Determination of Equilibrium Level 485 



natality and a low mortality, whereas another population of smaller 

 equilibrium size may display high birth and death rates. In anal- 

 ogous fashion the level of water in a tank (the standing crop) gives 

 no clue to the rate of inflow (natality) or the rate of outflow (mor- 

 tality). A high level of water in the tank may be maintained by a 

 trickle of water running in and out, or, in another instance, a low 

 water level may exist with a large inflow and a large outflow. 



Determination of Equilibrium Level. What then does determine 

 the magnitude of the standing crop at equilibrium? The total 

 biomass of organisms inhabiting an area always tends to increase. 

 This is due to the numerical increase of the population and to the 

 growth in size of the individuals, or both. In some kinds of organ- 

 isms, such as fish or trees, the individuals continue to grow larger 

 all their lives, but in others, such as rodents and insects, full size is 

 attained as soon as adult condition is reached. Although the number 

 of trees in a forest stand tends to decrease due to competition, the in- 

 crease in size of the individuals, for a time at least, more than makes 

 up for the loss in numbers so that here also the total biomass of the 

 population tends to grow larger. Since the size of the standing crop 

 is not limited by factors intrinsic within the population, the increase 

 in total biomass must be stopped by external factors. The biomass 

 of the population will eventually reach the maximum size possible 

 for the circumstances regardless of the speed of increase. The maxi- 

 mum standing crop that can maintain itself indefinitely in an area 

 is spoken of as the carrying capacity of the area. 



The ultimate limit to the growth of the population is determined 

 by the supply of nutrient materials and energy. If the supply of 

 these needs is large and if no other factor interferes, the population 

 will grow to a large size. If food and energy continue to be pro- 

 vided at a rapid rate, the large population can be continuously main- 

 tained. A clear example of such a situation is found where a continu- 

 ous upwelling of deep water supplies nutrients for the growth of the 

 marine organisms in the area. Conditions of this sort account in part 

 for the large carrying capacity of certain fishing banks. 



When a population lives up to the limit of its food supply, it is 

 living dangerously. Should the food supply be curtailed or should 

 the population increase still further, large-scale destruction of the 

 inhabitants would ensue. If populations of animals were checked by 

 food supply only, critical conditions would be widespread because we 

 know that animals can destroy their food supply faster than it can 

 grow. The essence of this idea in relation to man was pointed out by 

 an early "ecologist" by the name of Malthus. Because of the in- 



