Material Removed 489 



ments tends to be absolutely larger in high latitudes than in regions 

 nearer the equator. This difference is particularly prominent if the 

 standing crop in the tropics, which often remains uniformly small 

 throughout the year, is compared with the standing crop in temperate 

 or polar regions at the height of the growing season following the 

 winter replenishment of nutrients. For instance, the standing crop of 

 zooplankton in the antarctic summer was observed to be about 10 

 times greater than that in the tropical Atlantic. Again many excep- 

 tions exist in local areas-as in the Great Barrier Reef lagoon, where 

 the standing crop was found to be as great as in temperate coastal 

 regions. Under tropical conditions organisms grow faster and, after 

 death, their tissues are decomposed and nutrients regenerated at a 

 more rapid rate, but as already explained this difference in growth 

 rate does not in itself account for the difference in the standing crop. 

 Part of the explanation of the latitudinal difference in total abun- 

 dance of marine life is the fact that at low temperatures the metab- 

 olism of poikilotherms generally is much reduced. This means that 

 a larger total number of organisms can be supported on the same 

 amount of food. However, regional differences in metabolism are by 

 no means simple and clear cut, as pointed out by Riley, Stommel, and 

 Rumpus, (1949). Evidence exists that the rate of supply of nutrients 

 to the surface in temperate and polar regions is usually greater than 

 in the tropics owing to the nature of the vertical circulation of the 

 ocean. Nutrients may thus be supplied more rapidly in relation to 

 their use in the cooler water, with the result that a larger standing 

 crop can be supported. However, in tropical regions with exception- 

 ally great upwelling-as along the Peruvian coast-an enormous stand- 

 ing crop of plankton exists and supports the food chain leading to 

 the production of guano, as described in Chapter 8. These regional 

 differences in abundance of marine organisms are discussed in further 

 detail by Sverdrup et al. (1942, Ch. 19). 



Material Removed 



Various aspects of the material removed from an area, as the second 

 concept of productivity, are of interest to both the theoretical and the 

 practical ecologist. Some may believe that the yield to man is the 

 only component of the material removed that concerns farmers, fisher- 

 men, hunters, and foresters, but losses from the area in the form of 

 emigration and organic deposits must also be considered. For the 

 purpose of maintaining the yield to best advantage other concepts of 

 productivity must also be understood and applied. In subsistence 



