The Geography of Phytochemical Races

2.1 Africa (Including Madagascar) 5
producers. Only eight species appeared to lack both tannins and alkaloid- positive
compounds. One of these exhibited a high concentration of a bifl avonoid, a polyphenolic
compound based on the coupling of two fl avonoid monomeric units; another was
shown to be positive for iridoids (15-carbon terpenoid derivatives). As Gartlan et al.
(1980) discussed, these observations are in agreement with the idea that taxa from
habitats where plants undergo rapid growth (i.e., stem elongation and leaf replacement)
invest a comparatively smaller amount of resources in production of defensive
chemicals (allelochemics) than species in habitats where growth is slower. Growth
would be expected to be slower in habitats where soil nutrients are in short supply,
as observed in the Douala-Edea site in this study. It has been debated that plants in
nutrient-challenged habitats are more likely to produce tannins or other polyphenolic
compounds, rather than alkaloids or other “qualitative” allelochemics, as defensive
compounds. This wide-ranging study would seem to offer support for the idea.
A few years earlier, Levin (1976) discussed, in some detail, the distribution of
alkaloid-bearing plants in relation to geography. The basic premise was that plants
from tropical regions are under more concentrated attack by predators of one sort or
another, and thus invest more biosynthetic effort in chemical defense systems. His
survey of the literature provided information on alkaloid occurrence in 110 families,
representing 38 orders of dicots, and showed that: (1) nearly twice as many annuals
as perennials were alkaloid bearing; (2) tropical fl oras had almost twice the level of
alkaloid-bearing taxa as temperate fl oras; (3) Magnoliales and Ranales had higher
levels of alkaloid-bearing components than other dicot families; and (4) there is
a latitudinal gradient with higher percentages of alkaloid-bearing taxa nearer the
equator. Three values representing the extremes of the latter point are 40% alkaloidbearing
taxa in Kenya (0°), 24.9% in taxa sampled from Mexico (ca. 25°N), and
10.8% in taxa from New Zealand (ca. 43°S).
In a subsequent paper, Levin and York (1978) assessed the alkaloid situation in
greater detail — again based on an extensive search of the literature — by calculating
“alkaloid toxicity indices.” Analysis at the generic level revealed that herbs,
shrubs, and trees consistently showed higher levels of toxicity in tropical fl oras
(based on 159 genera), lesser levels in subtropical fl oras (based on 109 genera), and
the smallest levels in temperate fl oras (based on 210 genera). The same trend was
evident when the data were assessed at the family level (analysis was based upon
24 tropical families, 14 cosmopolitan families, and 14 temperate families).
2.1.2 The Senecio radicans Complex (Asteraceae)
The Senecio radicans complex, a somewhat amorphous taxonomic assemblage, is
a group of succulent groundsels, most of which are native to Africa. Glennie et al.
(1971) examined 25 “species” of this group for their fl avonoids. Although the overall
fl avonoid profi le of the group was comparatively simple, some interesting patterns of
distribution did emerge. The pigment profi le of the complex consisted of kaempferol
[1] (See Fig. 2.2 for structures 1–5) and quercetin [2] 3-O-mono- and diglycosides,