviata) for instance love to feed on the staghorn coral Acropora cervicornis. Several field studies have demonstrated that outplanting staghorn corals in mixed-species assemblages, rather than monospecific groups, can reduce the attraction of coral predators and subsequently the damage inflicted on restored corals. Successful approaches to deter predators are likely highly context-dependent and will vary among regions and reefs. In places like French Polynesia, important reef building corals like Massive Porites are intensely preyed upon when outplanted to a reef. Thus, providing some “cover” with fast-growing, hardy species like Pocillopora (cauliflower coral) could provide a reprieve from predation by fishes and allow these corals to persist. In the Caribbean, as more and more species become available for coral restoration, getting a better handle on coral predator preferences would provide guidance on species assemblages that can be outplanted to better resist coral predation.
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photo: Reef in the Conception Reserve, Bahamas (The Ocean Agency/XL Catlin Seaview Survey)
These examples really show that restoration practitioners need to know their reef sites inside out before planning restoration efforts. How can they make sure to know all these things?
Margaret Miller: Having local knowledge of reefs where restoration is to be undertaken is certainly important. Information such as the ecological history of environmental stressors – e.g. what may have caused coral mortality in the past – and the management regime as well as the strength of specific ecological interactions in a particular site are extremely useful. For example, the intensity of predation on corals can vary immensely over short distances. Valuable information regarding intensity of predation or disease among different sites can be gained in a pilot project, outplanting a small number of corals across a wide range of sites. Although this represents an additional effort, identifying sites where predation or disease will impair restoration success may be worthwhile. The Reef Resilience Network is offering some good advice on how to select a coral restoration site. Furthermore, Jeffey Maynard and colleagues give some guidance in their paper “Assessing relative resilience potential of coral reefs to inform management“.
Mark Ladd: It is important to highlight that many of the ideas we propose in our paper are untested in a restoration context. They are based on decades of work by colleagues on the ecology of restoration and coral reefs, our literature review of coral restoration efforts, and a survey of restoration practitioners we conducted as part of the paper. Further, we have outlined a series of important knowledge gaps that would greatly improve our ability to successfully restore corals. We hope that our suggestions encourage those in the field of coral restoration to test these ideas with robust field experiments and continue to advance the field of coral restoration.
This new study was published under the following title:
