Soil fertility and chemical defense

Janzen (1974) observed that tropical plant communities growing on nutrient-poor white-sand soils always had fewer herbivores than nearby plant communities based on fertile soils. He hypothesized that plants growing on infertile soils would have a difficult time replacing tissues lost to herbivores, due to their slow growth rates. Therefore, the permanent or "apparent" plants found on such soils should be heavily protected by quantitative defenses. Indirect evidence in favor of this hypothesis is that rivers draining such white-sand environments contain tea-colored waters. These brown- or black-water rivers contain large amounts of phenolic compounds that have leached from the surrounding plant communities. The famous Rio Negro of the Amazon basin drains such a white-sand area.

McKey and his colleagues (McKey et al. 1978) tested this idea in Africa. Plants from a white-sand region in Uganda had twice the phenolic compounds as compared with plants from a nutrient-rich soil in Cameroon.

Finally, Coley and others (Coley 1980, Coley et al. 1985) have shown that plants not limited by a physiological stress of some kind are fast-growing, have higher leaf protein contents, shorter leaf lifetimes, higher herbivory rates, lower amounts of defensive metabolites, and higher turnover rates of defenses as compared with plants that grow slowly. Furthermore, herbivores, especially generalist herbivores, have a definite preference for the fast-growing plant species.

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