Award: OCE-1546374

The impact of a large episodic disturbance on an invasive (outbreak) coral: Will Typhoon Maysak promote or suppress an invasive Montiporasp. coral on reefs of Ulithi, Micronesia?

Ulithi Atoll, in Yap State of Micronesia, is the fourth largest atoll in the world. The Atoll's healthy coral reefs have sustained local populations of approximately 1000 people for hundreds of years. Since the 1960?s, however, reefs have changed. The outbreak of a weedy coral, in the genus Montipora(locally called 'cabbage coral') has overtaken some reefs, smothering other corals and reducing structural complexity. The local people reported that these reefs were less productive, so we have been examining the impacts by comparing reefs with and without this coral. Curious about the cause, we interviewed islanders about when they first saw this coral. They believe that the beginning of this outbreak coincided with Typhoon Ophelia, which passed over Ulithi Atoll in 1960. Impact from that storm may have damaged other corals, providing an opportunity for this 'weedy' species of coral to take over, possibly exacerbated by climate change which may be creating more favorable conditions for this coral. We had an opportunity to test this hypothesis when Typhoon Maysak passed over Ulithi in 2015.

This project addressed several questions:

1. Is there a pattern to which reefs have become dominated by Montipora sp.?
2. Do reefs dominated by Montiporasp. have lower diversity and abundance of other coral species? 
3. Do reefs dominated by Montipora sp. have lower diversity, abundance, and biomass of reef fishes?
4. What impact will the typhoon have on the local distribution of Montipora sp? Will Typhoon Maysak break apart this brittle coral and cause it to spread by fragmentation, or will the storm provide a chance for other coral species to re-populate these reefs?

We validated the local people's impressions of Montipora sp. dominated reefs having lower productivity: Our reef surveys indicate that corals, fishes and key invertebrates are less diverse and abundant on Montipora sp. dominated reefs. 

The next round of inquiry was to determine the direct effects of the Typhoon on the coral and its dispersal. For this question, we used state-of-the-art genomic tools. We first did a set of experiments right after the Typhoon passed over Ulithi. We designed a grid on the ocean floor to collect a large number of Montipora coral fragments to determine: 1. If fragments were related to attached 'parental' colonies, and 2. How far from the parents the coral fragments had been scattered by the Typhoon. 

We then waited a year to visit Ulithi again and set up a similar experiment, but this time we looked at small fragments of corals that were attached to the bottom floor. These were pieces of corals that had been broken up by the Typhoon the previous year, had been dispersed by the Typhoon, and then had been able to survive and reattach themselves.

The first surprising findings were that, unlike what we expected, the majority of fragments and colonies are genetically different from one another, which means that most Montipora corals are the result of sexual reproduction, rather than cloning and fragmentation. In addition, our investigations showed that, indeed, large colonies had been broken up by the Typhoon and fragments had been dispersed as far as 40 meters (120 feet) from the parent colony. Some of these fragments survived and reattached themselves. Our main conclusion was, therefore, that Typhoon Maysak did in fact play a role in the dispersal of Montipora corals. 

Yet this is only one part of the story. A complex and unexpected picture emerged from our work. Overall, the abundance of Montiporasp. throughout the Atoll decreased after Typhoon Maysak. Although coral fragments were dispersed widely by the Typhoon, the large original colonies were broken up and other coral species were able to recolonize the spaces left open by the broken Montipora. We are closely monitoring reefs in the years following Typhoon Maysak to see whether these reefs will return to their original more diverse states, or if the fragmentation of Montipora will allow it to spread further.

Last Modified: 05/20/2019
Modified by: Giacomo Bernardi