Painted caves are karstic cavities here considered as stable physical systems in a state of dynamic equilibrium. From the example of the Gargas cave, we show that introduction of exceeding energy (visitors, lighting) causes a loss of stability and implies a risk of degradation in the cavity. From different approaches, we identify the periods and the causes of instability and we determine the maximum level of introduced energy which would preserve the conservative properties of the cavity. These results permit to define cave equipment and visitor capacity compatible with satisfactory conservation conditions.