methods for determining coral reef health
This topic submitted by Matt McWilliams at 4:58 PM on 6/4/08.
This barracuda coasts above the corals at
Molasses Reef, Key Largo, Florida.
methods for determining coral reef health
matt mcwilliams
Coral reefs are an important ecosystem. Not only are these ecosystems important to ocean organisms, but to all organisms on the planet. Coral is a unique organism in that it is a combination of zooxanthelle, coral, and calcium carbonate. This co-existence provides for and creates the coral reef ecosystem. This ecosystem is also connected to other ocean habitats such as sea grass beds and mangrove forests. These ocean environments are all tied to ecosystems on land. Thus, earth is an interconnected web of living and non-living objects all working together. So, when one ecosystem is damaged and begins to die, or when new ecosystems develop, these occurrences affect all the other ecosystems of earth. These habitats provide invaluable ecosystem services and functions. These include, shoreline protection of the shoreline, sustain biodiversity, bring in tourists, and breeding grounds for many marine creatures (Fisher 2007). These ecosystems are thus vital components of the oceans supporting much of marine life. Not only are they vital to other marine organisms, because of their ecosystem services, they have been recognized by the World Conservation Strategy as global life support systems necessary for human survival. The services provided are estimated to equal upwards of 53 trillion dollars (Souter 2000). Thus, correctly understanding health of coral reefs is of the utmost importance. A number of measurements have been developed to help humans know the health of coral reefs. Some of the most common measurements include biodiversity, percent coral cover, species composition, frequency of disease, and live/dead coral. However, one of these measurements does not stand alone in representing coral health, they only represent a sliver of that particular corals condition. To gain the most accurate and wholesome assessment of coral health, it is necessary to use all of the measurements available to scientists. Thus, by assessing the biodiversity, percent coral cover, species composition, frequency of disease, and live/dead coral cumulatively, one can gain a fairly accurate idea of how healthy coral is.
Before completing any of these measurements the area to be measured must first be chosen. There are a many ways in which to set up areas to measure. The idea is to set up these areas in a way that eliminates as many variable as humanly possible. Hoffman and Fisher both used transects as their own areas of study. A transect is a rectangular area in which the measurements are done. Generally, several semi-randomly placed transects are measured to generalize measurements for a larger area. This is done to save time and money and is a much more feasible way to gain knowledge and understanding rather than identifying every single piece of coral. Once it is known what is to be measured and how much, the particular measurements to take place must be decided.
One necessary measurement in understanding coral health is to assess the coral biodiversity. Biodiversity is an important measurement for the health of coral as a species and as an ecosystem. Like Souter mentions in his article, healthy coral reefs are some of the most diverse ecosystems on the planet hosting anywhere from 1 to 9 million species. Also, more important to coral health is the estimated 800 different species of coral that can be found on reefs. A wide variety of species which reefs are capable of having indicates good living conditions in which a number of organisms are able to thrive. Furthermore, a plethora of species is much more likely to survive adverse conditions. Thus, it indicates health in the sense that it is more durable. Biodiversity can be measured in a number of ways. However, three common ways are through species richness and diversity, the Simpson index, and the Shannon index. Species richness is simply the number of species in an area, while species diversity is the number of individuals present in each species in each area. The result in combining these two measurements is a simple measurement of biodiversity. While simple, it is still effective. Fisher in his study of coral health used species richness and diversity along with colony size to determine the health of reefs in the Dry Tortugas and Key West. The Simpson index also calculates biodiversity but uses a more complicated formula that takes into account additional variables. The Shannon index goes even further and factors probability into the biodiversity equation. Thus, biodiversity is one fundamental and necessary measurement in determining coral health.
Another indicator of coral health is measuring the percent coral cover. This is a measurement of the live coral in any given transect within a coral reef area. Furthermore, it is a spatial measurement that measures the amount of space that coral takes up in the given transect. This is important because it can help determine which sections of reef are the healthiest as well as to the density of coral populations in the area. Fisher used percent live coral to assess coral health around the Gulf of Mexico. He used this with other measurements such as colony size and species richness to determine the health of corals along the gulf coast. This shows that percent coral cover is a valuable and important aspect of resolving the health of coral.
