158 



AGGREGATION, COMPETITION AND CYCLES 



handicap to each successive layer, as well as to the seedlings of the 

 dominants. The last effect in particular is likewise exerted by the 

 progressive interception of falling rain from canopy to the lowermost 

 layer. In regions such as that about Pikes Peak where summer pre- 

 cipitation is mostly in the form of light showers, interception by the 

 coniferous crowns regularly accounts for a large portion of the rain- 

 fall, with a corresponding reduction in ground cover and the germina- 

 tion of tree seeds. Furthermore, though reaction and competition are 



Fig. 34. — Disoperation between Spanish moss {Tillandsia usneoides) and live-oak 

 {Quercus virginiana), northern Florida. (Photo by J. R. Watson.) 



regarded as the driving forces in succession, it is clear that disopera- 

 tion plays a regular though secondary role. 



Disoperation in Animal Communities. With the exception of 

 competition in the proper or strict sense, all coactions that result in 

 discomfort or disadvantage to individual or group may be regarded 

 as disoperative, if the effect falls short of the destruction of the or- 

 ganism. Here again the effect is naturally one of degree, since the 

 same parasite may weaken, cripple, or kill its host in accordance with 

 the intensity of its action. Hence, internal and attached parasites 

 have been considered in the chapter on coactions, leaving for the 

 present treatment the independent parasites, which exemplify the 

 original meaning of the word. Apart from human society, the most 



