DISTRIBUTION AND ABUNDANCE 8l 



If, now, we take events in their natural order, and begin with the 

 tadpoles in a pond, then it seems clear that in Lower Parkficld, and for 

 much of the time at Dagger Lane in 1947, we have just the kind of 

 events that Lack fmds in most birds. The population of tadpoles was 

 too large for the ponds, and in one, led to such disasters that the fmal 

 effect was not a larger number of frogs, but almost certainly a smaller. 

 There is very good evidence that food shortages occurred in many of 

 the ponds and must have provided a density-dependent factor limiting 

 the population. In other ponds, I feel sure that predation was respons- 

 ible for the catastrophic decline in numbers, often rcacliing complete 

 extinction. The assumption that predation is a density-dependent 

 factor has, I think, often been made in too facile a manner. If the 

 predators were dragon-fly larvae, I am prepared to beheve that, when 

 the numbers of tadpoles are large in one year, so many dragon-flies 

 survive that next year, the population of tadpoles meets with increasing 

 predation and so declines. But I cannot accept that this argument 

 applies to ducks. A pair of ducks could eat all the tadpoles in a pond in 

 a few days, and probably this often happens, but surely it cannot be 

 argued that the population of ducks next year is larger because of this 

 fleeting episode in the life of the ducks. Ducks are very probably a 

 most important controlling factor in tadpole and frog populations, but 

 they act catastrophically. 



In Chapter i, I showed from the work of Douglas, Moore and Volpe 

 that the southern hmit of distribution on this species is probably fixed 

 by the temperature, which is lethal to the eggs and embryos if it is 

 over some quite low hmit. The effect of temperature on large numbers 

 of organisms is the subject of very large-scale experiments every day, 

 for the canning industry carries out such work in the course of ordinary 

 production. Contrary to what might be expected, cans of food are 

 not subjected to temperatures certain to kill all organisms, for this 

 would damage the food. The processes are devised to kill or make 

 dormant so many organisms that the chance of one surviving to give 

 trouble is so low that complaints do not arise often enough to be 

 important. It is a statistical operation, depending for success on a 

 sufficiently low value for the probabihty. Bacterial populations vary 

 in their resistance from cell to cell and from species to species, the 

 variations are inherited and follow statistical laws. The larger the 

 number of cells present, the greater the chance of having a resistant 

 specimen in the can. The canner aims, therefore, at having his material 



