Populations of key species are usually considered as good biological targets to assess the potential impact of induced environmental changes, namely with regard to long term responses. In such case, it is particularly suitable to account for their population dynamics and reproductive biology. From the ecological point of view, the identification of inter-site variations between populations of the same species can be achieved through comparative population dynamics studies, including the estimation of energy budgets and its relation to available food sources. Sampling methods were agreed within partnership and contemporaneously conducted at three different sites in the Mediterranean and Atlantic for comparison. From this database models have been developed to understand adaptation to environmental gradients and changing habitats. Reproductive biology and population dynamics studies of key stone species is a useful tool to integrate environmental changes at different time scales. In fact, within certain limits, species tend to cope with environmental changes, and when they are under environmental stress, natural or induced by human activities, their life cycles, production, and energy budgets will reflect it. Starting from a calibrated population dynamics model, hopefully validated as well, different scenarios, corresponding to different environmental conditions, may be accounted for by changing the forcing functions and by changing the model parameters in accordance. The coastal line is constantly submitted to both natural and human sources of change and Talitrus saltator will tend to be affected by these changes. Since it is usually present on European beaches, namely along the Mediterranean coasts, they may serve as an indicator of the prevailing ecological conditions.