78 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



the remains of previous vegetation in the form of leaves, fragments of 

 twigs, fruits, seeds, and so forth, which occur in various stages of decom- 

 position, and of demohtion wrought by animals. 



Terrestrial soil, if abounding in animal life, is favourable to vegetation 

 because it is rich in humus bodies ; ^ but if wanting in animal life then 

 the vegetation is usually low and stunted. Animals, especially earth- 

 worms, work on the soil and thus on vegetation in four different ways : 



1. They comminute vegetable remains by means of their jaws or, 

 in the case of earthworms, by means of their alimentary canal with the 

 aid of ingested stones. 



2. In their intestines they mix their food with mineral particles of 

 soil, thus promoting the formation of humus by producing a finely- 

 mixed soil. 



3. They bury vegetable fragments in the soil. 



4. By the tunnels and passages due to their activity they render 

 soil more porous and better aerated the soil becomes ' mellow ' thus 

 promoting respiration in the roots and consequently growth in the plants. 

 The excrement deposited likewise serves to render soil friable and porous. 

 In this way animals also facilitate drainage. 



Earthworms play a special role in ordinary soil. In Denmark two 

 large species, Lumbricus terrester and L. rubellus ; as well as L. pur- 

 pur eus, Allolobophora turgida, and species of Euchytreus, are of signifi- 

 cance. They make burrows which descend vertically into soil to a depth 

 of two or more metres, and which reach down to deeply buried roots. 

 The burrows are filled with substances, fragments of leaves and excrement, 

 nutritious to the plant. Five other species live in arable soil. Some- 

 times they are so numerous that some 400,000 individuals may occur in 

 a hectare of land. 



At night, and in moist, dull weather they emerge from their burrows 

 and deposit excrement in the form of friable castings on the surface of 

 soil. They drag leaves into the ground, so that these decompose ; they 

 comminute vegetable remains, acting on these mechanically, swallowing 

 and intimately mixing them with mineral particles which they have 

 likewise swallowed. In addition, their alkaline digestive liquids neutralize 

 humous acids in soil. Shade, shelter from wind, and moist air contribute 

 towards a wealth of animal life in the soil ; shade and shelter from wind 

 are therefore of indirect importance to vegetation. When a forest-soil 

 is exposed to desiccation and the fallen leaves are carried away by wind, 

 the earthworms vanish, the soil becomes dry and hard, and the vegetation 

 suffers. In acid soil (bog, heath,) and dune, earthworms are wanting. 

 Upon their presence or absence depends the occurrence of a humus soil 

 or a raw humus soil in north temperate forest and heath. Conversely 

 they disappear upon the production of raw humus and humous acids. 

 Even upon the growth of rhizomatous plants in the forest do they exert 

 an action ; - their presence or absence causes a series of variations in 

 the kinds of soil that corresponds to a series of variations in the plants 

 clothing it.^ 



As an additional example, showing how animals may affect vegetation, 



* Seep. 42. ' P. E. Muller, 1894. 



^ The natural history of earthworms has been investigated by C. Darwin (1881), 

 P. E. Miiller (1878), and in the tropics by C. Keller (1887). 



