species and with Dactylis glomerata, as well as between Dactylis and 

 both Lolium and Tnfolium. The positive correlation may have 

 been caused by a higher nitrogen requirement of Lolium which 

 was supplied by Tnfolium. This correlation tended to disappear 

 as the community became increasingly integrated, ^^b 



Two or more species may be independently associated, i.e., the 

 various species are able to grow in the environment created by 

 the whole community rather than because of direct interrelations 

 between species; for example, in Norway Linum catharticum and 

 Briza media grow in calcareous soils, one seldom occurring with- 

 out the other^^ (Figure 1-16). Positive correlation in the distribu- 

 tion of species in an area may indicate that certain habitats are 

 more suitable for some species than for others, or that one kind 

 makes conditions favorable for the occurrence of others. Many 

 terms used in plant sociology, such as "association" itself, reflect 

 the importance of the subject of interspecific association. Certain 

 groupings in Australia such as Dodonaea and Vittadinia communi- 

 ties on ridges and Bassia and Cassia in the valleys, are related to 

 topographic conditions. Other pairs of species showing strong 

 association are Bassia uniflora-^gophyllum apiculatum, and Cassia 

 eremophila- Westringia rigida.^^ 



Association may favor the establishment of groupings under 

 difficult conditions as, for example, near the central California 

 coast, where trees, by the condensation of atmospheric moisture, 

 provide suitable conditions for the growth of certain orchids and 

 seedlings of Douglas fir, Monterey cypress, and Eucalyptus.^^^ 

 In arctic and alpine regions some species become established in 

 the cushions oi Silene acaulis and Arenaria obtusiloba. However, the 

 advantages of association are often not well understood, as in the 

 frequent association oi Diapensia lapponica and Vaccimum uliginosum 

 in Greenland, ^*^ or Cassiope tetragona and V. uliginosum in north- 

 western Alaska. ^'^^ 



The causes and processes involved in interspecific association 

 are exceedingly complex. Results secured in the study of animal 

 populations may be helpful; for example, instances of both 

 facilitation and interference have been found among 22 strains 

 of Drosophila melanogaster when they were living together as com- 

 pared to strains living separately, so it appears that the viability 



36 • Species and PopmElations 



