Page 696 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

660 part IV Soils, Ecosystems, and Biomes
▲Figure 20.18 Bison grazing in Grasslands National Park, Saskatchewan. [Marshall Drummond; (inset) Bobbé Christopherson.]
number of plants have also developed spines, thorns, or bad-tasting tissue to discourage herbivory.
Finally, some plants that grow along desert washes produce seeds that require scarification—abrasion or weathering of the surface—for the seed to open and permit germination. This can occur from the tumbling, churning action of a flash flood flowing down a desert wash; such an event also produces the moisture for seed germination.
Some desert plants are ephemeral, or short-lived, an adaptation that takes advantage of a short wet season or even a single rainfall event in desert environments. The seeds of desert ephemerals lie dormant on the ground until a rainfall event stimulates the seed germination. Seedlings grow rapidly, mature, flower, and produce large numbers of new seeds, which are then dispersed long distances by wind or water. Seeds then go dormant until the next rainfall event.
The vegetation of the lower Sonoran Desert of south- ern Arizona is an example of the warm desert biome (Figure 20.19). This landscape features the unique sa- guaro cactus (Carnegiea gigantea), which grows to many metres in height and up to 200 years in age if undisturbed. First blooms do not appear until it is 50 to 75 years old. In cold deserts, where precipitation is greater and tem- peratures are colder, characteristic vegetation includes grasses, xerophytic shrubs, such as creosote bush, and woody shrubs, such as sagebrush (Artemisia tridentata). Succulents that hold large amounts of water, such as the saguaro cactus, cannot survive in cold deserts that
▲Figure 20.19 Sonoran Desert scene. A saguaro cactus and other characteristic vegetation in the Lower Sonoran Desert west of Tucson, Arizona, at an elevation of about 900 m. [Robert Christopherson.]
experience consecutive days or nights with freezing winter temperatures.
The faunas of both warm and cold deserts are limited by the extreme conditions and include only a few resident large animals. Camels, occurring in the deserts of Africa and the Middle East, are so well adapted to warm desert conditions that they can lose up to 30% of their body weight in water without harm (for humans, a 10%–12% loss is dangerous). Desert bighorn sheep are another large animal, occurring in scattered populations in inaccessible mountains and canyons, such as the inner Grand Canyon but not at the rim. Desert bighorn declined precipitously from
Georeport 20.4 Biodiversity and Food Sources
Wheat, maize (corn), and rice—just three grains—fulfill about 50% of human planetary food demands. About 7000 plant species have been gathered for food throughout human history, but more than 30 000 plant species have edible parts.
Natural, but as yet undiscovered, potential food resources are waiting to be found and developed. Biodiversity, if preserved in each of the biomes, provides us a potential cushion for all future food needs, but only if species are identified, inventoried, and protected.