Other indicators of coral health include the composition of species. This harkens back to species diversity in portraying the populations of individual species but also the presence or absence of keynote species. For instance, certain species like the acropora palmata indicate a healthy barrier reef. In a study by Smith acropora species were specifically examined to determine the health of the reef. They studied the growth rates and size of acropora corals to determine health. Yet the composition does not have to be only limited to coral species. Perhaps even more beneficial is to count all species including starfish, urchins, and algae. These all interact with corals and high populations of one often times correspond to low populations of others. Thus, many algal species and few coral species would be an indicator of an unhealthy coral reef. However, this alone does not provide a cumulative or definitive answer to the question of coral reef health.
One such measurement that can also be added is the frequency of coral disease. In most cases the presence of coral disease indicates a coral reef that is not at 100% full health. However, nothing is truly ever at 100%, thus, even with the presence of disease, a coral reef on the whole can still be considered healthy. Measuring coral disease requires a comparison between non-diseased coral versus diseased coral. Obviously, a lower presence of disease suggests a healthier reef. Yet a reef with little disease can still be unhealthy if it is not deemed healthy through some of the other measurements.
A great way to help determine coral reef health is by measuring the ratio of live coral to dead coral. This measurement was used by Hoffman et. Al. as they assessed the coral reef health of the Fuji and Cook islands. This was not the sole measurement used though. It was used in tandem with several other measurements to determine the coral health of these islands. This can help determine the current health of the reef as well as indicate what the previous health of the reef may have been. A ratio in favor of dead coral could possibly suggest a dying reef. However, this reef could also be considered healthy in all the other measurements. In this case the only way to explain the high ratio of dead coral would be a sudden large disturbance. As McCabe hints at, one such disturbance could be tropical storms such as hurricanes. They may hit a healthy reef leaving a high ratio of dead coral but the individual coral and variety of species left can still indicate a healthy reef. So, while this ratio may seem as the only one necessary, it is not always a cumulative and accurate assessment of coral health.
Thus, it is apparent that there are a great number of ways in which one can measure the health of coral. It is also clear that using only one measurement will most likely not give an accurate or complete understanding of coral health. The only way to gain a complete understanding of coral reef health would be to measure every aspect of corals and their interaction. Since this is impossible, measuring as many factors of the coral reef ecosystems health as possible must suffice. Furthermore, measuring as many of these as possible is the only way to get a real sense of the health of coral reefs. A good way to accurately assess the health of a coral would be to assess the biodiversity, percent coral cover, species composition, frequency of disease, and live/dead coral. It is important and necessary to attempt to correctly understand the health of coral reefs because of their importance to the ocean ecosystems and also the larger ecosystem that is planet earth. The health of coral reefs affects us all, and the destruction or loss of these ecosystems will change all other ecosystems as well. Hopefully in assessing the health of coral reefs scientist will be able to help broadcast to the world the sad situation many coral reefs are in. there is already much documentation on the health of coral reefs, the question is, how much more is needed until change is demanded and protection is required.
Literature Cited
Fisher, W. S., Davis, W. P., Quarles , R. L., Patrick, J., Campbell, J. G., & Harris, P. S. (2007). Characterizing coral condition using estimates of three-dimensional colony surface area. Environ Monet Asses. 125, 347-360.
Hoffmann, T. C. (2002).Coral reef health and effects of socio-economic factors in figi and cook islands. Marine Pollution Bulletin. 44, 1281-1293.
McCabe, J. M. (2005).TEMPESTITES FROM HURRICANE FRANCES IN THE BEACH AND LACUSTRINE ENVIRONMENTS ON SAN SALVADOR, BAHAMAS. Geologic Society of America. 37, 66.
Reese, E. S., & Crosby, M. P. (1996). A manual for monitoring coral reefs with indicator species. 1-47.
Smith, L. D., Devlin, M., Haynes, D., & Gilmour, J. P. (2005). A demographic approach to monitoring the health of coral reefs. Marine Pollution Bulletin, 51, 399-407.
Souter, AuthorD. W., & Linden, O. (2000). The health and future of coral reef systems. Ocean and Coastal Management. 43, 657-688.
Wolanski, E., Richmond, R. H., & McCook, L. (2004). A model of the effects of land-based, human activities on the health of coral reefs in the great barrier reef and in fouha bay, guam, micronesia. journal of marine systems. 46, 133-144.
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