Niche Width and Niche Overlap: A Method for Measuring Them

Suppose r different animal species are found in some or all of c different habitats. Observations on the numbers of occurrences of the animals in the habitats can be tabulated in an r ° c table. The overall "diversity" of the table, H T say, is defined as Brillouin's information measure with the r ° c table's cell frequencies of the different "symbols." It is shown that H T may be split into two components in two different ways. The first way splits H T into H A , the diversity of the animal species, plus the weight mean of the within—species habitat diversities. The second way splits H T into H B , the diversity of the habitats, plus the weighted mean of the within—habitat species diversities. The weighted mean of within—species habitat diversities constitutes a measure of the average niche width of the r species of animals. The weighted mean of within—habitat species diversities constitutes a measure of the average niche overlap among the c habitats examined. The measures are of actual, realized niche widths and niche overlaps and are therefore inevitably affected by the relative abundances of the species and the habitats. A method of standardizing them is proposed. An example is given using data on the occurrences of several species of aphids on several host plants of the genus Solidago (goldenrods).