What does science tell us about our Northern Islands coral reef ecosystems?

Amongst all of the grit and glamour that has been posted in the local media regarding the northern islands monument there has been little scientific insight offered. This, despite the numerous voyages that local and federally sponsored scientists have embarked upon to gather data, represents my concerns as a marine biologist. Here, I will try to offer as much insight as I am able to, picking upon the dataset that I collected over the past 5 years, 10,234 individual coral colony measurements from the nearshore reef slopes, 0 – 50 ft. deep, along the nine emergent islands.

The situation is best put into perspective with some preliminary background information about two highly influential scientists, Edward O. Wilson and Robert MacArthur, who taught the world a great deal about islands and the distributions of biological species. They first taught us that there are three major drivers of species diversity along a chain of islands: 1) the size of the island, 2) the rate that new species immigrate into the local species pool, and 3) the rate of extinction. The smaller islands that are furthest away from the replenishing larval sources hold fewer species, while the number of species rises when more space becomes available for plants and animals to find happy homes in. In the case of the northern islands, the source of replenishing larvae comes locally from the standing stock of reproducing individuals, as well as regionally from the islands east of us, most notably the Marshall Islands. Given this information, I guessed that island size and extinction rates would be the strongest drivers of species diversity in the NMI, as immigration rates are relatively similar throughout the island chain. Island perimeter is easy to measure and is shown on the map next to the island names. This is a good approximation for how much shallow coral reef exists. For a proxy to extinction rates I substituted volcanic disturbance events that have occurred infrequently along the island chain over the past century.

Several graphs are presented below to illustrate some of the key findings. First, there exists a tight relationship between island perimeter and the number of species of coral and coral reef associated plants and animals. The first graph shows Uracus (blue), the smallest, northernmost island holds the lowest diversity, while Maug and Pagan (brown and green), two of the three biggest islands, hold the greatest diversity.

So why should we care about the number of species that exist on these islands anyway? One very clear result of modern ecological studies is a relationship between resiliency, or the ability to recover following a disturbance, and diversity. The more species you have the better equipped you are to deal with unexpected change, because you have more functional redundancy (more kinds of algae eating fish, photosynthesizing corals, and filter feeding invertebrates). This is similar to the common flu passing through a population; some individuals get sick and others do not. The organisms (people) that remain healthy continue to maintain the ecosystem services (business and society).

So what are the impacts of disturbances? Disturbances come in many shapes and forms, I will start with the most obvious one, volcanic activity. Eruptions have been occurring along our northern islands chain for hundreds-of-thousands of years, but just in the last century were records kept. I’ve found that volcanic disturbances have their strongest impacts upon species diversity and the amount of living coral. The second graph shows that live coral increases on islands where volcanic activity has not recently occurred. But more than just coral coverage, the predictable relationship between island size and species diversity is also impacted by volcanic disturbances. The third and final graph shows that the number of coral species expected to be present on each island, based upon its size, is reduced by volcanic disturbances. Anatahan and Pagan (blue and green), where the two most recent eruptions have been documented, show the greatest reduction in diversity.

In addition to volcanic disturbances, typhoons and outbreaks of coral eating starfish are two additional disturbances that can alter our reef ecosystem. In the winter and spring of 2003 and 2004 our data found significantly higher populations of coral eating starfish on CNMI’s reefs compared with all other years during this decade. These animals prey upon live corals and leave only dead skeleton footprints. Coral colony measurements detected similar negative impacts between Maug and Saipan, however recovery has been significantly different. By 2007 the average coral colony size had increased to pre-disturbance levels at Maug while at LaoLao Bay colony sizes still have not. Diversity too, has followed the same patterns. While some questions linking causes and effects will always remain, it is clear that the clean waters and functionally redundant food webs present on Maug have aided coral reef recovery. In culmination, this information sets the stage for desirable resource management options in the NMI.

In the northern islands the most desirable, and economically important reefs thrive on big islands where volcanic activity is currently low. Thus, it is not surprising that Maug, the third largest island without recent volcanic activity, holds the largest and most diverse coral and reef associated invertebrate assemblages. Clearly Maug deserves enhanced consideration for management, but strategies are best focused at the island level, where the most significant ecological relationships exist. In an ideal world management strategies should match the scale of ecological relationships. In the NMI the data show no relationship between individual watersheds, land-use, or feral animal populations and coral assemblages in the NMI. Its not that these forces don’t alter coral reefs, just that their impact is small in comparison to volcanic activity and the availability of habitat (the amount of coral reef). As such, the data suggest that management options that encompass entire islands, such as MPA’s, are ideal for the volcanic island chain. Exactly which islands and what type of ‘island-based’ management is subject to debate, however, it is my hope that this information would be used by the people, government, and businesses before making important decisions that will dictate the future health of CNMI’s coral reefs.