91 



interfere with the welfare of the animals present, and it is probable 

 that they also exist in soils. The older naturalists elaborated the idea 

 that if organisms were not such active agents in the destruction or 

 transformation of plant and animal bodies such remains would soon 

 encumber the earth. Thus organisms themselves are among the most 

 active agents in influencing directly and indirectly the ventilation of 

 animal habitats. 



8. The Tree Trunk as a Habitat 



A living tree trunk is composed of wood, sap (moisture), and 

 bark, all of which are relatively poor conductors of heat. When the 

 trunks are cooled, as in winter, they are slow in warming, not only 

 because of poor conduction but also because of the slow circulation of 

 sap, which is derived from the cool ground-water. As the season 

 progresses, the trunks warm up, this process being retarded in part by 

 the shade and the cool forest conditions ; and in the fall, radiation of 

 the heat accumulated also takes place slowly. The tree trunk therefore 

 changes its temperature slowly, as does the soil. The animals which 

 live within wood thus live in a relatively cool and stable environment. 

 In living trees the humidity is relatively high, as it may also be in 

 fallen, decaying logs. Relatively dry logs, before progress of decay, 

 on the other hand, form a relatively dry and uniform habitat. (Cf. 

 on the temperature of trees : Harrington, '93, pp. 72-75 ; Packard, 

 '90, p. 23; and Jones, Edson, and Morse, '03, pp. 97-100.) 



p. Prairie and Forest Vegetation and Animal Life 



The dependence of animals upon plants for food is one of the most 

 fundamental animal relations. It is a world-wide relation, but its 

 mode of operations varies greatly in different envirotiments. For ex- 

 ample, many years ago, Brooks gave us a graphic picture of the role 

 of marine vegetation in the economy of marine animals. In the sea 

 there are no forests or grasslands, and no corresponding animals as- 

 sociated with these conditions, as on land ; but in the sea great numbers 

 of minute plants float, and upon these feed an immense number of 

 small crustaceans and other small animals. These small creatures 

 occur in such large numbers that at times the sea is a sort of gruel 

 which sedentary and stationary kinds may appropriate by simply al- 

 lowing the sea to flow into their mouths. The food here circulates in 

 their environmental medium, as plant foods do in the soil and air. This 

 condition has made it possible for vast numbers of plant-like animals 

 to grow over the sea floor as plants do over rocks and plains. The 

 living meadows of animals thus furnish pasture for a host of preda- 



