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CADDIS Volume 4

Estimating Taxon-Environment Relationships: Central Tendencies

Central Tendencies

Taxon-environment relationships can be expressed in terms of central tendencies that describe the average environmental conditions under which a taxon is likely to occur. Using this method, the single-value descriptor of the taxon-environment relationship, uwa, for taxon j is estimated by computing the mean of the environmental variable of interest at the sites in which the taxon is observed:

Equation 1:  For a given taxon, the weighted average central tendency estimate of its taxon-environment relationship is calculated as the mean of the environmental variable of interest at the sites at which the taxon is observed.  EQUATION 1where N is the total number of sites and xi  is the value of the environmental variable of interest at site i. For presence/absence data, Yij  is equal to 1 when taxon j is present at site i and 0 when taxon j is absent. For abundance data, Yij  is the abundance of taxon j at site i. Statistical scripts for computing central tendencies are available under the R Scripts tab of this section.
Once central tendencies are computed for each taxon, one can use them to infer conditions at a new site based only on biological observations, by computing a weighted average inference.

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