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Alkaloid-Bearing Plants: An Ecogeographic Perspective

The incidence of alkaloid-bearing plants is dependent upon their habit and ecogeographical distribution. Among annual species the incidence of alkaloids is nearly twice that of perennials, among tropical floras nearly twice that of temperate floras, and a latitudinal cline is evident. In New Guinea, disparate communities differ in the incidence of alkaloid-bearing species and in the amount of alkaloid contained in their vegetative tissues. Families primarily distributed in the tropics have a higher percentage of alkaloid-bearing species than do those of temperate regions or those with cosmopolitan distributions. The most primitive orders, Magnoliales and Ranales, have a higher percentage of alkaloid-bearing species than the remainder of the dicots. These findings are discussed from the vantage point of coevolutionary theory. It is suggested that the ecogeographic patterns may be the result of differences in pest pressure, the alkaloids playing a defensive role in plants.