Coral reef ecosystems are among the most diverse and highly productive ecosystems on the planet yet are currently threatened by a number of natural and human-induced factors. Regardless of the cause, reef degradation generally results in an irreversible phase-shift from dominance by reef-building corals to dominance by fleshy macroalgae or seaweed. These shifts are believed to be irreversible and lead to communities that are less diverse and much less complex. While a number of natural disturbances can cause localized coral mortality, reduced top-down control (caused by overfishing) and increased bottom-up control (caused by nutrient pollution) are the most frequently implicated causes of human-induced reef degradation (aside from global warming). The relative importance of each of these factors in causing reef decline has been the subject of debate and much research among scientists. However, despite much effort there is still not consensus in the scientific community as to how these factors independently and interactively influence phase-shift formation or the loss of reef building coral. Further, not all phase-shifts are alike; some result in blooms of a single species of algae while others result in a more diverse mixed species assemblage and still others involve exotic species. Through analysis and synthesis of data from the literature, this project will develop conceptual models to determine the relative strength of top-down versus bottom-up control on coral reefs. Ultimately these results will help to identify the importance of overfishing and nutrient pollution on reefs by specifically identifying how these factors influence benthic reef community structure. The information generated by this project will be highly useful in implementing sound science-based management decisions for conservation of coral reef ecosystems across the globe.
Background Information
One of the most intriguing questions in coral reef ecology has been to understand how and why corals dominate the landscape (Figure 1) while plants dominate most other ecosystems on the planet. Reef ecologists generally believe that a combination of physical and biological factors favor the dominance of corals over algae on healthy or pristine reefs. Humans, however, can have dramatic effects on coral reef communities by altering any of the above factors, often shifting the competitive edge away from the corals and in favor of faster growing fleshy algae (Figure 2).
From a biological perspective, dense populations of grazing fish and sea urchins on healthy reefs help keep algae cropped to low levels (Figure 3). Generally on reefs with abundant fish and invertebrate populations, the algae that are present are either very small turfs that have fast growth rates or are either chemically and/or physically defended against grazers. Experimental evidence suggests that the removal of ggazers from reefs through overfishing or disease can allow algae to begin overgrowing corals and eventually cause a phase shift where long-lived, slow-growing corals are replaced by fleshy, fast-growing seaweeds (Figure 2).
From a physical perspective, the availability of nutrients, primarily nitrogen and phosphorus, can also influence the growth rates and abundance of algae on reefs. Most reefs have developed in the clear, warm and nutrient deficient waters of the tropics. The coral organisms and their symbiotic zooxanthellae (microscopic single celled algae that live inside the coral tissue) are extremely efficient at using nutrients and are able to survive with very little external input. Reef algae and seagrasses, on the other hand, need external sources of nitrogen and phosphorus to survive. These organisms are able to absorb nutrients from the water column, sand or sediment. On healthy reefs the low input of nutrients prevents algae from being "fertilized". In extreme cases where nutrient inputs are high (from sewage outfalls, terrestrial runoff containing agricultural fertilizers, landscaping, etc.), algae may be able to overgrow corals and become dominant, even occasionally forming large blooms (Figure 4).
The introduction of exotic, alien or non-native species has caused numerous problems around the globe able to out-compete native species. In places such as the Hawaiian Islands, this is particularly a problem because there are so many endemic species (species which are only found in Hawai'i). Over the years several species of marine algae have been introduced to Hawaiian reefs, primarily for experimental aquaculture. Unfortunately many of these species have spread from their initial points of introduction and have become quite abundant on the shallow reef flat
environments around the Hawaiian Islands. These invasive species are seen as one of the largest threats to global biodiversity, as they are often (Figure 5). While invasive species are known to have negative effects on ecosystem structure and function in forests, grasslands, lakes and some cold water marine environments much less is known about invaders on coral reefs.
