12 . Viewpoint of Modern Ecology 



environment causes the death of most young organisms brought into 

 the world, and sometimes it may vary so as to kill all members of a 

 population. Man is an unavoidable part of the environment. With 

 intelligence he may help guard the danger spots— he may even improve 

 the productivity of natural environments. Without intelligence man 

 accelerates destruction. 



In the preceding examples of mass mortality and extinction the 

 devastating action which the environment can have, with and without 

 the presence of man, is obvious to all. For every spectacular instance 

 of this sort, there are thousands of instances in which no cataclysmic 

 destruction is taking place, but in which the forces of the environ- 

 ment are nevertheless exerting a crucial influence in some subtle or 

 obscure manner. 



This undercover work of the agents of the environment may perhaps 

 be most easily appreciated first in its action on individual species. 

 The geographical range of each species, for example, is controlled by 

 the pruning action of external forces. Every species is pressing 

 against its boundaries and is always tending to extend its range. 

 It is held in check by the physical and biological factors of the environ- 

 ment which kill off those individuals which spread out too far— into 

 areas where conditions are no longer tolerable. During periods when 

 no unusual fluctuations occur at the limits of the range inhabited by 

 a species, the destructive effect of the environment may not be spec- 

 tacular because ordinarily only a few individuals are eliminated at any 

 one time or place. 



The environment may thus control crucially, but quietly, the 

 geographical range of a species. Fluctuations in the environment 

 may even allow the limits of distribution to change somewhat as 

 conditions vary without a particularly noticeable mortality. One 

 example of that possibility is found in the commercial fishery records 

 for the landings of the weakfish along our middle Atlantic coast. 

 This fish, which is sold in New York markets as "sea trout," was 

 caught only in waters south of Cape Cod previous to 1895. During 

 that year and in succeeding seasons weakfish were taken north of the 

 Cape, and by 1901 the catch was large enough to support a regular 

 fishery. By 1907, however, the numbers of this fish had dropped 

 again to such a low figure that the fishery was abandoned, and never 

 established again north of Cape Cod. There was no dramatic 

 change in the ocean waters around the Cape during those six years. 

 Some subtle, unnoticed variation in the ecological conditions allowed 

 the weakfish to extend its range a short distance for a little while, and 

 then caused the northern limit of the species to be drawn back again. 



