  We have recently introduced a measure of the bipartite entanglement of
identical particles, E_P, based on the principle that entanglement should be
accessible for use as a resource in quantum information processing. We show
here that particle entanglement is limited by the lack of a reference phase
shared by the two parties, and that the entanglement is constrained to
reference-phase invariant subspaces. The super-additivity of E_P results from
the fact that this constraint is weaker for combined systems. A shared
reference phase can only be established by transferring particles between the
parties, that is, with additional nonlocal resources. We show how this nonlocal
operation can increase the particle entanglement.
