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l" .., -. ')'J I j.j ..J' Primarily because of its mid-continent location, the Basin experiences weather that is known tor its fluctuations and extremes. Averages are misleading for seldom does aver- age weather actually occur. Instead, weather tends to fluctuate widely around the annual averages, with the occurrence and the degree of fluctuations being unpredictable. Winters are re- latively long and cold; summers are fair and hot; spring is cool, moist, and windy; autumn ;s cool, dry, and windy. Normal annual precipitation varies from west to east in the Basin, averaging over 35 inches in the Rocky Mountains of the western boundary, about 14 inches on the Great Plains, about 26 inches on the Central Lowlands, and over 36 inches in the Ozark Highlands. Figure 4 shows normal annual precipi- tation for the period 1931- 1960. About 70 percent of the pre- cipitation occurs as rainfall during the growing season. One of the climatic factors of great importance because of the agricultural activity in the Basin is the length of the frost- free period. The definition of the frost-free period is the average number of days each year between the last freezing tempera- ture in the spring and the first frost in the autumn. While the frost-free period does not completely define the growing sea- son for all crops and grasses, it is a general indicator of the most favorable period. The length of the frost-free period in the Basin is about 30 days in the higher elevations of the Rocky Mountains, about 140 days on the Great Plains, and about 180 days in the Ozark Highlands. The subbasin descriptions include a discussion of their average winter and summer temperature ranges.. Environmental Resources TERRESTRIAL BIOLOGICAL RESOURCES Ecosystems <if the Missouri River Basin originally were dominated by various grassland types in the pra[ries and plains region, and by forests, shrublands, mountain grasslands, and alpine tundra in the mountainous regions, Gallery forests, with most biotic diversity in the Middle Missouri and Lower Missouri subbasins, developed along the major rivers and streams. In recent times, much of the various grasslands types of the plainS and prairie region have been converted to cropland wherever favorable soils, topography and climate exist, al- though natural grasslands of sizable acreages do persist in several States in areas of less favorable topography, substrate, or climate. The shrubland, woodland, and forest iand of the mountains have not been subjected to intensive disturbances and retain more of the natural characteristics. River bottomland forests in the prairies and plains have been eliminated or have deteriorated in many parts of the Basin, but many still afford key habitat for native plants and animals. The agricultural lands now support a mixed array of some of the native biota, having broad ecological tolerances, and a host of tolerant or cultural exotics Including herbs, shrubs, trees, insects, birds, and mammals. Occasionally, populations of some native species explode, and the species becomes a new pest. This is related to their ability to extract energy and raw material from new cul- tural environments at an accelerated rate due to lack of checks and balances. However, other native populations typically have more narrow ecological requirements. and often decline or be- come extinct regionally. The native terrestrial biota in the Missouri River Basin have been exposed to a succession of changing environments in the geological past, and only a few known endemic species have survived the extremes of the Pleistocene epoch. During this time successive gla~jers scoured much of the mountains and caused a direct or indirect deposition or reassortment of massive amounts of sediment. Affected biota were forced to migrate or become extinct. Biotic relicts of that period still per- sist in some areas of the Basin, and populations contain poten- tially important genetic information. In historical periods this array of biota has been exposed to stresses differing in type or extent including new chemicals, artificial lights and noise. megadisturbance of substrate or soils, animals, and direct ex- posure to large human populations. As a result, some species have become extinct, still others have lost most of their habitat, while Qthers have been declining over broad areas. Only a comparatively few species have benefited from special conser- vation measures. Fortunately, ecosystems in the Basin still are of sufficient size and integrity to support large populations of the native flora and fauna, and, in addition, allow for sizable sustainable harvests. The economic and social worth of this resOurce in the past, however, has not been fully understood or appreciated within an economic system where short-term exploitation of nonrenewable resources has often received priority over conservation and management of less marketable renewable resources. American buffalo on South Daleota range Terrestrial big game species, such as moose, elk. deer, Rocky Mountain goats, big horn sheep, and grizzly and black bears, continue to be in much demand in the Basin by both residents and nonresidents, and are in short supply in most States with the exception of Wyoming, where surplus popula- tions of pronghorn antelope occur. In Montana, a State with the highest overall population of big game resources in the con- tiguous 48 States, the sale of nonresident licenses leads the 17 I I