Feldmann index, as proposed by its author, has been employed in a great deal of phytogeographic studies, most notably by algologists investigating the Atlantic and the Mediterranean Sea. However, since it does not take into account the number of green algae, particularly abundant in tropical floras, Cheney has suggested to replace the R value by the R + C value (namely red algae plus green algae). Testing (R + C)/P ratio on more than 25 floras from the western coasts of North Atlantic, he obtained values increasing from north to south and ranging from 1.4 for the flora of Labrador to 6.1 for that of Jamaica. In Cheney's view, even though the variation trend of its index is similar to that of R/P index, it is more sensitive and accurate. In fact, if we calculate the Cheney's index on floras previously considered by Feldmann, values obtained fall within a wider range than values of R/P ratio (from 1.7 to 7 vs. 1.08 to 4.59) (Figure 2).
Cheney's index was used by Kapraun in studying the floristic affinities of North Carolina marine algae. Values obtained demonstrated that inshore waters of North Carolina belong to a warm temperate region.
Despite close correlation with specific geographical areas, Feldmann index, or the one modified by Cheney, cannot be effectively used in describing floras of geographical areas where conspicuous upwelling occurs, as along the coasts of South Africa or Chile, or again along coasts where brackish waters prevail, such as those of Uruguay. As a matter of fact, in all these regions, floristic richness appears to decrease considerably, as opposed to neighboring regions, by selection of few species tolerant to the inherent environmental variability in these regions. In addition, since such a decrease affects mainly brown algae, the Feldmann index is misleading, because it yields a very high value, equivalent to that in tropical or equatorial floras in the boreal Atlantic.
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