ИСТИНА

Stability of species depends on its ability to survive in non-stable environment. The resulting survival of individuals in the course of ontogenesis is an optimal indicator of species stability. The common toad populations undergo an influence of many different environmental factors simultaneously at each developmental stage. It is very difficult to separate the effects of different factors in nature. During long-term investigations I simulated effects of the egg density, the type of egg cord packing and the oxygen deficiency in lab conditions. I study development of the common toad from fertilization to hatchling. In the ‘egg density’ experiments I revealed that spawns of the common toad use two different developmental strategies (Dmitrieva, 2005, 2007). ‘Egg cord packing’ experiments showed that dynamics of mortality and developmental rate depend not only on the egg density (egg number in a certain volume of water), but also on the type of packing and stretching of egg cord. The toad spawn structure coupling with the ecology of toad spawning causes very high heterogeneity of conditions within the same spawn. For example, in the convoluted cords, embryonic mortality was higher and developmental rate was lower than in the straightened ones. The ‘oxygen deficiency’ experiments showed that embryos developed in the experimental (oxygen deficiency) and in the control conditions did not differ significantly in resulting mortality at the hatchling stage. However, dynamics of mortality were different. Oxygen deficiency did not lead to increase of the total mortality, but shifted the mortality to earlier developmental stages. It is very interesting that resulting survival were the same in the majority of experiments and did not differ from control. Thus, embryos adaptively react on the simulated environmental factors using one of alternative developmental strategies. Such ontogenetic plasticity seems to provide stability of the common toad populations.