sistence. Its requirements of water and mineral nutrients can be 

 secured in moist to fairly dry soil, ranging from loam to soil of a 

 sandy or rocky texture. Thus little bluestem has a wide range of 

 ecological amplitude in respect to soil fertility and moisture, as 

 well as duration of light and length of growing season, but it does 

 not tolerate much shade. A number of ecotypes are known, and 

 probably many more exist (Figure 1-26). Its numerous leaves and 

 extensive fibrous root system make it efficient in photosynthesis 

 and absorption of water, so it has a well-developed capacity for 

 using the resources of the environment, as well as for competing 

 with other plants except where it is overtopped. However, in very 

 dry habitats such as the High Plains of Colorado, it cannot com- 

 pete with the lower-growing shortgrasses. Although occurring 

 usually as a dominant, its bunch life-form permits other species 

 to grow in association with it. Flowers are produced on stalks 50 

 to 150 cm tall, so they are well exposed to pollination by wind, 

 and the fruits are adapted to wind dissemination although they 

 may also be carried by animals. Large yields of seed, maturing 

 in late summer or early fall, are dependent upon suitable tem- 

 peratures and moisture supply during blooming and later, but 

 such conditions do not obtain every year. The seed is dormant 

 for two to four months after maturity, germination is slow, and 

 the small seedlings grow slowly. Little bluestem tolerates grazing 

 well unless cropped too closely, and it also withstands burning 

 during the nongrowing season. Usually it is not attacked seriously 

 by parasites. The stages encountering greatest hazards for suc- 

 cess are those of seed production and seedling establishment. 

 The capacity to occupy special habitats and the persistence of 

 these habitats are important in the ecological success of many 

 species (Figure 1-27). The requirements and amplitudes of such 

 species have become so well adapted that they cannot compete 

 well elsewhere, but in their own habitats they often have so much 

 competitive capacity that other species cannot usually invade. 

 Species in some communities characterized by Ammophila, Cakile, 

 Salicornia, and Spartina, have been closely associated on exposed, 

 as well as on protected seashores of the North Atlantic since the 

 Miocene Epoch. ^^ Certain parts of northern Alaska and Scan- 

 dinavia were not glaciated during the Pleistocene, so there has 



56 * Species and PopvalatioxKS 



