Patterns in time

Although reef fish faunas vary according to their location, the most profound and interesting changes are those that occur over time. Reef fish diversities vary in response to processes that occur at different timescales.

• Intergeneration changes driven by the variation in the recruitment of juvenile fishes. Reef fish populations are subject to an open water dispersive stage before commencing life on the reef. This influences the number and type of fish that constitute the next generation of reef life.

• Decadal-scale changes associated with climatic variation such as that encountered in El Niño years, cyclonic activity and in some cases biological agents such as the crown-of-thorns starfish, Acanthaster (see Chapter 26). Pulses of temperature increase Acanthaster feeding, and destructive cyclones result in declines of living coral with concomitant changes in the numbers of many small reef fishes.

• Century-scale changes associated with longer term shifts in ocean temperature or current systems as exemplified by the little ice age (1200-1800 CE). Such climatic shifts will modify distributional patterns of fish, including migration into warmer sections of their range.

• Geological and oceanographic processes and long term climatic trends that usually operate over thousands or millions of years. These include changes to the geographical location of reefs (plate tectonics) and drastic modification of sea level (glaciation cycles).

The most informative approach to understanding the present day GBR fish fauna is to summarise the history of the reef through geological time. A comprehensive description of the geological history of the GBR is provided in Chapters 2 and 3.

At the start of reef fish history, 55 My, much of eastern Australia lay well south of the tropics. Over the next 50 million years tectonic processes moved the Australian continent northwards with the northern boundary of the reef reaching present tropical latitudes about 25 My. However, the entire extent of the reef was not wholly tropical until 3 My. Three things are important about this historical pattern. First, unlike the continents of the northern hemisphere, the Australian coast was not subject to episodes of extensive glaciation during the mid to late Cenozoic. In contrast to the temperate coasts of north America and Europe, the southern Australian fish fauna underwent periods of extensive diversification resulting in lineages of reef fish especially within the wrasses, morwongs and leather jackets (Monacanthidae) that are unique to the southern hemisphere. Some of these temperate-water groups have been able to penetrate tropical environments.

Second, over this period temperatures of the surrounding oceans have varied substantially. From 60 to 45 My the GBR was subject to water temperatures ranging from 9°C to 19°C, thus inhibiting coral reef formation. The combination of northwards continental movement and increasing oceanic temperatures provided a period of 17 My to the present day when an increasing proportion of the reef enjoyed temperatures that permitted coral reef growth.

A third factor, however, further inhibited tropical reef formation. Glaciation cycles drive major fluctuations in sea level and over the last 1.8 My 32 glaciations have been recorded. Over the last 430 thousand years, there has been an increase in the magnitude of cycles, resulting in four episodes of rapid sea level variation with maximum amplitudes of 120-140 m. Cycles of sea level fluctuation reduce and alter habitats that in turn modify reef fish populations. For the GBR, the most dramatic changes have occurred over the last 130 ka. At that time sea levels were equivalent to those of the present day. Sea levels then declined in a series of steps to 125 m below present day levels 20 ka ago. There fol lowed an abrupt rise to present levels commencing 16-18 ka ago.

Given the configuration of the GBR and association with the continental shelf over much of the previous 130 ka there would have been no reef formation on the north-east Australian coast other than a fringing reef at the continental margin. The characteristic mid- and outer-shelf reefs that harbour most of the fish species would have been nonexistent. The best estimates of the rate of sea level rise place the age of the GBR in its present configuration as less than 7 ka old.

In summary, the present GBR is surprisingly young and has been subject to enormous changes over the last 200 ka. Over this period, characterised by cyclic changes in sea level, the reef has ceased to exist during low stands and then been reconstituted through the rising seas. In the periods of rising seas the reef must have been recolo-nised by reef fishes from the reef systems to the east and north. The present-day configuration with the system of midshelf platform reefs is only 6-8 ka old, although the fish species themselves are far more ancient.

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