  Populations exhibiting partial migration consist of two groups of
individuals: Those that mi- grate between habitats, and those that remain fixed
in a single habitat. We propose several discrete-time population models to
investigate the coexistence of migrants and residents. The first class of
models is linear, and we distinguish two scenarios. In the first, there is a
single egg pool to which both populations contribute. A fraction of the eggs is
destined to become migrants, and the remainder become residents. In a second
model, there are two distinct egg pools to which the two types contribute, one
corresponding to residents and another to migrants. The asymptotic growth or
decline in these models can be phrased in terms of the value of the basic
reproduction number being larger or less than one respectively. A second class
of models incorporates density dependence effects. It is assumed that increased
densities in the various life history stages adversely affect the success of
transitioning of individuals to subsequent stages. Here too we consider models
with one or two egg pools. Although these are nonlinear models, their
asymptotic dynamics can still be classified in terms of the value of a locally
defined basic reproduction number: If it is less than one, then the entire
population goes extinct, whereas it settles at a unique fixed point consisting
of a mixture of residents and migrants, when it is larger than one. Thus, the
value of the basic reproduction number can be used to predict the stable
coexistence or collapse of populations exhibiting partial migration.
