METHOD OF ARTIFICIAL HABITATS 3O9 



Quadrat, transact, and migration circle are equally valuable for ecesis and 

 competition. The quadrat is best adapted to work in a homogeneous area, 

 while the transect is suited to a heterogeneous one characterized by zones, 

 societies, or communities. It is an advantage to replace the denuded tran- 

 sect by a series of denuded quadrats, one for each zone or society, when the 

 transect would be too long for convenience. The denuded migration circle 

 is invaluable for aggregation and ecesis, since it makes possible the study 

 of migration as a distinct function. A series of denuded quadrats, con- 

 sisting of one or more in the different stages of a succession, furnishes 

 important evidence concerning the development of each stage. By far the 

 best method, however, for making a comparative study of the stages of a 

 succession is the quadrat sequence. A quadrat is denuded each year, thus 

 yielding a complete sequence of miniature stages through the whole course 

 of succession. This method is especially valuable when a succession is 

 represented by a single example, and there is no opportunity of reconstruct- 

 ing it by the comparison of various stagCvS. A quadrat sequence is naturally 

 of the greatest value if begun at the time when the first invaders appear. 



360. Modification of the formation by transfer. The study of partial 

 and intermittent invasion into an estal^lished vegetation is made through 

 the transfer of a species or group of species by means of seeding or plant- 

 ing. The process differs in no way from that described for experimental 

 evolution, except in so far that an endeavor is made to establish a family or 

 a community, and not merely a few individuals. Transfer makes possible 

 the critical investigation of ecesis under conditions of intense competition, 

 as well as the study of aggregation and the origin of plant groups under these 

 conditions. Perhaps its greatest value is in the experimental study of al- 

 ternation and zonation, especially the former. It is practically impossible 

 to determine whether alternation, especially when corresponsive, is due to 

 physical or historical causes, i. e., migration and competition, except by 

 means of the reciprocal transfer of the species concerned. 



Field cultures for the careful study of ecesis and competition are made bv 

 transferring seeds or plants to new or denuded soils. This is practicallv 

 a combination of the methods of modification and transfer. It has a unique 

 value in making it possible to initiate artificial successions of almost any 

 character that is desired, and to carry them out with the reactions more or 

 less under control. This opens up an extremely important field of ex- 

 perimental inquiry, which promises to put the study of succession upon a 

 much more exact basis. Competition cultures in the field are not essentially 

 different from those under control, and they will be considered under the 

 next method. 



