The natural context of changes in the Great Barrier Reef - The Great Barrier Reef: An Environmental History - page 34

patchy and dynamic ecosystem in which specific impacts may be very difficult to
discern and attribute with confidence.
To aid the understanding of historical changes in the Great Barrier Reef,
therefore, this chapter begins with an outline of the large-scale, geomorphological
changes that have occurred in the region as a result of the Holocene evolution of
the continental shelf, which has involved profound changes in sea level and in
sedimentation patterns. This outline represents a morphogenic approach to the
evolution of the Great Barrier Reef and provides a context for the narratives of
anthropogenic, historical changes that follow. Such an approach indicates that
anthropogenic changes are influenced by geomorphological factors that have made
some reefs highly vulnerable to degradation and decline. Another natural source
of change in the Great Barrier Reef - the tropical cyclones that occur frequently in
the region - is considered briefly next. Within that natural context, the accounts
of human activities (such as the early European fisheries, coral mining, coral
collecting and shell collecting) that follow suggest that some already-vulnerable
reefs were intensively exploited, over a long period of time. In addition to those
sustained activities, other, more sporadic human activities (such as military target
practice and channel blasting) have also been superimposed, contributing to the
creation of complex patterns of vulnerability, degradation and decline of some
parts of the Great Barrier Reef. Consequently, some parts of the ecosystem were
probably far from pristine at the time of the formation of the GBRMP, in 1975.
The Holocene evolution of the continental shelf
The Great Barrier Reef is a dynamic ecosystem characterised by almost
continuous change at a wide range of geographical and temporal scales; those
changes are largely controlled by geomorphological and climatic factors (Hopley,
1982, 1994; Hopley et al., 2007). Coral growth in the region has varied in rate
and extent during the Holocene epoch, accelerating as hydro-isostatic processes
adjusted sea level and as sea surface temperature (SST) increased, but also
inhibited by mechanical erosion and ecological processes, such as bioerosion,
which denude coral. Therefore, coral reefs represent the outcome of a balance
between constructive and destructive processes, and they are typically patchwork
assemblages of living, dying and dead corals (and coralline algae). However, such
spatial and temporal patchiness does not necessarily indicate the overall decline
of coral reefs. In addition to the natural variability that occurs on coral reefs due
to the balance between coral (and algal) growth and bioerosion, coral reefs are
also subjected to periodic changes as a result of variations in geomorphological,
climatic, meteorological and biological factors. One of the most significant of
those factors is the Holocene evolution of the continental shelf in the region,
including variations in sea level and sedimentation patterns, which has profound
implications for the vulnerability of the Great Barrier Reef to other impacts and
which represents an important part of the large-scale context in which other,
historical changes in the ecosystem have occurred.