DISTRIBUTION AND ABUNDANCE 85 



for they remained in flit condition all the summer on food that entered 

 the cage. The population density was far higher than ever occurs in 

 nature, and the only conclusion is that the animals were in the presence 

 of a great surplus of food. The other example is to be found in the 

 paper on Scapliiopus already cited. The population of these toads was 

 enormously greater than ever occurs in R. temporaria. It is, of course, 

 true that these examples involve the assumption that the food and 

 other ecological factors of the different species were the same. This 

 cannot be quite true, but there is in most species a considerable overlap 

 in the kinds of food taken, and I do not believe that in the presence 

 of so m.uch food for toads, frogs would starve. 



Mortality 



By making approximate estimations by eye, I concluded that in 

 most years, there were only about i per cent of the original number 

 of tadpoles at metamorphosis. A rough but quite independent 

 estimate, based on the rate of infestation by P. integerrimum (Chapter 

 10, Appendix ij), gave a mortality estimate of the same order. When 

 it is remembered that in half the ponds the mortality is 100 per cent, 

 we must conclude that the average mortality is well over 99 per cent. 

 For the sake of argument, let us suppose that the mortality is 99 per 

 cent. Then of the 1,500 eggs laid by the frogs, fifteen become htde 

 frogs. Frogs become sexually mature at three years old, perhaps 

 sometimes at two (Smith, 195 1). This can also be checked by the hfe 

 history of P. integerrimum for the bladder form of the parasite becomes 

 mature at three years old, and is therefore ready to lay eggs just when 

 its host does so for the first time. 



If, as I assume for the sake of simphcity, the number of frogs remains 

 stationary, then the total mortahty of the frogs that are the progeny 

 of one pair must be no more than thirteen frogs in three years. It is 

 probable that frogs breed in more than one season (in R. clamitans, there 

 is definite evidence (Martof, 1953) that some may breed for three 

 seasons^uszczyk (195 1) found the same in R. escnlenta) so that this 

 mortahty is probably over-stated at about 30 per cent per annum. 

 Lack estimates that the mortahty of most small birds is about 30 to 

 60 per cent per annum, so that the frog lives by comparison a fairly 

 safe life. Robins (Lack, 1954) have an expectation of life of just over a 

 year, so that it would be most exceptional for any robin hatched at the 

 same time as a tadpole to be ahve when the frog that the tadpole 



