A number of species have pronounced capacity to form new 

 stands following fire or other disturbance (Figures 1-17 and 1-18); 

 for example, seeds may be protected within cones as in the 

 lodgepole pine (Pinus contorta ssp. murrayana), or they may have 

 great mobility as in the fireweed {Epilobium angustifolium). The 

 crowns or rhizomes may survive fire and produce new shoots, e.g., 

 Populus tremuloides, Salix pulchra, many grasses, and some plants of 

 Purshia tridentata. The mobility and high viability of many seeds 

 enable some species to form pioneer communities such as Populus 

 sargentii, Salix nigra, and S. interior on sandbars, Russian thistle 

 {Salsola kali) and other weeds in abandoned fields, Betula populi- 

 folia in devastated forests, and Rubus occidentalis in burned-over 

 forests (Figures 1-17 and 1-18). 



Birds are important in dispersing seeds. Certain cereals and 

 weeds grow on the feeding grounds of gulls, ^"^ and in Norway the 

 tits {Farus spp., Sitta europaea) contribute to the dispersal of 

 Galeopsis tetrahit, G. bifida, and Juniperus communis by gathering 

 and storing seeds. ^^ The rarity of Mercurialis perennis in Ireland, 

 in spite of having been widely introduced, is evidently the result 

 of the absence or extreme infrequency of any agent of dispersal. 

 This species does not lack genetic variability nor is there lack of 

 suitable habitats. In Britain, where certain kinds of mice and 

 ants occur which may be the agents of transport, the plant is ag- 

 gressive and common.'" 



Under severe conditions where production of seed is hazardous, 

 vivipary and other forms of apomixis may be advantageous. 

 Vivipary, or the formation of vegetative buds in place of florets, 

 is frequent in the grass family, for example, in Poa bulbosa and 

 Festuca vivipara. Apomixis and self-pollination may result in the 

 formation and perpetuation of single biotypes, so that the popu- 

 lation has a high degree of genetic homogeneity, well adapted to 

 a particular habitat, but even in such a population differences 

 in microhabitat conditions and competition will induce pheno- 

 typic variations among the individuals. 



If the biotype possesses a high degree of adaptability it can live 

 in a variety of microhabitats and survive unfavorable conditions 

 better than if it possesses less. The greater the number of biotypes 



Ecological CHaracteristics of Species & Popmslatioxfts • 41 



