Aspects of the population ecology of the painted frog, discoglossus pictus in the Maltese Islands

Abstract

Field and laboratory observations on the Painted Frog Discoglossusoictus were undertaken to study its population biology under local conditions. The effect of population density on D.nictus development was studied in the laboratory by culturing tadpoles from the same spawning at 8.5, 15, 40, 84 and 96 tadpoles/litre (1-1). Tadpoles develop at a slower rate at high densities and metamorphose at a lower body weight than those at low densities. The rate of development varies between individuals from the same spawning. The higher the culture density, the greater the variability. Population density had no effect on either tadpole mortality or the number of tadpoles completing metamorphosis or the time taken for the tail to be resorbed. No direct relationship was found between climatic factors and spawning date. Frogs in different places spawned at different times but at any one site they spawned at the same time. At Chadwick Lakes they spawned twice in a year and tadpoles took two months to complete metamorphosis. Rainfall is directly related to D.nictus adult and tadpole distribution but not to their abundance. Tadpoles attained comparable mean sizes at nine different sampling sites investigated, except for one sample obtained from Bidnija (31-5-86) which had a mean body length significantly larger than any of the others. No difference was found between D.nictus adult male and female wet weight and body length at Chadwick Lakes. There was a predominance of male (sex ratio 6:1). A significant positive correlation was found between D.pictus tadpole body length and tail length, body length and wet weight, tail length and wet weight, and dry weight and wet weight in laboratory and field samples. A significant positive correlation was found between cultured D.pictus juvenile body length and wet weight.