84 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



were none. Moreover, Upper Parkfield and Lower Parkfield, a short 

 distance away, have also lost their frogs, possibly because swans were 

 placed on Upper Parkfield, and sometimes move to Lower Parkfield. 

 In i960 there were no swans and spawn was found in Lower Parkfield. 



Passing now to the factors regulating the abundance of the adults, 

 it is easy to see that the upper limit is set by the numbers of meta- 

 morphosing tadpoles. Nothing that happens in the frog population 

 can have any influence in raising the numbers above this ceiling, but 

 the numbers may be reduced. I shall later show that food shortage 

 among the adults is probably never a limiting factor. Predation may 

 perhaps be important in some places, and is mostly catastrophic when 

 it occurs, but it is more appropriately discussed later. On the whole, 

 when the tadpole has metamorphosed, the main risks are over. 



So far, I have dealt with relative numbers. It is not difficult to carry 

 out a census of breeding frogs in an area, for the spawn is laid in 

 conspicuous parts of the pond. With a little practice, it is easy to 

 estimate the number of clumps. If all the ponds in the area are known, 

 the census is probably at least as accurate as those on other animals. 

 By these methods, I calculated that the area of about a square mile round 

 Large Totteridge in the thirties contained about three or four breeding 

 frogs per acre. In 1949, I estimated that a similar area near Hospital 

 Pond contained rather more, say five or six. I know of no other 

 census for tliis species, but for comparison, Syrrophus marnockii, a 

 species of toad with the unusual habit of laying its eggs in the soil, was 

 recorded by Jameson (1955) as having populations of this order, but 

 that Pearson (1955) found numbers of the Spadefoot Toad, Scaphiopus 

 holbrookii, of many hundreds and even more than a thousand per acre 

 in some areas. 



Whether one can regard the name "Common Frog" as appropriate 

 for tliis species is perhaps a matter of taste. It is not nearly as common 

 as many people think who gain their impressions from the conspicuous 

 breeding habits of the animals, and assume that the countryside is as 

 densely populated as the small part of it that they happen to see in the 

 spring. 



Food for the adults is, I think, never a limiting factor. I have no 

 direct evidence for this statement in this species, but there are two 

 suggestive reports in other species. A colony o£ B. bufo, required for 

 physiological experiments, has been kept in a wire enclosure where the 

 toads sheltered under turf. It was found unnecessary to feed them. 



