Coral reefs have amazing species diversity, which depends, in part, on a mutualism between the coral animal and a group of symbiotic algae that live inside the coral. The algae provide the coral host with approximately 90% of the energy it needs (from photosynthetic product).  In return, the algae are rewarded with a place to live and a generous allotment of nitrogen (mostly fecal matter) from the coral.  Unfortunately, coral are under attack from a variety of sources. Most problematic, humans are releasing massive amounts of carbon dioxide into the atmosphere, which is increasing ocean temperatures and also making the ocean more acidic.  Both processes can kill coral by causing coral to eject their symbiotic algae, making it impossible for the coral to get enough nutrients.

The coral reef at Mo’orea. Credit: Moorea Coral Reef LTER site.

But other factors threaten coral ecosystems as well. For example, the reefs of Mo’orea , French Polynesia (pictured above), were attacked by the voracious seastar, Acanthaster planci, between 2006-2010, which reduced the coral cover (the % of the ocean floor that is covered with coral when viewed from above) from 45.8% in 2006 to 6.4% in 2009.  Then, in Feb 2010, Cyclone Oli hit, and by April, mean coral cover had plummeted down to 0.4%.

Researchers survey the reef at Mo’orea. Credit: Peter Edmunds.

Peter Edmunds has been studying the coral reef ecosystem at Mo’orea for 14 years, and has observed firsthand the sequence of reef death, and the subsequent recovery.  Working with Hannah Nelson and Lorenzo Bramanti, he wanted to document the recovery process, and to identify the underlying mechanisms.  Fortunately Mo’orea is a Long Term Ecological Research (LTER) site, one of 28 such sites funded by the United States National Science Foundation.  Consequently the researchers had long term data available to them, so they could document how coral abundance had changed since 2005.  Their analysis showed the decline in coral cover from 2007 to 2010, but a remarkable rapid recovery beginning in 2012 and continuing through 2017.

% cover (+ SE) of all coral , Pocillopora coral (the species group that the research team focused on). and macroalgae at Mo’orea over a 13 year period based on LTER data. The horizontal bar with COTs above it represents the time period of maximum seastar predation.

What factors caused this sharp recovery? One general process that could be part of the answer is density dependence, whereby populations have high growth rates when densities are low and there is very little competition, and low growth rates when densities are high and there is a great deal of competition between individuals, or in this case, between colonies. The problem is that though density dependence makes intuitive sense, it is difficult to demonstrate, as other factors could underlie the coral recovery.  Perhaps after 2011 there was more food available, or fewer predators, or maybe the weather was better for coral growth.

High density (top) and low density (bottom) quadrats of Pocillopora coral established by the research group.

To more convincingly test for density dependence, Edmunds and his colleagues set up an experiment, establishing 18 1m² quadrats in April, 2016. The researchers reduced coral cover in nine quadrats to 19.1% by removing seven or eight colonies from each experimental quadrat (low density quadrats), and left the other nine quadrats as unmanipulated controls, with coral cover averaging 32.5% (high density quadrats).  They then asked if, over the course of the next year, more recruits (new colonies < 4cm diameter) became established in the low density quadrats.

Returning in 2017, the researchers discovered substantially greater recruitment in the low density quadrats than in the high density quadrats. This experiment provides strong evidence that the rapid recovery after devastation by seastars and Cyclone Oli was helped by a density dependent response of the coral population – high recruitment at low population density.

Density of recruits just after (left), and one year after (right) the quadrats were established. Solid bars are means (+ SE) for high density quadrats, while clear bars are means (+ SE) for low density quadrats.

In recent years, many coral reef systems around the world have experienced declining coral cover, a loss of fish and invertebrate diversity and abundance, and an increase in abundance of macroalgae.  While many of these reefs continue to decline, others, such as the reefs at the Mo’orea LTER site, are more resilient, and are able to recover from disturbance.  The researchers argue that we need to fully understand the mechanisms underlying recovery – in other words what is causing the density dependent response? Is it simply competition between coral that cause high recruitment under low density, or may interactions between coral and algae be important?  And what types of interactions influence recruitment rates under different densities?  One possibility is that at high densities, coral are eating most of the tiny coral larvae as they descend from the surface after a mass spawning event.  This raises the important question of why many reefs around the world do not show this density dependent response.  Clearly there is much work remaining to be done if we want to preserve this critically endangered marine biome.

note: the paper that describes this research is from the journal Ecology. The reference is Edmunds, P. J., Nelson, H. R. and Bramanti, L. (2018), Density‐dependence mediates coral assemblage structure. Ecology, 99: 2605-2613. doi:10.1002/ecy.2511. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.