  In this Article we perform a systematic study of the global entanglement and
exciton coherence length dynamics coherence in natural light-harvesting system
Fenna-Matthews-Olson (FMO) complex across various parameters of a dissipative
environment from low to high temperatures, weak to strong system-environment
coupling, and non-Markovian environments. A non-perturbative numerically exact
hierarchical equations of motions method is employed to propagate the dynamics
of the system. We found that entanglement is driven primarily by the strength
of interaction between the system and environment, and it is modulated by the
interplay between temperature and non-Markovianity. In contrast, coherence
length is found to be insensitive to non-Markovianity. Furthermore, our results
do not show the direct correlation between global entanglement and the
efficiency of the excitation energy transfer in the FMO complex
