Suggested answers for case study 10


Characteristics of the associations

The associations are best plotted using the 'Graph' option in the Plot menu, with plot style set to Barchart and the 'X labels' box ticked.

The deeper-water assemblages seem to have higher diversity, as shown eg. by the Fisher alpha values. The trend is a little 'messy' however, with high diversity in the Oyster assemblage and somewhat lower in the Myophorella assemblage.

The onshore-offshore gradient

The onshore-offshore gradient is reproduced in the dendrogram.

The onshore-offshore gradient can also be glimpsed along axis 2 in the detrended correspondence analysis, where the order is MYOPH-OYSTE-CORBU-PLEUR-PINNA-PSEUD. The order along axis 1 is CORBU-OYSTE/MYOPH/PSEUD-PLEUR-PINNA, which roughly follows a grain size gradient (clay to sand). These interpretations of the axes are however not entirely obvious, and may be debatable.

The onshore-offshore gradient appears along axis 3 of PCA, but this axis explains only 9 percent of the variation in the data.

Support for the associations

The best method to use is perhaps cluster analysis, because the DCA biplot can get quite cluttered with this many taxa. Cluster analysis with a distance measure based on correlation shows six very distinct clusters, corresponding to the six associations.