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What are the ecological impacts of coral bleaching?
| Coral reefs are the most diverse ecosystems on Earth, and this due to a bottom-up effect from the heterogeneity of coral species found on most reefs (Connell 1978). Global warming has caused changes in the temporal and spatial patterns of numerous terrestrial organisms, and the same effects are shown in marine anthozoans. As ocean temperatures continue to rise, zooxanthellae and corals that are more tolerant to increased temperatures prevail while other species are lost (see figure A below). In a study conducted by Fabricius et al. (2004) in Palau, it was shown that surviving reefs in atypically warm regions comprised much less coral heterogeneity than reefs in normal ocean temperatures. Therefore, while the reef as a whole did not bleach and die altogether, a few species of coral had a competitive advantage due to their tolerance to elevated sea temperatures and completely colonized the region; the warmer region contained only two different zooxanthella communities while the two reefs in lower temperatures were represented by between ten and twelve zooxanthella communities. Marshall and Baird (2000) observed the percentages of various coral types that survived a mass bleaching episode on the Great Barrier Reef in 1998. Hydrocorals were found to be particularly susceptible to bleaching due to elevated ocean temperatures, with eighty-five percent of hydrocorals dying. Cyphastrea, Turbinaria, and Galaxea were the three coral genera found to be most resistant to the rise in temperature. Thus, it is predicted that as ocean temperatures increase, coral reefs will become more homogeneous and the threshold temperature for bleaching will rise due to the prevalence of temperature-tolerant zooxanthellae and coral (see figure 2 below) (Hughes et al. 2003). |
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California Hydrocoral |
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This homogeneity in the producer and primary consumer levels of the food chain and the overall reduction in coral will without a doubt cause a decrease in the biodiversity of organisms that rely on coral for survival. A large reduction is expected in the population of reef-dwelling fish and invertebrates and their predators, as well as small pelagics that inhabit reefs for a portion of their life cycle. The abundance of herbivorous fish, such as parrotfish, will most likely increase, while certain species of fish that rely on temperature-sensitive corals, such as hydrocorals, should decrease.
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