Reflections on Claims about Coral Cover (Part 1)

The Australian Institute of Marine Science (AIMS) put out a media release earlier today claiming to have scientifically documented changes in coral cover for the northern section of the Great Barrier Reef, particularly in the vicinity of Lizard Island.   Claims include that:

The losses of coral we’ve recorded so far are significant. Despite these losses, coral cover on most reefs is at moderate levels of between 10 and 30%.

In fact, the method used to survey the corals makes it impossible to determine whether the loss is statistically significance or not, and the surveys were only of the reef perimeters, specifically avoiding habitats with higher levels of coral cover including the reef crest.  So, claims that coral cover is between 10 and 30% really needs to be qualified.

Many textbooks have been written about the type of survey needed to know if there is a statistically significant change in some aspect of the distribution and abundance of an organism.  This could be the population of a city, or even the extent of coral at the Great Barrier Reef.   Knowing the distribution and abundance of organisms and how this might change over time is fundamental to understanding the world around us.  It is fundamental to the biological sciences.

Rarely is it possible to count every person or every coral.  Instead, scientists rely on samples that must be representative of the entire population of interest, and samples must be large enough to generate meaningful data that gives unbiased results.

A population will always include some natural variability and knowing how this is distributed in space and time is important before any conclusions are drawn about the significance of a perceived increase or deterioration, for example, in the corals.

Importantly, in order to be able to quantify statistical significance the data needs to be gathered as an actual count.  By which I mean the number of corals needs to be counted per unit area, and given the extent to which coral cover changes naturally with habitat it is important that the survey method distinguishes between the different habitats at a reef.

Of concern, the long-term monitoring being undertaken by the Australian Institute of Marine Science (AIMS) of coral cover is by way of perimeter surveys that do not include, for example, the reef crest that is often the habitat with the highest coral cover.  This is the equivalent of claiming to know how the population of a city has changed but never actually going into the central business district (CBD) or even the inner-city suburbs or industrial area.  The AIMS surveys that have been reported upon in the media today are surveys of only the reef perimeter, which is the equivalent of skirting around the outer suburbs of a city and from this limited information having an opinion about how the population of a city might have changed.

It is also very important to note that the data is not collected as an actual count.   Rather AIMS coral cover data is categorical data, with the survey collected by towing a person behind a boat and noting whether the extent of the coral cover falls into one of the following categories:  0-10%, 10-30%, 30-50%, 50-75%, 75-100%.

These categories have a very large range, up to 25%.   It doesn’t matter how many surveys are undertaken, given the way the data is collected it is impossible to calculate average coral cover.   It follows that it will be impossible to know how average coral cover changes.

Because the AIMS coral cover data is categorical data it is not possible to calculate a standard error or any other such statistic. Calculating an uncertainty value, for example a standard error, depends on knowing the sample mean.   The method used by AIMS precludes this, it is therefore not possible to draw any conclusions about the statistical significance of the recent survey results.