Entropy production in the human body as a whole can be calculated from observed energetic data. The entropy production in humans thus calculated shows a two-stage character over the human life span, that is an early increasing stage and a later decreasing stage until death. Similar methods for calculating entropy production are also applied to lake ecosystems. Entropy production in two lakes (Lake Biwa in Japan and Lake Mendota in USA) has been calculated, and it is found that entropy production per year per volume of lake water in the eutrophic Lake Mendota is larger than that in the oligotrophic Lake Biwa. Because ecological succession (evolution) in a lake always proceeds from oligotrophy to eutrophy, the present results suggest that processes of ecological succession accompany the increase in entropy production. This situation is parallel to the trend in the early stage of the human life span. Based on the above discussion, a hypothesis is presented on the entropy principle in living systems.