ECOLOGICAL 203 



drops when the lemming population is much reduced by starvation 

 or epidemic. 



Of great importance in connection with agriculture is the balance 

 between small rodents and small carnivores. An upsetting of this 

 spells disaster, as the rabbits in Australia illustrate tragically. For 

 in the main the prodigious multiplication of those that were 

 imported to the great Island Continent was and is due to the 

 absence of the natural carnivorous checks. The same is true in 

 regard to the calamitous increase of European sparrows in the 

 United States, into which small numbers were imported on repeated 

 occasions, in the hope of countering the attacks of elm-tree 

 caterpillars. 



Whether natural checks are exterminated or were never present 

 is immaterial; and the introduction of newcomers into a country 

 without adequate natural checks will have, of course, the same 

 results as the elimination of the natural checks in the old 

 country. 



Everyone knows the instructive story of the introduction of the 

 mongoose into Jamaica. There are several kinds of these energetic, 

 fearless carnivores in Africa and India, and they serve a very useful 

 purpose in checking the increase of small rodents and of snakes. 

 To check the imported Oriental rats, which have followed man 

 like a shadow in all his voyages, the mongoose was introduced into 

 Jamaica, where it did good service. It not only counteracted the 

 rats, but it turned its attention effectively to the native "cane-rats", 

 small murine rodents very destructive in the sugar plantations. 

 But having finished with the rodents, the indefatigable carnivores, 

 who had now multiplied considerably, began to attack the poultry 

 and the young of ground-nesting birds. They also attacked certain 

 lizards and snakes, and several species were exterminated. But 

 both the birds and the reptiles had been playing a useful part 

 in checking the multiplication of various injurious insects, which 

 now began to increase, to the great detriment of various crops. 

 Thus the cure began to evolve a new disease, and this particular 

 case is but one out of many. Consequences are not single, but 

 multiple. 



Operating on Nature is like playing chess, one has to try to see 

 the distant consequences of a move. Some years ago, in the North 

 of Scotland, a price was put on the squirrel's head, because of the 

 serious damage it was doing in eating off the tops of young forest 

 trees. But as the squirrels' heads came tumbling in, month after 

 month, for two or three years, all with the forester's approbation, 

 a cloud rose in the sky, just as with the mongoose in Jamaica, for 

 there was an alarming increase in the numbers of wood-pigeons. 

 And these birds are on the black-list as far as agricultural interests 

 are concerned. The connection, far from obvious at first, is that 



