As we saw, the propagation, or cloning of corals, reduces the genetic diversity of the restored reefs and can contribute to the eventual collapse of the coral population. This is similar to the cloning of trees and food crops on land, which has resulted in great losses due to pests and disease epidemics.
Genetic variability means that individuals in a population have different tolerances to stresses. Coral colonies with identical DNA have the same environmental conditions and disease susceptibility tolerance. In a mass bleaching or similarly destructive disturbance event, all of the coral in the monotypic population may be lost simultaneously.
Despite the expansive scientific literature on the long-term consequences of cloning corals or other animals and plants, the practice is still very common among governments, NGOs, and community-based reef managers. Often, the same mistakes are repeated in different locations due to a lack of understanding of the basic theories on genetics and reproductive strategies.
When large disturbance events occur, and coral mortality is high, there are generally 3 factors required for the recovery of the area:
In the case of monotypic reefs (those produced through asexual means), reproductive failure occurs because most corals have barrier against ‘selfing,’ they cannot self-fertilize. Furthermore, fitness of the larvae is reduced due to inbreeding depressions. Essentially the gene pool becomes too small to react to environmental changes, and delirious genes may persist through generations, much like the problems caused by inbreeding in pets or humans.
Ecosystem recovery is dependent on sexual reproduction, new corals are needed to replace those that were lost. As you can see, asexual reproduction is not a major factor in the return of a reef to a stable and productive state. Thus, projects focusing solely on the asexual cycle of coral reproduction will have little long-term success in enhancing reef resilience.
High genetic diversity and successful reproduction and recruitment are essential for the adaptation of corals to the increasing frequency and severity of many natural disturbances.
This is not to say that all reef management programs using methods similar to the 3 Step coral restoration system are ineffective. Those that utilize only naturally created coral fragments can achieve very high survival rates and tend to accomplish short-term goals focusing on increasing the health, abundance, and diversity of corals and related reef organisms in the restoration area.
These programs should, however, not be confused with programs utilizing so-called ‘donor’ colonies’ to propagate and rear mono-specific feedstocks of corals. Excessive use of coral propagation rarely leads to positive long-term ecosystem effects and often negatively impacts both the ‘donor’ and the restored reef areas.
Community-based reef managers need to be aware of the genetic effects of different restoration methods for corals or other organisms and should keep in mind how the projects they carry out will affect the long-term survival of the ecosystem. In some cases, the positive long-term effects of such programs remain debated, but it is clear that they create little to no negative impact on the ecosystem and are generally very effective at raising awareness and community involvement in marine projects.
If done well, they have the potential to greatly expedite coral reef recovery while increasing species and genetic richness.
Going back to the study by Hein we discussed in the last topic, the programs analyzed showed increased coral cover. But, ours showed the greatest increase in coral coverage, with about twice the percentage of hard corals in the restored sites then in the unrestored sites. For structural complexity, ours was again the most successful, with a two-fold increase. Of the four programs investigated, ours was the only one to show significant increases in the natural settlement of juveniles and coral species richness. Lastly, ours was the only program to show gains in coral health, with all coral propagation projects showing decreases in coral health between restored and unrestored sites.
We think that all this discussion of genetic diversity is pretty self-evident and hope you now do, also. But why do so many of the largest coral restoration programs in the world still rely on asexual propagation from a limited number of genets to produce restoration feedstocks?
One of the reasons why mainstream coral restoration has failed for so long is due to a lack of long-term monitoring. The other stems from costs and a focus on quantity. It is easy (and cheap) for any group, trained or untrained, to propagate corals asexually and claim to have made hundreds or thousands of new colonies. But as was pointed out by Clark and Edwards in a 1995 article based on a ten-year study of coral propagation:
“[after a decade] transplanted areas will only be distinguishable from un-transplanted ones by the greater amount of dead coral in the former.”Clark and Edwards, Coral transplantation as an aid to reef rehabilitation: evaluation of a case study in the Maldive Islands.
But why are so many programs failing to learn from these mistakes?
In 2020 a collaborative coral restoration database was created, and the authors of the first study to analyze that database found that in 60% of restoration projects, monitoring was only carried out for less than 18 months. Not nearly long enough to show reliance or long-term success. Those authors, Boström-Einarsson et al., stated: “owing to poor communication and collaboration between coral restoration practitioners, coral reef managers, and scientists, a large proportion of coral restoration work to date has been undertaken with little or no scientific input or detailed monitoring.”
That same study found that the majority (60%) of programs only monitored their work for 18 months or less. It’s easy to claim success on such short times scales when major disturbances occur only every few decades (prior to the last two decades).
Furthermore, that same study found that of the 44 genera included, 3 genera made up 50% of all the corals being used in restoration projects. For the most part, all the other 41 genera of coral on the reef were being completely ignored.
Lastly, we think it’s important to note that Acropora makes up 30% of all corals being restored. There is good reason for this in the Carribean, as they were recently listed as an endangered species. However, in general this speaks to the short sighted nature of many programs, who use only the fastest growing corals to achieve short-term results.
This is why long-term monitoring of coral restoration projects is so vital and will be covered in our next lesson. Regardless of whether or not your project is successful, you should be tracking it and sharing your experiences, both positive and negative. This is one of the most important things we can do as reef managers in an industry still in its infancy if we are going to save reefs around the world.