316 PLANT SOCIOLOGY 



the individual species react individually to these changes within the 

 association. If this were not the case, one association would be 

 immediately and completely replaced by another. Experimental 

 studies of succession in the Swiss National Park and elsewhere show, 

 however, that the replacement takes place gradually. A sudden and 

 pronounced overturn of species occurs only when through external 

 influences or by means of certain species of great dynamogenetic vigor 

 the life conditions, and thereby the competitive relations, of the habitat 

 are fundamentally changed. If this happens, all species with slight 

 ecological plasticity succumb to the change in the habitat; at the 

 same time, many others remain as before. Thus a similar degree of 

 aggressiveness of the species has been assumed, when in reality we have 

 a profound ecological revolution in the conditions of the habitat. This 

 revolution is often due directly or indirectly to human agencies, but it 

 may be due to natural causes, such as changes in the level of ground 

 water, splashing or overflowing with salt water, influx of calcareous 



water, etc. 



Species which greatly influence the habitat, or in other words species 

 of high constructiveness, are mostly determinative also for the existence 

 of numerous satellites. Their appearance or disappearance has as a 

 necessary consequence the exchange of many companion species. 



Expression of Constructiveness.— FayiWsird (1919, 1920) first ener- 

 getically urged the examination of the causative value of species. He 

 distinguished in plant communities constructive, conserving, indiffer- 

 ent, and destructive species. These most important dynamogenetic 

 phenomena may be expressed by the following signs: 



1 = constructive t 



[I = conserving / ■ = conserving and ■ = constructive, 



H = consolidating) consolidating conserving, and 



n = neutral consolidating 



I = destructive 



Great constructive or destructive activity is expressed by under- 

 scoring the appropriate sign. 



Constructiveness is a character which expresses itself sociologically, 

 as distinguished from aggressiveness, which, being more autecological, 

 helps the plant in carrying on and winning in the struggle for existence. 

 Very persistent species often are not of genetic value in the beginnings 

 of the association to which they belong. Examples are seen in many 

 annual dry grassland communities as Carex tomentosa in the Molinie- 

 tum caricetosum tomentosae, and Pinus halepensis in the Erica 

 multiflora facies of the Rosmarinus-Lithsperniumfruticosum association. 

 The tenacious Genista scorpius and Juniperus oxycedrus hold their 



