  The spatial dispersal of individuals is known to play an important role in
the dynamics of populations, and is central to metapopulation theory. At the
same time, local adaptation to environmental conditions creates a geographic
mosaic of evolutionary forces, where the combined drivers of selection and gene
flow interact. Although the dispersal of individuals from donor to recipient
populations provides connections within the metapopulation, promoting
demographic and evolutionary rescue, it may also introduce maladapted
individuals into habitats host to different environmental conditions,
potentially lowering the fitness of the recipient population. