  The properties of competition models where all individuals are identical are
relatively well-understood; however, juveniles and adults can experience or
generate competition differently. We study here less well-known structured
competition models in discrete time that allow multiple life history parameters
to depend on adult or juvenile population densities. A numerical study with
Ricker density-dependence suggested that when competition coefficients acting
on juvenile survival and fertility reflect opposite competitive hierarchies,
stage structure could foster coexistence. We revisit and expand those results.
First, through a Beverton-Holt two-species juvenile-adult model, we confirm
that these findings do not depend on the specifics of density-dependence or
life cycles, and obtain analytical expressions explaining how this coexistence
emerging from stage structure can occur. Second, we show using a
community-level sensitivity analysis that such emergent coexistence is robust
to perturbations of parameter values. Finally, we ask whether these results
extend from two to many species, using simulations. We show that they do not,
as coexistence emerging from stage structure is only seen for very similar
life-history parameters. Such emergent coexistence is therefore not likely to
be a key mechanism of coexistence in very diverse ecosystems, although it may
contribute to explaining coexistence of certain pairs of intensely competing
species.
