306 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



than by the cHmatic vagaries which are indubitably the usual cause 

 of such major distributional change or extinction ? The fan-shaped 

 and ' feathery ' Elms of New and Old England, respectively, which 

 are such an important feature of the landscape of those two delightful 

 parts of the world, are threatened by the so-called Dutch Elm disease 

 which has already considerably changed the local environment in 

 many areas. The White Pine Blister-rust is apt to kill the dominant 

 species of some of the most widespread and important forests of 

 North America. These are but a very few instances of devastating 

 parasitic attacks on natural or semi-natural vegetation. 



The small herbivores include Snails, Slugs, Locusts, and the larval 

 stages of Insects such as caterpillars (the larvae of Butterflies and 

 Moths). These often cause considerable local damage and even 

 devastation — particularly to individual crops but sometimes to 

 native species or whole tracts of vegetation.^ But their exclusion 

 even for experimental purposes is often difficult, taxing the ingenuity 

 of the investigator. The effects of the smaller vertebrate animals, 

 such as Voles and Mice, tend to be most conspicuous where Man 

 has upset the balance of nature by destroying their enemies, such 

 as Hawks and Foxes. Rabbits may be especially troublesome, often 

 converting heaths or even forest into grassland by their gnawing and 

 prevention of regeneration of such dominant trees as Beech on chalk 

 in England. This is easily demonstrated experimentally by eff"ec- 

 tively wiring oflF areas (the netting must be sunk some inches in 

 the ground) which soon come to exhibit the early stages of forest 

 regeneration, whereas the pastured surrounding tracts remain close- 

 cropped and grassy. Care must be taken also to exclude small 

 Rodents and Birds which can destroy tree seedlings, for example, 

 by nipping off the tender young shoots. It will be interesting in 

 due course to observe the effect on the local vegetation of the virtual 

 (if only temporary ?) extermination of Rabbits by myxomatosis in 

 several European countries. 



Of large browsing Mammals such as Buffaloes, which used to 

 inhabit the great grassy plains, the natural populations are now 

 widely destroyed. They were probably important in the mainten- 

 ance of the grasslands, at least in the damper areas where trees are 

 able to grow, but are nowadays largely replaced by domestic herds, 



^ It has even been suggested that a plague of caterpillars may have been a 

 leading cause of the dying out of the ancient Norse settlements in West Greenland 

 — by devouring the vegetation and hence starving the Sheep on which the Norse- 

 men were largely dependent. 



