148 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



I think that the evidence is conclusive. The frogs are influenced by 

 something that happens to their ponds in the winter, even if they are 

 not in those ponds. Now, what could be happening that is consistent 

 with the facts? 



It has been known for many years that the spring growth of algae is 

 accelerated by the presence of plant nutrients brought in to the ponds 

 by the run-off. Rain does not usually dilute a pond; on the contrary, 

 it is a fertihzing agent (See the rises in phosphate after rainfall in Fig. 

 24). Among these nutrients are, of course, potassium and phosphates. 



-45-39-33-27-2 1 -15-9 -3 0+3 +9 +15+21+27+33+39+45+51+57 

 DEPARTURE FROM MEAN SPAWN DATE IN DAYS 



Fig. 41 . Histogram Showing the Distribution of Spawn Records 



They are plotted as departures from the mean date in each year, for 24 years 



in a fairly uniform area, SE England. This curve is not of the normal form 



being sharply peaked and negatively skewed. 



Phosphates tend to be the limiting factor, and in fact water undertakers 

 use the level of phosphate as an indication that the peak of algal 

 production has been reached, so that their troubles with consumers over 

 tastes and smells are nearly over. Light is obviously an important 

 factor, and, indeed, with certain underground waters, every effort 

 must be made to avoid exposing the water to light before passing it 

 into the mains, for, otherwise, algal growth is certain to be troublesome. 

 In spite of modern methods of control, it is still necessary sometimes 

 to take a reservoir temporarily out of service until the algal peak has 

 passed. There have been cases of reservoirs proving so troublesome 

 that, in spite of the cost, they have been abandoned. If indeed it is 



