142 ANIMAL ECOLOGY 



rate, its enemies will also do so, but will lag far behind and 

 be too late to have any predominant effect on the numbers of 

 the prey. 



19. It seems, therefore, that the food relationships of animals 

 result in the numbers being controlled in the majority of cases 

 by carnivorous enemies, but that when disturbances- in the 

 environment cause a sudden acceleration in the rate of increase 

 of some smaller species, its enemies no longer act as an efficient 

 control. Actually the numbers of an animal are ultimately 

 very often controlled by organisms smaller than itself, i.e. by 

 parasites which produce epidemics. The following parallel 

 may perhaps make this argument clearer. In ordinary 

 weather the fire-brigades in a town are sufficiently numerous 

 to keep down any outbreaks which may occur, but if there 

 occurs a spell of very dry weather for several months there 

 will be a sudden increase in the number and spreading 

 powers of fires ; they will flare up more quickly and be more 

 difficult to extinguish. In such an emergency the number of 

 fire-brigades will be inadequate for dealing with the situation, 

 and there will be a long delay before new firemen can be 

 properly trained and equipped with helmets and hoses. Mean- 

 while the fires will go on spreading and setting light to other 

 houses, and in bad cases the whole town may be burnt down 

 and have to be rebuilt again, as in the great fire of London in 

 1666. In just the same way, once a mouse population has 

 *' broken out " and escaped from the control of its enemies, it 

 will give rise to a number of other mice, which in turn will 

 increase and spread Hke the fire. The firemen (represented 

 here by owls and foxes) cannot bring up and train more of their 

 species in time to be able to stop the outbreak. 



zo. In the case of a bad fire the situation can usually be 

 saved by a migration of fire-engines from neighbouring towns, 

 but if it is a dry summer everywhere the fire-brigades will be 

 busy putting out their own fires, and so unable to help any one 

 else. Similarly, in a mouse plague there is often a huge 

 migration of owls to the spot, which increase the chances of 

 reducing the numbers. But here again the importance of 

 climatic factors is seen, since the latter may cause plagues over 



