To understand how geology affects biology one needs first to consider the forces that influence the edaphic environment. Jenny (1941, 1980) described soil formation as a function of climate, organisms, topography, parent material, and time. Pedogenesis (the evolution of soil) begins with the geologic processes responsible for the formation of the three basic types of parent material: sedimentary, metamorphic, and igneous. Variations in topography and climate affect rates of weathering as do the types of organisms associated with the soil. Finally, the amount of time these forces have had to interact ultimately affects soil formation. Major (1951) adapted Jenny’s factors of soil formation to describe the evolution and distribution of vegetation types. He reasoned that the same five factors-climate, topography, organisms, parent material, and time-are primarily responsible for the distribution of species. Kruckeberg (1986) attributed botanical (or biological) diversity to these five factors as well.

The presence of many rare taxa in one place is often associated with soil properties unusual for the surrounding region. Such substrates include serpentinite, granitic, carbonate, volcanic, and alkaline soils. Soils derived from serpentinite often support highly restricted and diverse plant taxa (Mason 1946b; Kruckeberg 1969). Serpentine endemism has been linked to deficiencies in calcium and abnormally high concentrations of magnesium, chromium, and nickel (Kruckeberg 1969; Fiedler 1985). Properties like these have been suggested as being sufficiently unique to account for this high degree of endemism (Kruckeberg 1951; Walker 1954; Whittaker 1954; Tadros 1957), although opinion as to their specific effects varies.

Kruckeberg (1984) described specific taxonomic and evolutionary responses by populations confronted with such a “geo-edaphic challenge.” These include: 1) taxa that are excluded from serpentine, 2) indifferent taxa that occur on or off serpentine, 3) taxa largely confined to serpentine in parts of their range, and 4) edaphic endemism, or taxa completely restricted to serpentinite. Excluded taxa are those that lack the ability to become established on a particular soil due to mineral deficiencies or toxicities and/or peculiar physical properties. Edaphic endemism, on the other hand, is the result of specific physiologic requirements of a taxon being met by a particular geologic formation (Kruckeberg 1984). Endemism has also been attributed to a lack of competitive ability on soils supporting the regional vegetation (Billings 1950; Gankin and Major 1964; Wood 1989). One example of a mechanism whereby low competitive ability could lead to endemism was offered by Tadros (1957) and Wicklow (1964) who suggested that increased microbial activity inhibits the spread of Emmenanthe rosea, a serpentine endemic, onto more fertile, non-serpentine soils.As you wander the hills of the Bay Area, look at the patterns in the vegetation. Slope aspect, exposure to or shelter from prevailing winds, fog drip, and substrate are the dominant climatic and geologic factors affecting the distribution of most of our local plant communities. Can you think of more? Happy ecologizing!

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