DISTRIBUTION OF ANIMAL COMMUNITIES 9 



is very sparse vegetation, which does not always cover the 

 ground at all completely. As we go south the vegetation 

 becomes more dense and higher until we reach a zone with 

 scattered trees. These are separated by wide intervals owing 

 to the fact that the soil is too shallow (being frozen below a 

 certain depth) to allow of sufficient root development except 

 by extensive growth sideways. Then we find true forests, but 

 still not very luxuriant. Finally, there are the immense rain 

 forests of the equatorial belt. The gradation consists essentially 

 first of a gradual filling up of the soil by roots, and then a cover- 

 ing up of the surface by vegetation. 



6. There is a further important way in which plant 

 communities affect animals. When we look at two plant 

 communities growing next to one another, it is usually notice- 

 able that the junction between the two is comparatively sharply 

 marked. Examples of this are the zones of vegetation round 

 the edge of a lake or up the side of a mountain. The reason 

 for this sharp demarcation between plant communities is 

 simple. Plants are usually competing for light, and if one 

 plant in a community manages to outstrip the others in its 

 growth it is able to cut off much of the light from them, and 

 it then becomes dominant. This is the condition found in 

 most temperate plant communities. Examples are the common 

 heather {Calluna), or the beech trees in a wood, or the rushes 

 {Juncus) in a marshy area. The process of competition is not 

 always so simple as this, and there may be all manner of com- 

 plicated factors affecting the relative growth of the competing 

 species, but the final battle is usually for light. In some cases 

 the winning species kills off other competitors, not by shading 

 them, but by producing great quantities of dead leaves which 

 swamp the smaller plants below, or by some other means. 

 The main phenomenon of dominance remains the same. 

 Now at the junctions of two plant communities there is also a 

 battle for light going on, and it resolves itself mainly into a 

 battle between the two respective dominants. Just as within 

 the community, so between two different dominants, no 

 compromise is possible in a battle for light. If one plant 

 wins, it wins completely. Now every plant has a certain set 



