  The Lagrangian formalism is developed for the population dynamics of
interacting species that are described by several well-known models. The
formalism is based on standard Lagrangians, which represent differences between
the physical kinetic and potential energy-like terms. A method to derive these
Lagrangians is presented and applied to selected theoretical models of the
population dynamics. The role of the derived Lagrangians and the energy-like
terms in the population dynamics is investigated and discussed. It is suggested
that the obtained standard Lagrangians can be used to identify physical
similarities between different population models.
