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Unit 13: Looking Forward: Our Global Experiment // Section 3: Multiple Stresses on Interconnected Systems

As we discuss solutions to environmental challenges, we must acknowledge that many environmental problems are interconnected, so we cannot protect one part of the natural environment without considering all the different stresses on the system as a whole. A good example of this is the multiple threats to coral reefs.

Coral reefs occur in a variety of forms throughout tropical and subtropical seas and account for half of the Earth’s present production of calcium carbonate (Wood, 1999). The scleractinian coral, the most important Order of reef building corals (also called "stony corals"), contains symbiotic algae within its tissues known as zooxanthellae. While the coral animal is capable of ingesting nutritious particulate material from seawater, an important component of its nutrition is derived from the photosynthetic products of the algae. The algal requirement for light limits the depth to which reef-building corals can live and form new reef complexes.

Coral reef communities are among the most diverse assemblages in the marine environment. Coral reefs occupy less than 0.25 percent of the marine environment but contain more than 25 percent of marine fishes, and in terms of total species diversity, coral reefs are the marine analogue of tropical rainforests. Because coraline skeletal material provides the physical substrate for the reef, the loss of the coral makes the balance of the reef community vulnerable.

Many studies have documented the decline in coral reefs around the world. However, identifying the primary cause of coral reef decline is very difficult because there are so many different environmental stresses on coral reef ecosystems. First, there is the direct destruction of coral reefs by anchoring. Most species of corals grow very slowly—usually around 1 cm of extension per year. This means that when fragments of coral are destroyed by ship anchors, even when the destruction is relatively minor, the cumulative effect of many different ships over several years can ultimately destroy a reef because the reef requires so much time to repair the damage.

A more extreme way of destroying the reef is through dynamite fishing. Dynamite fishing involves setting off charges in the water that stun or kill fish, making them easy to gather with basic skin-diving equipment. Although it is banned in most tropical countries, it is still quite common, especially in poor regions with limited access to deep-water fisheries. A by-product of the blast that kills the fish is the total demolition of coral within many meters of the blast. Left behind is a pile of coral rubble, unsuitable for supporting the diverse communities of organisms that live in a healthy coral reef ecosystem.

Another threat to coral reefs is overfishing. Some of this fishing is not even for food, but for live tropical fish for aquaria in homes. In addition, even pelagic fishing can cause problems for coral reefs because marine food webs are very complicated. Depletion of one species—large, predatory fish, for example—can lead to unforeseen consequences resulting in other species collapsing, ultimately affecting the coral reef ecosystem.

Coral reefs are also threatened by human land use. In many tropical regions, development near the coast has led to an increase in erosion of soil, which can kill coral reefs either by the direct effect of terrestrial soil material on the coral or by adding excess nutrients, which stimulate algal populations that can outcompete the stationary corals for light. Other forms of pollution associated with development are also a problem for coral reefs.

The final threat to coral reefs comes from climate change, although there are two separate impacts. Changes in ocean chemistry resulting from higher CO2 levels are likely to be more serious threats to the health of coral reef communities. Aragonite—the mineral used by corals for their skeleton—is supersaturated in seawater today by about 400 percent. This will decline as CO2 concentration in the atmosphere rises, with corresponding reduction in pH. Calculations suggest that aragonite saturation will decline by approximately 30 percent at atmospheric CO2 concentrations twice the pre-industrial level, and this will lead to lower calcification rates for corals. It is possible that reduced rates of calcification will make reef corals more susceptible to storm damage.

An additional threat to coral reefs from climate change is a condition known as “coral bleaching,” which occurs when the corals lose their symbiotic algae in response to environmental stress (Fig. 3). Some corals do recover following brief periods of bleaching, although the means by which the algae become reestablished is highly speculative. If this fails to happen, the coral tissue dies, leaving the calcareous reef substratum exposed to physical damage and dissolution. Experiments have shown that this condition can be caused by elevated temperatures, reduced salinity, and excessive suspended fine particulate matter, and one or more of these factors has been associated with numerous observed bleaching events. There is also evidence that at elevated temperatures virulence of bacterial pathogens of corals may increase and that these may be involved in the bleaching process.

Coral reef after a bleaching event

Figure 3. Coral reef after a bleaching event
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Source: © 2003. Reef Futures. Courtesy Ray Berkelmans, Australian Institute of Marine Science.

Corals in today's tropical and subtropical oceans are very near their upper limits for temperature (some within 2°C) during the warm seasons of the year. The response of different species of coral to warmer temperatures is probably sensitive to both the magnitude of the increment of temperature and the rate at which this increase is experienced. Bleaching that isn't necessarily fatal can occur in response to an increase as small as 1°C above normal seasonal maxima. There is evidence that thermal anomalies greater than 3°C are fatal to several coral species. With the death of coral tissue, the reef substrate is subject to erosion from physical and dissolution processes and colonization by other organisms, especially seaweeds.

With so many different threats to coral reefs, how can they be protected? The challenge is to solve many of the different threats simultaneously. Protecting coastal marine ecosystems by setting aside marine preserves will not solve the issue by itself. This is because under predicted climate change conditions, raised carbon dioxide levels and warmer temperatures will destroy reefs even in protected areas.

This basic problem can be generalized to many types of environmental issues—in particular, the relationship between biodiversity loss and climate change. The primary strategy for protecting endangered species has been to set aside natural habitat, either as national parklands or wilderness areas. However, climate change threatens to undo much of the good work accomplished by conservation efforts, as the same barriers to that keep people and development out of these natural habitats also serve to prevent many species of plants and animals from migrating to preferred climate zones as the climate changes. Isolating natural ecosystems into specific protected areas bounded by agricultural or urban areas means that migration of these ecosystems in response to climate change becomes impossible. Thus, if we cannot avoid the most extreme climate change scenarios, many of the conservation efforts will fail. This does not mean that preservation of habitat is not important. Human appropriation of land continues to be the major threat to biodiversity, particularly in tropical forests. However, conservation is not enough when faced with the grand challenge of global climate change.

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