200 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII 



some distance. Animals with long, soft hair are the most effective 

 agents. The clothing of man is likewise well adapted to such 

 transport, as anyone knows who spends time in the field during 

 late summer and fall. Some of these devices are simple hooks, ef- 

 fective because of sharpness or strength; others are elaborate 

 structures with several features insuring their transport. The fruits 

 of awn and needle grasses are illustrative, since they have sharp- 

 pointed, retrorsely-barbed fruits, which easily penetrate cloth, 

 fur, or wool, and an awn which twists with changes of moisture 

 and thus pushes the fruit forward to a secure anchorage. These 

 may cause severe damage to grazing animals by penetrating skin, 

 lips, or even internal organs. 



Soil Animals.— The microfauna of the soil, concentrated in the 

 upper strata, consists of great numbers of protozoa, nematodes, 

 and rotifers. In addition, there are various macroscopic worms and 

 insects. 263 In general, the numbers of animals vary in response to 

 the same factors affecting the microflora, and the greatest numbers 

 are always found in soil with high organic content. All contribute 

 to organic decomposition and use a part of the products for food. 

 Several protozoa probably consume bacteria, and some nematodes 

 are parasitic on the roots of plants, causing much trouble in culti- 

 vated soils where they are present. 



Of the macroscopic fauna, earthworms are most active. Their 

 constant burrowing facilitates aeration and drainage and their use 

 of fresh or partially decomposed organic matter as food contrib- 

 utes to decomposition. Since mineral matter is also ingested in 

 feeding, the earthworm moves quantities of soil about, and this 

 tends to mix mineral and organic materials. In cultivated soils this 

 has no great significance, but for natural soils the advantages are 

 obvious. Earthworms are found in the best soils and best sites but 

 rarely in poor soils. It would appear, then, that they serve to make 

 good soils better but that poor soils derive little from them. 



A very high proportion of all insects spend part of their lives in 

 the soil. Their larvae tunnel through the soil and, thereby, con- 

 tribute to organic decomposition and distribution. 



Larger Animals.— The principal effect of larger animals upon 

 plants results from grazing or other feeding habits. Carnivorous 

 animals affect communities onlv indirectly by keeping down the 



