necting the subalpine scrub with the laurel forest below, so that 

 populations oi Adenocarpus spp. come into contact with one an- 

 other and hybridize successfully. The populations are highly 

 variable, especially in the lower pine forest, and hybrids are able 

 to survive on burned-over areas. These hybrids are often asso- 

 ciated with Cistus monspeliensis, a good fire indicator.'^'* The 

 crossing of two formerly geographically separated species (allo- 

 patric) may result in a highly competitive hybrid where a suitable 

 habitat occurs; for example, the fertile Spartina townsendii (2n = 

 126), in Southampton Water, England, is a hybrid of 5. maritima 

 (2n = 56) and the North American S. alterniflora {2n = 70).^°^ 

 In this way hybridization and introgression may cause the sub- 

 mergence of small isolated populations. 



Differences of Species in Resistance to Grazing, 

 Mowing, or Other Treatment 



This fourth group of disjunctive interrelationships between 

 plants and other organisms includes the feeding on many plants 

 by many kinds of animals, from nematodes and slugs to cattle, 

 trampling by larger animals, and clipping or mowing by various 

 animals and man. Because of the long period of adaptation of 

 grassland species in the Great Plains to both adverse physical con- 

 ditions and grazing, the resulting grassland types are some of the 

 most durable of any found today. Stands comprising such species 

 as Agropyron smithii, Stipa comata, Bouteloua gracilis, and Carexjilifolia 

 are capable of enduring prolonged drought, severe infestations 

 of grasshoppers, and much overgrazing. In the arctic tundra 

 perennial herbs possess great capacity to recover from grazing 

 throughout the year by lemming, even though the growing season 

 of the plants lasts only seven to ten weeks. ^^^ 



Grasses, especially those with rhizomes or runners, withstand 

 grazing, trampling, or mowing better than forbs. The latter, hav- 

 ing growing points above the surface of the ground, are usually 

 more susceptible to damage from these causes as well as from 

 desiccation and freezing than the former, with less exposed grow- 

 ing points. Excessive plant growth in some species such as peren- 

 nial ryegrass, unless it is moderately grazed or mowed, may result 

 in elevated crowns, inhibition of tillering, and greater suscept- 



Species ai%<l Popvmlatioims 



