Cross-references
Cay Formation
Coral Cays-Geohydrology
Diagenesis
Hydrodynamics of Coral Reef Systems
Lagoons
Reef Structure
Waves and Wave-Driven Currents
Wave Set-Up
INTRINSIC AND EXTRINSIC DRIVERS
ON CORAL REEFS
Richard B. Aronson
Florida Institute of Technology, Melbourne, FL, USA
Synonyms
Autogenic and allogenic drivers on coral reefs
Introduction
The morbidity and mortality of reef-building corals have
increased worldwide in recent decades, resulting in
declines of both living coral cover and the density of coral
colonies (Gardner et al., 2003; Bruno and Selig, 2007;
Wilkinson, 2008; Schutte et al., 2010). Coral populations
have not declined uniformly in time or space, and on some
reefs they have remained stable or increased (Porter and
Meier, 1992; Murdoch and Aronson, 1999; Aronson
et al., 2005); nevertheless, the overall downward trend is
indisputable. Because there are so many potential drivers
of change on coral reefs, assigning causality to observed
patterns is difficult. Ecology strives to ascertain the relative importance of multiple processes that are causing
change, rather than to falsify individual explanations or
classes of explanation (Quinn and Dunham, 1983). The
complexity and scale dependence of pattern and process
have led to considerable controversy over what is happening to coral reefs.
Causal typologies
Ecologists have tended to classify stresses, disturbances,
and other processes as either intrinsic or extrinsic. Intrinsic
and extrinsic causes can be further classified based on
whether they are natural or anthropogenic, and the scales
at which they operate (local or regional–global). Strict
adherence to these typologies has led to confusion over
complex cases. The types, intrinsic and extrinsic, natural
and anthropogenic, and local and regional–global, are
usefully viewed as the endpoints of continua, rather than
as specific targets of classification.
Intrinsic drivers act at small, local spatial scales and
include processes such as predation, herbivory, competition, recruitment, juvenile mortality, adult mortality,
succession, framework accretion, and framework
bioerosion. Changes in these processes generate changes
in reef communities. Ecological succession following
a disturbance such as a cyclone (an extrinsic disturbance)
would appear to be a clear example of a natural, localized,
intrinsic process in coral reef communities. The upward
vertical accretion of a reef, which is based on coral growth,
raises the living community into shallower water, so in
the absence of changes in sea level, the shallowingupward replacement sequence of corals should be considered autogenic succession (Aronson et al., 1998). (In contrast, reef drowning from rapid sea-level rise, which
results in a deepening-upward sequence, is extrinsically
driven.) Even succession, however, is not without its classificatory problems. The trajectories of successional
sequences will vary in accordance with varying regimes
of extrinsic disturbances such as cyclonic activity and
global change (Connell, 1978; Done, 1999).
Extrinsic drivers of change on coral reefs include local
anthropogenic stresses such as pollution, sedimentation,
and nutrient loading (eutrophication); local anthropogenic
disturbances such as ship groundings, coral mining, and
blast fishing; local natural stresses such as upwelling and
the off-platform transport of hot/hypersaline or cold/
hyposaline, inimical waters; local natural disturbances
such as bioerosion, coral mortality caused by damselfish
(Pomacentridae) and other corallivores, tectonic activity,
and lava flows; large-scale natural stresses such as
increased turbidity following heavy rains and variations
in insolation; large-scale anthropogenic stresses such as
global warming and ocean acidification; large-scale natural disturbances such as regional disease epidemics of
corals and herbivorous echinoids, interruptions in connectivity (i.e., larval supply), and regional outbreaks of the
corallivorous seaster Acanthaster planci; and large-scale
anthropogenic disturbances such as introductions of alien
species, increased turbidity from large-scale deforestation,
and pan-tropical overfishing. The limited utility of these
simplistic classifications for science and management is
highlighted by the example of perturbations to larval supply, which can be natural or anthropogenic, and which can
act on local to regional spatial scales (Sammarco, 1985;
Hughes et al., 2002). Overfishing is clearly anthropogenic,
but it can be viewed as a local, regional, or global source of
disturbance, and it is extrinsic or intrinsic depending on
whether or not human fishers are considered part of the
reef ecosystem.
Evolving models of reef dynamics
Despite these difficulties, broad generalizations are possible about intrinsic and extrinsic drivers of reef dynamics.
