484 Dynamics of the Ecosystem 



high equihbrium level for the standing crop. In the second situation 

 the growth of the population is much slower, but the same ultimate 

 size of standing crop is attained. In the third situation the same 

 period of time is required to reach equilibrium as in the second, but 

 a lower equilibrium level is established. If a census of the standing 

 crop is taken before the populations have reached their maximum 

 equilibrium levels, the larger standing crop will clearly occur in the 

 area in which the rate of production has been the greatest. Usually, 



Equilibrium standing crop 



Short season 



Long season 



Time 



Fig. 13.8. Relation between size of standing crop and rate of growth of popula- 

 tion. (Clarke, 1946.) 



however, the observer will enter a natural area long after the initial 

 growth of the population has been completed and will measure the 

 standing crop at or near its equilibrium level. At this point the 

 magnitude of the standing crop is not an index of growth rate. The 

 size of the population at any moment is the result of the accumulation 

 of production minus the amount of destruction which has taken place 

 up to the time of observation. 



In the initial growth of the population the length of life of the indi- 

 viduals influences the steepness of the population curve because, the 

 longer each individual lives, the faster will the total numbers increase. 

 Once equilibrium has been reached, however, individual length of 

 life has no effect upon the size of the standing crop. As discussed 

 in Chapter 9, natality is equal to mortality at equilibrium, or A = M. 

 If 50 animals are born per unit of time, 50 animals will die in the same 

 period, and it makes no difference whether those animals have lived 

 5 days or 5 years. Similarly, the size of the population is not de- 

 termined by the magnitudes of A and M. As indicated in the dia- 

 gram, a population with a large standing crop may exhibit a low 



