  Using techniques of non-autonomous dynamical systems, we completely
characterize the persistence properties of an almost periodic Nicholson system
in terms of some numerically computable exponents. Although similar results
hold for a class of cooperative and sublinear models, in the general
non-autonomous setting one has to consider persistence as a collective property
of the family of systems over the hull: the reason is that uniform persistence
is not a robust property in models given by almost periodic differential
equations.
