  Let $X$ be a Hadamard manifold, and $\Gamma$ a non-elementary discrete group
of isometries of $X$ which contains a rank one isometry. We relate the ergodic
theory of the geodesic flow of the quotient orbifold $M=X/\Gamma$ to the
behavior of the Poincar{\'e} series of $\Gamma$. Precisely, the aim of this
paper is to extend the so-called theorem of Hopf-Tsuji-Sullivan -- well-known
for manifolds of pinched negative curvature -- to the framework of rank one
orbifolds. Moreover, we derive some important properties for $\Gamma$-invariant
conformal densities supported on the geometric limit set of $\Gamma$.
