O. Gy. Dely szerk.: Vertebrata Hungarica 22. (Budapest, 1984)

Korsós, Z.: Comparative niche analysis of two sympatric lizard species (Lacerta viridis and Lacerta agilis) 5-14. o.

In the earlier or later .parts of the day with more favourable temperature conditions in the open areas the situation is quite opposite. This behaviour is due to the lack of complementary tolerance aspect of the population, I.e. the active regulation of body temperature, which would be the direct answer to the exterior effects of the ambient temperature. A similar relationship has been described by OUBOTER (1981) for L. sicula salffl

. Hence Fig. 7 may actually be interpreted as a niche response surface of L. viridis (MAGUIRE 1973), which reflects its population reply to the constant vegetation density and the changing temperature as environmental factors. The physical structure of the soil and the velocity of the wind was found to have no distinct effect on the two lizard populations. The lizards occur frequently on soil types with 27-29 % of clay content, obviously because this is the most recurrent soil type in the study area. Hence the lizard populations seem not to make distinction between the soil types. Differentiation In activity periods has already been observed in terrarium by SAINT GIRONS (1976). The less rigorous requirement of L. agilis for air temperature enables it to begin foraging earlier, but on the other hand the intolerance of high soil temperature forces it to finish It sooner. The strange trimodal activity curve of L. viridis requires further studies, but the functions itself is not unknown In bimodal form (for L. taurica

, CRUCE 1970). For a possible explanation, let us compare the shapes of the activity diagrams of the two species around 13.00 hrs (Fig. 3).We can see an opposite situation: while the sand lizard has an activity maximum then, the foraging activity of the green lizard declines abruptly. In accordance with SCHOENER'

s hypothesis, temporal resource partitioning is determined by an energy balance (SCHOENER 1974). The question is whether the energy gained while feeding exceeds the sum of energy lost in foraging plus In the inactive periods, or not. If the answer Is positive then the animal will be active and forage for food; If negative then the lizard will wait at rest. In our case, after Increase in activity of the sand lizard It would not be worth for the green lizard to forage since it could not get enough prey, and so its loss of energy would be greater than the gain from the food. When the sand lizard retires because of the high soli temperature the green lizard may become active again, thus Increasing its competitive ability. I relative frequency % Fig. 7. Niche response surface of Lacerta viridis