348 Relations within the Species 



the population will result in the disadvantages of overcrowding. 

 Another consideration is the fact that the longer an animal or a plant 

 remains unharvested, the longer it will be exposed to environmental 

 dangers and the greater the possibility that it will die from other 

 causes or be removed by predators other than man. If natural mor- 

 tality is very high, the crop may have to be harvested at an earlier 

 age than would be necessary should the reverse be true. The optimal 

 level for the population thus varies widely according to the ecological 

 relations of each species. The optimal yield is obtained when the 

 rates of reproduction, growth, and mortality are so adjusted as to pro- 

 duce the greatest annual increment with due regard to availability and 

 other modifying considerations. More detailed discussion of both 

 the theoretical and the practical aspects of this important topic is 

 available in Russell (1942), Trippensee (1954), and the pubUcations 

 of the United States Department of Agriculture. 



SPATIAL RELATIONS OF POPULATIONS 



Space Requirements 



As a population grows in numerical strength and as its members in- 

 crease in size, the individuals tend to come closer and closer together 

 as long as the population occupies the same area. In many instances 

 the habitable area is sharply limited, as it is for aquatic populations 

 developing in a pond, or for land organisms multiplying on an island. 

 Although in other instances populations can spread out at their mar- 

 gins for a time as they develop, eventually the unoccupied area is 

 completely taken up and a condition of overcrowding begins to ap- 

 pear. 



Every organism requires a minimum amount of space within which 

 it can carry on its necessary exchange with the external world. This 

 minimum space must be sufficient to provide food and other necessi- 

 ties, to absorb metabolites, and to permit reproduction. A relation 

 obviously exists between the size of the organism and the minimum 

 size of the space that it can inhabit. Whales do not live in ponds. 



The space required by some organisms is quite small and may be 

 no larger, in one plane, than the actual dimensions of the body. The 

 number of adult barnacles that can inhabit a rock, for example, is 

 determined simply by the number of barnacle bases for which the 

 area of the rock affords room. Barnacles can grow with their sides 

 closely appressed to their neighbors because water currents bring 

 food particles and oxygen and take away metabolites and progeny. 



I 



