The Great Barrier Reef
86
are much less obvious and harder to measure than catastrophic mortality, but they nonetheless play a crucial role
in the long term dynamics of reefs.
Two categories of stress, acute and chronic, are useful for assessing human impacts and natural disturbances. Acute disturbances act suddenly, and usually
for a short time, although their impacts may have long
term repercussions. Examples include a ship grounding, an oil spill, or a nuclear bomb test. Chronic impacts
occur over an extended period and are often difficult to
stop. For instance, subsistence overfishing in densely
populated developing countries, deforestation leading
to coastal runoff of nutrients and sediment, or discharge
of sewage from a coastal city are ongoing, chronic disturbances. There is some evidence that recovery from
some types of human impacts is more difficult or slower
than recovery from natural disturbances. According to
a recent review, coral assemblages suffering from
chronic (usually human) impacts recovered in only
27% of cases, compared to 69% for acute impacts.
Another useful way to view impacts on reefs is to
consider how human activities affect the structure of
food webs. The removal of species near the top of a food
chain by fishing can lead to an increase in abundance of
their prey (called a top-down effect). Many reefs worldwide have been severely overfished. Megafauna such as
sharks and turtles are increasingly rare worldwide, and
fisheries have moved lower down the food web targeting increasing numbers of herbivores such as parrotfish.
Similarly, the addition of nutrients can stimulate growth
of species at the bottom of the food web (primary producers such as phytoplankton and fleshy algae). This
bottom-up effect can propagate upwards in a food web
by providing more food for herbivores and their predators. Top-down and bottom-up distortions of food webs
typically happen simultaneously.
Can we identify reefs that are most at risk? Anticipating and preventing damage is likely to be more effective than restoration afterwards (Box 9.1). Obviously,
the number of people near a reef is crucial, for example,
BOX 9.1 CORAL REEF RESTORATION
The number of restoration projects is increasing as governments and NGOs attempt to
‘do something’ about the worldwide decline of coral reefs. Excluding artificial reef
projects, nearly two hundred coral reef restoration studies have been undertaken worldwide over the past three decades, at a combined cost exceeding US$200 million. Most of
them are small scale transplant experiments, where one or two species of corals are removed from one reef and relocated at a damaged site (e.g. after a ship grounding, cyclone, or the Asian tsunami). The total area of all of these projects is less than one square
kilometre, while globally the amount of reef that has been degraded in the past few decades is about 10
5 times greater.
Coral reefs are much more diverse and complex than other systems such as mangrove stands, grasslands, or lakes, where restoration has sometimes been possible. However, no one has been successful at artificially restoring the biodiversity or ecological
functions of a coral reef at a meaningful scale. A better outcome for sustaining coral reefs
will come from addressing the root causes of reef degradation and from targeted interventions that build resilience to phase-shifts (e.g. by improving land-use practices in reef
catchments, by establishing alternative employment options to reduce fishing pressure,
and by reducing greenhouse gas emissions).
International Coral Reef Initiative resolution on artificial coral reef restoration and rehabilitation. 2005: available at http://www.icriforum.org/library/ICRI_resolution_
Restoration.pdf [Verified 21 February 2008].
86
are much less obvious and harder to measure than catastrophic mortality, but they nonetheless play a crucial role
in the long term dynamics of reefs.
Two categories of stress, acute and chronic, are useful for assessing human impacts and natural disturbances. Acute disturbances act suddenly, and usually
for a short time, although their impacts may have long
term repercussions. Examples include a ship grounding, an oil spill, or a nuclear bomb test. Chronic impacts
occur over an extended period and are often difficult to
stop. For instance, subsistence overfishing in densely
populated developing countries, deforestation leading
to coastal runoff of nutrients and sediment, or discharge
of sewage from a coastal city are ongoing, chronic disturbances. There is some evidence that recovery from
some types of human impacts is more difficult or slower
than recovery from natural disturbances. According to
a recent review, coral assemblages suffering from
chronic (usually human) impacts recovered in only
27% of cases, compared to 69% for acute impacts.
Another useful way to view impacts on reefs is to
consider how human activities affect the structure of
food webs. The removal of species near the top of a food
chain by fishing can lead to an increase in abundance of
their prey (called a top-down effect). Many reefs worldwide have been severely overfished. Megafauna such as
sharks and turtles are increasingly rare worldwide, and
fisheries have moved lower down the food web targeting increasing numbers of herbivores such as parrotfish.
Similarly, the addition of nutrients can stimulate growth
of species at the bottom of the food web (primary producers such as phytoplankton and fleshy algae). This
bottom-up effect can propagate upwards in a food web
by providing more food for herbivores and their predators. Top-down and bottom-up distortions of food webs
typically happen simultaneously.
Can we identify reefs that are most at risk? Anticipating and preventing damage is likely to be more effective than restoration afterwards (Box 9.1). Obviously,
the number of people near a reef is crucial, for example,
BOX 9.1 CORAL REEF RESTORATION
The number of restoration projects is increasing as governments and NGOs attempt to
‘do something’ about the worldwide decline of coral reefs. Excluding artificial reef
projects, nearly two hundred coral reef restoration studies have been undertaken worldwide over the past three decades, at a combined cost exceeding US$200 million. Most of
them are small scale transplant experiments, where one or two species of corals are removed from one reef and relocated at a damaged site (e.g. after a ship grounding, cyclone, or the Asian tsunami). The total area of all of these projects is less than one square
kilometre, while globally the amount of reef that has been degraded in the past few decades is about 10
5 times greater.
Coral reefs are much more diverse and complex than other systems such as mangrove stands, grasslands, or lakes, where restoration has sometimes been possible. However, no one has been successful at artificially restoring the biodiversity or ecological
functions of a coral reef at a meaningful scale. A better outcome for sustaining coral reefs
will come from addressing the root causes of reef degradation and from targeted interventions that build resilience to phase-shifts (e.g. by improving land-use practices in reef
catchments, by establishing alternative employment options to reduce fishing pressure,
and by reducing greenhouse gas emissions).
International Coral Reef Initiative resolution on artificial coral reef restoration and rehabilitation. 2005: available at http://www.icriforum.org/library/ICRI_resolution_
Restoration.pdf [Verified 21 February 2008].