Coral Reefs and Climate Change

Introduction

Research on the current and future impacts of human-induced climate change on reef-building corals is causing scientists and managers to become increasingly concerned about the future of coral reefs. A healthy reef ecosystem literally buzzes with sounds, activity and colors and is populated by incredibly dense aggregations of fish and invertebrates. In this respect, tropical reefs are more reminiscent of the African Serengeti than of the tropical rainforest they are often compared to, where the resident birds and mammals can be secretive and difficult to see. A coral reef can contain tens of thousands of species and some of the world’s most dense and diverse communities of vertebrate animals. Unfortunately, very few remaining coral reefs resemble this pristine condition; on most, corals and fishes are much less abundant than they were only a few decades ago.

The Role of Corals on Coral Reefs

Healthy coral reefs are dominated by various forms of reef-building corals, which fill the role of trees in a forest, by generating the physical framework of the reef, benefiting thousands of associated plants and animals. Ecologists refer to corals, trees, and other organisms (e.g., kelp, oysters, etc.) that literally create habitats as “foundation species” and recognize that their loss can be catastrophic for the community and ecosystem that is built around them. The structure built up by corals over thousands of years provides complex refuges in which animals can hide from predators. When corals die, the abundance of reef fish quickly decreases, mainly due to the lack of places for larval (baby) fish to settle as they leave the open water and settle on the reef where they will spend their adult lives. For example, in Papua New Guinea reef fish communities were greatly impacted by coral loss due to ocean warming and sedimentation run-off from the conversion of forest to oil palm plantations.

Patterns of Coral Loss

We know very little about the historical biological baseline of coral reefs because we didn’t really begin to study them until they were already being degraded. Based on surveys in the 1960s and early 1970s and recent studies of relatively “pristine” reefs, it appears that historically, coral cover (the percentage of the ocean floor covered by living coral) on undisturbed reefs was 70% or higher. However, even before people began degrading reefs, natural disturbances such as storms and predator outbreaks reduced coral cover locally. Thus the historical average (i.e., across all reefs in a region, including disturbed and undisturbed reefs) was surely lower and probably closer to 50 or 60%.

There are many examples of coral loss on individual reefs and we can get a rough idea of how coral cover has changed at regional and global scales over time by combining data from many sources. Several such “meta-analyses have been performed recently by pooling survey data from the scientific literature with unpublished data from governmental and non-governmental monitoring programs. Examples of such organizations include the Australian Institute of Marine Science’s Long Term Monitoring Program and Reef Check, which trains volunteer divers to survey hundreds of reefs a year around the world. The resulting picture is of widespread coral loss, even on some of the world’s very isolated and intensively managed reefs. For example, coral cover in the Florida Keys declined to only ~ 8% by 2006. Similar losses since the late 1970s have been documented throughout the Caribbean, although some subregions such as the Lesser Antilles still have fairly high coral cover. The Pacific generally has higher coral cover than the Caribbean, although the picture is not that much brighter. An unpublished analysis of the most recent survey data indicates that Pacific coral cover is roughly 30%, probably around half of what it was several decades ago and coral loss on some Pacific reefs is on par with Caribbean loss.

Climate Change and Coral Loss

There are many causes of local and global coral loss but human-induced climate change is one of the main and undeniable threats. Climate change is having negative effects on coral populations via at least three mechanisms.

Coral Bleaching

First, ocean warming is directly reducing coral cover through coral bleaching. Reef-building corals contain plant-like organisms called zooxanthellae that live symbiotically within their tissue. Zooxanthellae provide their coral host with food and oxygen and in return, the zooxanthellae receive nutrients, carbon dioxide, and an enemy-free shelter. This symbiotic relationship evolved tens of millions of years ago and has been critical to the success and evolutionary radiation of corals and to the development of reef ecosystems. When summertime water temperatures are just a degree or two warmer than usual for a few weeks, this critical yet delicate symbiotic relationship breaks down and the zooxanthellae are expelled, often leading to the coral’s death. (The greater the magnitude or duration of the warming, the greater the mortality and effect on coral populations.) The phenomenon is called “coral bleaching because the coral animal appears to turn white after the zooxanthellae loss. This is because without their zooxanthellae symbionts, which contain various photosynthetic pigments, corals are nearly transparent and the white, external calcium carbonate skeleton that the coral polyps live on becomes plainly visible.

Carbon dioxide, methane, and other greenhouse gases trap heat, leading to global warming. The increase in ocean temperature is variable and quite subtle: on the order of 1° C over the last several decades. But even such modest changes have caused mass-coral mortality events around the world during some of the especially warm summers we have all experienced over the last ten years. In 1998 when an intense El Niño greatly warmed much of the western Pacific and Indian Oceans, coral bleaching was widespread, causing mass coral mortality in many countries. For example, in Palau, more than 90% of the corals on some reefs bleached and at least 50% perished. Even some isolated reefs were impacted. In the Maldives, in the east Indian Ocean, bleaching caused coral cover to plummet to only about 5%.

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