The previous examples have shown how global factors can influence biodiversity, but it is important to note that regional factors on a smaller spatial scale than the entire globe can also strongly influence biodiversity. In this section, we will consider regional impacts of topography, the physical structure of the landscape, including mountains, valleys, etc. Consider, for example, the complex topography that results from California’s numerous mountain ranges (Fig 2.6.1). Air masses moving across the California landscape must rise to go over these mountain ranges. As they rise, the air masses cool, expand, and drop much of their moisture as rainfall on the ‘front’ or windward side of the mountain range. When the air masses fall on the ‘back’ or leeward side of the mountain range, they are dry and so rainfall in this area is minimal. This process is called the rainshadow effect because the leeward side of the mountain ranges are ‘shadowed’ from receiving rainfall by the windward side of the range. Which side of a mountain range is the windward or leeward side depends on which direction the air masses are moving.
In Northern California, air masses generally come from the Pacific Ocean and move east over the landscape (Fig 2.6.2). These air masses first rise over the coast ranges and deposit some rainfall on the western sides of these mountains. The central valley is in the rainshadow of the coast ranges and so receives less rainfall. The air masses then rise over the Sierra Nevada mountains, which are much taller than the coast ranges, and so the air masses deposit their remaining moisture on the western side of these mountains. Consequently, the great basin of Nevada is in the rainshadow of the Sierra Nevadas and is a desert, receiving very minimal rainfall. This pattern can be clearly seen in maps of the average annual precipitation of northern California (Fig 2.6.2)
The rainshadow effect influences biodiversity due to the influence it exerts on climate on the windward and leeward sides of mountains. The pattern of precipitation and elevation across a topographically complex landscape like northern California leads to differences in the plant and animal communities that inhabit these areas. For example, the western windward side of the coast ranges are famous for their redwood forests, while the eastern leeward sides are primarily oak forest. Similarly, the western windward side of the Sierra Nevadas are primarily mixed pine and fir forest, while the eastern leeward side contains a mix of juniper and pine woodlands, sagebrush scrub, and alkaline scrub systems as the landscape transitions to the desert flats of western Nevada. In both cases, the plant communities on the leeward side are more drought adapted than the communities on the windward side. The central valley between the coast ranges and Sierra Nevadas was historically grassland, though much of it has now been converted to agricultural and urban landscapes.