lO] ENVIRONMENTAL FACTORS 305 



the community or any part thereof. These are largely external in 

 origin. For practical purposes such factors manifest themselves in 

 the totality of direct and indirect effects of animals on plants. It 

 is, however, also convenient to include with the animals such plants 

 as the lowly scavengers and agents of chemical change, and, in 

 addition, those which are damagingly parasitic. Also mentionable 

 here are ' insectivorous ' (carnivorous) plants, climbers, epiphytes, 

 and mycorrhizas — w^hich last, like Lichens and other symbioses, are 

 concerned with the existence of more than one species at a point. 



Apart from changes engendered by the main or subsidiary com- 

 ponent plants themselves, organisms of many other sorts may affect 

 a plant community in numerous and diverse ways. There are the 

 soil Bacteria, causing all manner of important chemical changes ; 

 the Protozoa, which devour the Bacteria ; the Earthworms which 

 help disintegrate organic matter and aerate the soil ; the Fungi 

 which carry this breakdown further ; the Snails which eat plants ; 

 and the browsing Mammals which may have the most profound 

 effect — for example in turning forest into treeless pasture. Other 

 important effects may be produced by Man and his domestic animals 

 and fires, by the Beavers which fell trees and turn valleys into lakes, 

 by the caterpillars and Locusts which may devastate whole areas, by 

 the Insects and Birds which pollinate flowers and carry diseases, 

 by the various animals which are so important in dispersing seeds 

 and other disseminules, and by the parasitic Fungi, etc., which in 

 extreme cases may even kill the dominant plant and change the 

 whole aspect locally. 



Parasitic lower plants (or sometimes invertebrate animals or 

 vascular plants) are, as we have already seen, very important to 

 crops whose mode of growth often encourages them or their ' carry- 

 ing ' insects. Less known is their effect on ' natural ' vegetation ; 

 seemingly it tends to be less drastic as equilibria are approached, 

 though we still do not know what organisms may be kept out by 

 potential predators, or perhaps so weakened in competition as to 

 fail to become established. Certainly the death-rate of seedlings 

 tends to be very high. If we had not plentiful records and even 

 recollections of the Sweet Chestnut forests of eastern North America, 

 how could we know that they had been devastated by blight and 

 that their absence nowadays is not due to some inimical climatic 

 or other factor ? And is it not possible that some other plants whose 

 remains tell us that they once flourished in areas where they do not 

 now grow, were ousted by parasites or other biotic impress rather 



