In many cases, more complex statistical analyses are needed which deal with more than one variable at a time - these are called multivariate analyses. For instance, it may be necessary to divide the collecting sites into separate groups according the overall climate (e.g. mean annual and monthly rainfall, mean minimum/maximum temperature etc.), rather than use a single environmental variable such as rainfall.
A wide range of multivariate analysis techniques exist for analysing groups of ecogeographic or taxonomic variables. No single method will give predictive results in all situations - the actual method selected will depend on the aim of the analysis, the structure of the data, the availability of software and any advice given by a statistics expert. These analyses are only possible with robust data.
There are two main types of multivariate analysis of particular relevance to ecogeographic data - hierarchical clustering methods and ordination methods.
The clustering process may be of two types: (1) agglomerative
- in which objects are joined into progressively larger clusters;
or (2) divisive - in which the cluster of all objects
is progressively divided into smaller clusters (not discussed
here).
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