486 Dynamics of the Ecosystem 



efficiencies of feeding and digestion and because of the great amount 

 of energy used for the various Hfe processes, each animal destroys a 

 much larger mass of food than its growth increment. 



These facts clearly indicate that animals could quickly eat them- 

 selves out of house and home. Temporarily unchecked populations 

 of insects or rodents sometimes increase to such numbers that every 

 green thing within the area of their abundance is destroyed. Plagues 

 of locusts in the Middle East have been known to consume all vegeta- 

 tion over areas as great as 600 sq km. Even deer, which reproduce 

 relatively slowly, when protected from other checks, soon multiply to 

 a point where the population needs more food than can grow within 

 their home range. 



When one realizes the fact that animals are able to destroy their 

 food supply faster than it can grow, the question arises why the 

 herbivores of the world have not destroyed all the vegetation, and 

 why carnivores have not killed all of their prey and thus brought 

 about their own destruction. One reason is that climate and other 

 conditions vary locally, so that the animals do not find conditions 

 suitable for maximal increase everywhere at once within their geo- 

 graphical ranges. If a local population does completely destroy itself 

 by starvation, the area can be repopulated from neighboring regions. 

 Another reason that extinction is usually avoided is that other factors 

 frequently stop the increase in the population before the entire food 

 supply is exhausted. For example, excessive grazing tends to change 

 the species composition of the vegetation, and this condition, recog- 

 nized as range deterioration, curtails the growth of the herbivores. 



Increase of some animals is checked by accumulation of metab- 

 olites, by disease, or by intensified predation, as discussed in previous 

 chapters, long before a food shortage is threatened. In special in- 

 stances lack of breeding sites may be the chief factor limiting popula- 

 tion growth. A striking illustration of the operation of this factor is 

 provided by studies of the wood duck made at the Great Meadows 

 Wildlife Refuge in Massachusetts. In an area of 120 hectares, in- 

 cluding a large swamp, 13 tree holes were located that appeared to 

 be suitable for nest sites in regard to size, shape, and location, and 6 

 of these were occupied by breeding ducks. After wildlife managers 

 had put up 65 nesting boxes in the area, ducks built nests in 43 of 

 these, thus revealing the extent of the previous underpopulation; the 

 number of wood ducks had been much smaller than could have been 

 supported by the supply of food and other necessities in the area. 



The tendency for some species to limit themselves to home ranges 

 or territories and thus avoid excessive concentration of the population 



