The Geography of Phytochemical Races

2.3 Europe 27
Fig. 2.14 Generalized pattern of distribution of cyanogenic glycosides in Europe
badly disturbed ski development) the frequency of cyanogenic plants was highest in
the low-level populations (53.8%), intermediate at middle elevations (25.7%) and lowest
at the high elevations (11.6%). The differences are highly signifi cant (P < 0.0001).
The ratio-cline parallels clines found in the European studies. An interesting point
about the system in western North America is that both white clover and the putative
herbivores (primarily slugs) are comparatively recent introductions, and that the sorting
out of cyanogenesis in these systems has occurred in less than a century.
There has been a good deal of discussion concerning the nature of the selection
factors involved in this system. An early observation by Daday (1954a) involved
the correlation between frequency of cyanogenesis in Europe and temperature.
Although herbivory involving slugs and snails has been discussed as a major factor
leading to the production of cyanogenic glycosides as defensive substances, the picture
is apparently more complex. Jones (1972) discussed early efforts to unravel this
system. Ganders (1990) pointed out that three factors—herbivores, temperature, and
moisture stress—are likely involved, and that it is not possible to explain the clinal
patterns on the basis of any one of these factors alone. He concluded by suggesting
the need for more complex theoretical models. Although the mechanism of how this
system originates is far from completely understood, the cyanogenic polymorphism
remains one of the most thoroughly studied cases in the literature.
Reports continue to appear dealing with the antiherbivore properties of cyanogenic
plants. For example, Saucy et al. (1999) demonstrated, using laboratory as
well as outdoor enclosure experiments, that voles (Arvicola terrestris) showed clear