Written by Alison Barrat and Andy Bruckner
On a scientific expedition to Palau this January we saw thriving coral reefs that contained many species of large, healthy corals, and only a few miles away we found desolate looking reefs that had virtually no coral at all.
Our science team recorded conditions that were optimal for coral growth; clear aquamarine waters, no sedimentation and limited nutrients, typical water temperatures, and a gentle reef slope covered in crustose coralline algae – all conditions that are about as good as it gets for coral.
But the Republic of Palau lies just outside of the northern Pacific typhoon belt. Normally tropical storms bypass the islands. However, in December 2012 Super Typhoon Bopha battered Palau. Although the eye of the storm was over 50 miles to the south storm waves of 35 feet or more pummeled the eastern barrier reef system. Once vibrant reefs were turned into fields of rubble as storm waves shattered fragile branching acroporids and overturned immense (2 m) massive boulders of Porites. In places underwater it was as if the reef surface had been scoured clean.
This gave our research team a chance to explore the transformation of a coral reef from bare ground to a mature ecosystem, which scientists call a climax community. After an event that wipes the slate clean a reef begins to recover, but repopulating a reef is not a single event. Algae, corals and other invertebrates arrive and take root, over the years some species flourish while others get replaced. Scientists have a name for this too, it’s called ecological succession. It has been well studied on land following events like a forest fire but underwater it’s less well understood.
In some respects a coral reef is like a forest. Just as plants need soil, baby corals, or recruits, need the right kind of substrate to settle onto and attach – sand is no good but the dead skeletal remains of corals killed by the storm work well.
As does something called ‘crustose coralline algae’, studies have shown that this helps to stabilize the substrate and encourages coral to settle. The first things to settle are pioneer species, the weeds, grasses and other small plants – in the case of the reef, these are the small, short-lived coral species, many of which brood their eggs and release well developed larvae ready to settle almost immediately. On Pacific reefs, the first corals to settle are early colonizers such as Pocillopora, the small digitate acroporids, and encrusting corals such as Pavona varians and certain species of Montipora
On land, bushes follow the weeds. On a reef, the equivalent of the bushes are the fast growing, branching corals that form thickets, such as the staghorn and bottlebrush acroporids, Seriatopora, as well as some of the plating and foliaceous corals such as Montipora, Leptoseris, Turbinaria, Merulina, and Pachyseris. These corals outcompete the first species to arrive, gradually making the reef less hospitable to them and pushing them out.
Sticking with the forest analogy next come the larger trees. Comparable species on the reef would include some of the sturdier massive corals such as Goniastrea, Favia, Favites, and Astreopora and the thick branched acroporids, table acroporids, Isopora and certain larger species of Pocillopora. This is a moment with the highest diversity of an ecosystem. In some studies of ecological succession in coral reefs the system never matures beyond this stage because disturbances are just too frequent nowadays. But we were able to see evidence of the next and final stage on the reefs in Palau.
Finally, in a forest, hardwoods and the likes of the majestic redwood trees move in, the largest and longest living species. Now the climax community becomes established and stable, the ecosystem has reached equilibrium and there are few opportunities for new species. On a reef, the parallel species are the longer-lived massive corals, especially the important frame building corals like Porites. These species take the longest to become established and over hundreds of year they grow to several meters in size. Fortunately, many of these longer lived corals tend to be more resistant to strong waves, and they often resist all but the severest of storms.
Palau’s reefs seem to be very resilient, the ideal reef growing conditions that we found there have already cultivated the recovery of the reef with many small recruits beginning to colonize the open ground. Our Palau research mission is part of a five year project called the Global Reef Expedition. During the course of this expedition we have witnessed many damaged reefs caused by bleaching, overfishing, and Crown of Thorns Seastars among other things. But witnessing the recovery of damaged reefs in Palau gives us great hope for the future and reminds us that coral reefs can be just as resilient as other ecosystems on earth.