Tipping Points

The ‘Tipping Point’ and ‘Death by a Thousand Cuts’ concepts

Tipping Point: The GBR has been pushed to the edge of an ecological cliff and a very small push could tip it over the edge. It implies that the GBR is a very unstable system which can easily flip to a different state.

Death by a Thousand Cuts (DBATC): The GBR is affected by multiply stressors, each by itself not being very important or even measurable, but taken together the effect is deadly.

These terms are often used to make the erroneous argument that the reef is on the brink of a disaster.

The GBR faces perceived threats from agricultural runoff (sediment, nutrients and pesticides), rising temperatures, falling pH and fishing, amongst others.  The fact that there are a large number of perceived threats each of which by itself may be tolerable but which together are very dangerous, poses many challenges for science.  Firstly, it can be very difficult to show that a particular threat by itself is having any measurable effect. Indeed it is notable that in the literature on the GBR which claims significant decline, rarely are statements made positively attributing a quantitative loss of coral, seagrass or other aspects of the GBR ecosystem to a particular cause.  Thus, it is not possible to attribute a proportion of coral loss to sediments, nutrients, or dredging.  This is not a criticism of the science but is a clear recognition that the GBR system is a highly complex one, with many linked processes, operating at different spatial and temporal scales, and that there multiple potential stressors.  In contrast, it is relatively easy using aerial photographs to make a quantitative attribution to the loss of North Queensland rainforest to clearing for sugar cane, clearing for pasture, clearing for urban development and roads.

Unfortunately, despite its appeal and apparently reasonable nature, the DBATC concept is also virtually impossible to counter and any argument that cannot be falsified (or be disprovable) must be viewed with caution.  It also virtually impossible to prove a negative, i.e. to prove that a specific factor isn’t the cause.  Essentially the question must be asked “how small must the proven damage from a particular stressor be before it is not considered to be a ‘cut’ which could send the GBR over the edge of a Tipping Point?”  Without quantitative knowledge of the contribution of the effect of each stressor, the ultimate conclusion of the DBATC argument is that only zero effect from a particular stressor can be tolerated.   This implicitly prevents discussion of what level of ‘stress’ caused by any effect is tolerable.

The Tipping Point argument, applied to the GBR, has other problems, less philosophical in nature.  There is a presumption that the GBR ecosystem is a very unstable system which is highly sensitive to small perturbations; however the scientific evidence for this proposition is weak.

• There are many reef systems around the world which are far more ‘stressed’ than the GBR. For example the reefs of the Caribbean lie adjacent to a population of around 50 million people, and support a large fishery.  In contrast the GBR has a population of 0.5 million living adjacent to it, there is no herbivorous fishery, total fishing pressure is minimal, and most coral reefs are distant from the coast and rarely visited.  Although the Caribbean reefs are in some cases highly degraded, they still mostly support live coral assemblages.  We do not want the GBR reef system to approach the state of the Caribbean reefs, but the evidence from the Caribbean does not strongly support that the proposition that the GBR is near a tipping point.
• Further, comparing the inner-shelf turbid-zone coral reefs of the GBR with the reefs elsewhere on the shelf also indicates that the GBR system is not approaching a tipping point. The inner-shelf reefs, which comprise perhaps 1% of the coral of the GBR experience far higher naturally high turbidity than the main reef matrix –often hundreds of times higher. In addition they have double the nutrient concentrations and much larger temperature fluctuations. These inshore reefs, by some definitions, could be considered to be highly stressed but are functional ecosytems often with very high coral cover. There is no tipping point between the corals that live in the sparkling clean water of the main reef matrix (99% of the corals), and the corals that live in the brown water, naturally turbid environments, on the inshore reefs despite the massive change in conditions between the two environments.
• The re-initiation of the GBR after the last ice age 20,000 years ago, further counters arguments about high sensitivity to small perturbations. During the last ice age, sea level was around 120 m lower than today, the previous reefs were small flat-topped hills on a vast flat coastal plain, that would later become the GBR ‘lagoon’.  Sea level rose sufficiently fast that coastlines were eroding by up to 50 metres per year, releasing large quantities of sediment into the marine system over thousands of years. Temperatures were also about 1 degree higher than present during the mid-Holocene, around 5,000 years ago, but the GBR thrived under these conditions which by today’s standards would be considered to be stressing the corals.
• It is sometimes claimed that the inshore reefs which are likely to be more affected by river runoff than the offshore reefs are a crucial part of the GBR ecosystem, implying that without them the GBR could fail (go over the tipping point). It should also be noted that around 10000 years ago, as the sea levels rose, the present inshore reefs did not exist while the main reef matrix was thriving. This indicates that the inner-shelf reefs, although important in their own right, are not crucial to the survival of the main GBR reef matrix (the 99% of corals) on the outer shelf reef tract. The main GBR reef matrix would thus survive even if all the inshore reefs, which are presently in excellent condition, were somehow annihilated – i.e we are nowhere near a tipping point.

In conclusion, used together, the tipping point and DBATC arguments can be used to claim significant damage due to any stressor, and it is an argument virtually impossible to counter because even the smallest extra stress can, by the definition of a tipping point, send the system over the edge.  Nonetheless, based on the existing scientific evidence, there is considerable doubt that the GBR is anywhere near a tipping point.

 

For more detail see Chapter 10 of Sedimentary Geoscience of GBR-Larcombe-Ridd-Final(151202)