Types of Coral Reef Restoration

Since Darwin’s time, coral restoration has evolved, but the fundamental principle remains the same—ensuring the survival of fragmented corals. Corals have a unique characteristic—they reproduce asexually and can regenerate even after partial colony mortality. For example, corals like branching or bushy colonies propagate new colonies through a process known as propagation.

In the modern era, we use robust materials such as concrete, steel, ceramics, and other materials to secure or replant the corals in natural reef areas. The coral, once secured in suitable physical conditions, can thrive and grow.

A naturally broken coral is secured to a modular artificial reef, with pictures taken at 1 and 3 years.

To summarize, corals will flourish given the right conditions for growth and stability. The idea here is to expedite natural processes on the reef, thus reducing the recovery time of the ecosystem—a process known as ecosystem succession.

Natural Succession in a Coral Reef

Ecological succession is the sequential change in the species structure of an ecological community over time. In coral reefs, this change progresses towards a stable state, known as a “climax ecosystem,” similar to a mature forest on land.

During the climax state of many Indo-Pacific fringing reefs, the ecosystem is home to an abundance of genetically diverse corals. Corals of varying growth forms create a complex topography, offering an environment for associated fishes and invertebrates. Fragmentation and regrowth, particularly in branching and corymbose corals, can lead to vast fields of monotypic stands—areas where all the colonies share the same mother colony.

Corals like the massive and submassive growth forms are less prone to propagation through fragmentation, growing slowly but surely, and building solid structures over time. These corals primarily propagate sexually, and their larvae can end up in the same reef or travel hundreds of kilometers away.

However, major disturbances like bleaching events during an El Nino year can disrupt this harmony, leading to significant local mortality. The ecosystem then shifts to a new state, wherein macro algae dominate in the absence of herbivore grazers. This results in the inaccessibility of the existing structure to incoming coral larvae, leading to a recruitment failure and eventual breakdown of coral skeletons due to mechanical and chemical processes.

Still, all hope is not lost. Some robust species like the brooding Pociliopora, Porites, and Favia fragum (Atlantic), or corals from the Fungiidae family (mushroom corals), can eventually stabilize the rubble field, reinstate the ecosystem’s function and pave the way for more coral resettlement and growth.

Accelerating Natural Ecological Succession

Though recovery from a mass mortality event is a challenging task, we can lend a helping hand by accelerating the natural succession of coral reefs. When dead reefs become recruitment-limited due to poor conditions, restoration begins with addressing local sources of pollution and nutrient loading, followed by assisting coral settlement via algae removal and herbivore reintroduction.

When the dead reef starts turning into rubble or sand, it becomes necessary to stabilize the reef and add solid structures. Once the reef begins to regrow, we can enhance its recovery by increasing the living components of the reef through various restoration methods.

The three key objectives of coral restoration over the last few decades have been: (1) increasing solid structures available for coral growth, (2) enhancing coral coverage, and (3) improving growing conditions. These objectives correspond to structural, biological, and physical restoration and will be explored further in the next few topics.