32 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



evidently already begun, for tadpoles were to be seen almost every- 

 where, at a density of about 2-10 per 100 cm^ feeding on the bottom. 

 On 29th May, many of the tadpoles were feeding on the surface film. 

 On 7th June, there were very large differences in size between the 

 tadpoles and there was still no sign that any of them were meta- 

 morphosing. Fig. 8 should now be compared with Fig. 5 representing 

 the events at Dagger Lane in the same year, and with Fig. 9 showing 

 the events in Lower Parkfield in the next year, 1948. The contrast is 

 very striking, for what is now being described is a catastrophe. The 

 average weight of the tadpoles in this pond, collected at "Spawn 

 Site" was only 270 mg. At Dagger Lane on the same date, the average 

 weight from the lightest collection was 345 mg and metamorphosed 

 frogs had already been seen. 



At the next visit on 13 th June the duckweed canopy was complete 

 and movements of the tadpoles were not easily seen. A net sweep at 

 "Spawn Site" showed that the animals were still abundant, but they 

 were small (average 140 mg) and were still far from metamorphosis. 

 A net sweep at "Seat," only 8 m away, brought up many much 

 larger tadpoles (average weight 480 mg) in a much more advanced 

 state of development. 



On 19th June, feeding movements could be seen among the duck- 

 weed. A net dip at "Spawn Site" collected many tadpoles, most of 

 them large. At "Seat," a colony of much smaller tadpoles was found. 

 Another collection was made alongside the wall at the pomt called 

 "West." On 27th, the collections made at "Spawn Site" and at 

 "Seat" differed Uttle in weight but, on 7th July, the mean weight of 

 the "Seat" tadpoles was only 190 mg, although those at "Spawn Site" 

 were more numerous with an average weight of 400 mg — that is, 

 almost the reverse of the conditions of 13 th June. The population 

 density in the pond was still very high — about one hundred tadpoles 

 were collected in one net sweep at "Spawn Site." The first meta- 

 morphosed frog was seen on 7th July. Fig. 8 shows that, up to this date, 

 there were fairly large tadpoles in this pond, but that a decline in weight 

 had now set in. Tadpoles in continuously decreasing numbers and 

 having a decreasing weight continued to inhabit the pond until loth 

 October, far beyond the normal date, although by this time, the 

 miserably emaciated specimens were so few that they were not easy 

 to fmd. On a final visit on 26th October, none could be collected. I 

 beheve that the number of httle frogs leaving the pond was far fewer 