One widely accepted scenario for the Caribbean region
emphasizes the role of human predation in causing degradation of reefs in the 1980s and 1990s. Historical
overfishing reduced populations of herbivorous fishes,
and then in 1983–1984 a regional disease outbreak decimated the ecologically important echinoid Diadema
antillarum, with the cascading consequence that declining
herbivory released fleshy and frondose macroalgae
610
INTRINSIC AND EXTRINSIC DRIVERS ON CORAL REEFS
Cay Formation
Coral Cays-Geohydrology
Diagenesis
Hydrodynamics of Coral Reef Systems
Lagoons
Reef Structure
Waves and Wave-Driven Currents
Wave Set-Up
INTRINSIC AND EXTRINSIC DRIVERS
ON CORAL REEFS
Richard B. Aronson
Florida Institute of Technology, Melbourne, FL, USA
Synonyms
Autogenic and allogenic drivers on coral reefs
Introduction
The morbidity and mortality of reef-building corals have
increased worldwide in recent decades, resulting in
declines of both living coral cover and the density of coral
colonies (Gardner et al., 2003; Bruno and Selig, 2007;
Wilkinson, 2008; Schutte et al., 2010). Coral populations
have not declined uniformly in time or space, and on some
reefs they have remained stable or increased (Porter and
Meier, 1992; Murdoch and Aronson, 1999; Aronson
et al., 2005); nevertheless, the overall downward trend is
indisputable. Because there are so many potential drivers
of change on coral reefs, assigning causality to observed
patterns is difficult. Ecology strives to ascertain the relative importance of multiple processes that are causing
change, rather than to falsify individual explanations or
classes of explanation (Quinn and Dunham, 1983). The
complexity and scale dependence of pattern and process
have led to considerable controversy over what is happening to coral reefs.
Causal typologies
Ecologists have tended to classify stresses, disturbances,
and other processes as either intrinsic or extrinsic. Intrinsic
and extrinsic causes can be further classified based on
whether they are natural or anthropogenic, and the scales
at which they operate (local or regional–global). Strict
adherence to these typologies has led to confusion over
complex cases. The types, intrinsic and extrinsic, natural
and anthropogenic, and local and regional–global, are
usefully viewed as the endpoints of continua, rather than
as specific targets of classification.
Intrinsic drivers act at small, local spatial scales and
include processes such as predation, herbivory, competition, recruitment, juvenile mortality, adult mortality,
succession, framework accretion, and framework
bioerosion. Changes in these processes generate changes
in reef communities. Ecological succession following
a disturbance such as a cyclone (an extrinsic disturbance)
would appear to be a clear example of a natural, localized,
intrinsic process in coral reef communities. The upward
vertical accretion of a reef, which is based on coral growth,
raises the living community into shallower water, so in
the absence of changes in sea level, the shallowingupward replacement sequence of corals should be considered autogenic succession (Aronson et al., 1998). (In contrast, reef drowning from rapid sea-level rise, which
results in a deepening-upward sequence, is extrinsically
driven.) Even succession, however, is not without its classificatory problems. The trajectories of successional
sequences will vary in accordance with varying regimes
of extrinsic disturbances such as cyclonic activity and
global change (Connell, 1978; Done, 1999).
Extrinsic drivers of change on coral reefs include local
anthropogenic stresses such as pollution, sedimentation,
and nutrient loading (eutrophication); local anthropogenic
disturbances such as ship groundings, coral mining, and
blast fishing; local natural stresses such as upwelling and
the off-platform transport of hot/hypersaline or cold/
hyposaline, inimical waters; local natural disturbances
such as bioerosion, coral mortality caused by damselfish
(Pomacentridae) and other corallivores, tectonic activity,
and lava flows; large-scale natural stresses such as
increased turbidity following heavy rains and variations
in insolation; large-scale anthropogenic stresses such as
global warming and ocean acidification; large-scale natural disturbances such as regional disease epidemics of
corals and herbivorous echinoids, interruptions in connectivity (i.e., larval supply), and regional outbreaks of the
corallivorous seaster Acanthaster planci; and large-scale
anthropogenic disturbances such as introductions of alien
species, increased turbidity from large-scale deforestation,
and pan-tropical overfishing. The limited utility of these
simplistic classifications for science and management is
highlighted by the example of perturbations to larval supply, which can be natural or anthropogenic, and which can
act on local to regional spatial scales (Sammarco, 1985;
Hughes et al., 2002). Overfishing is clearly anthropogenic,
but it can be viewed as a local, regional, or global source of
disturbance, and it is extrinsic or intrinsic depending on
whether or not human fishers are considered part of the
reef ecosystem.
Evolving models of reef dynamics
Despite these difficulties, broad generalizations are possible about intrinsic and extrinsic drivers of reef dynamics.
One widely accepted scenario for the Caribbean region
emphasizes the role of human predation in causing degradation of reefs in the 1980s and 1990s. Historical
overfishing reduced populations of herbivorous fishes,
and then in 1983–1984 a regional disease outbreak decimated the ecologically important echinoid Diadema
antillarum, with the cascading consequence that declining
herbivory released fleshy and frondose macroalgae
610
INTRINSIC AND EXTRINSIC DRIVERS ON CORAL REEFS