CAUSES OF SUCCESSION 247 



Second stage (Fig. 248): Bark loosened; wood still solid or barely 

 softened. Under the bark were the flattened Pyrochroidae larvae, the 

 small snail (Zonitoides arbor eus), a few of the four-legged larvae of 

 the passalid (Passalus cornutus), many larvae of fungus-gnats (Myceto- 

 philidae), and a single specimen each of the beetle (Penthe pimelia) and 

 the slug (Philomycus carolinensis) . None of these were abundant. 

 The flattened beetle larvae were most characteristic. 



Third stage (Fig. 249): The wood is thoroughly softened and the 

 bark generally loosened. Here the animals present in the earlier stage 

 are increased in numbers. The passalid larva is more abundant. 

 Slugs are numerous. Snails (Pyramidula alternata) are found in such 

 situations as are large enough for them to enter. Fungus-eating beetles 

 are present (Megalodacne heros Say). A click-beetle larva (Thar ops 

 ruficornis Say) bores into the softened wood. 



Fourth stage (Fig. 250) : The bark fallen off; the log a mere mass of 

 rotten wood. Such a log is only shelter for the regular inhabitants of 

 the forest floor which we have already enumerated on the preceding 

 pages. 



VI. GENERAL DISCUSSION 



A study of the tables shows several points of interest. Take first 

 the ground stratum. Beetles which live under decaying wood are 

 common on the beach where the decaying wood is common, but are 

 absent through the cottonwood, pine, and black-oak stages. They 

 appear again with the fallen leaves and moist logs of the black oak-red 

 oak stage. Vegetation in itself is not directly important. Moist 

 decaying wood is common, both on the beach and in the woods. Wood 

 and moisture are evidently essential to such animals. Turning to the 

 snails, which probably all come out into the open to feed during the night 

 and during moist weather, we note that they do not appear until the 

 under-log beetles put in their second appearance. In general the total 

 number of species and of individuals increases until the oak-hickory 

 stage is reached and falls off again in the beech and maple stage. 



In general we note that as the forest passes from the bare-sand stage 

 to the beech-maple stage, there is a great increase in the space to be 

 inhabited by animals and the diversity of possible habitats, at least up 

 to the oak-hickory stage. 



I. CAUSES OF SUCCESSION 



The causes of succession in forests are chiefly changes in physical 

 condition with increase in denseness of vegetation, such as the increase 



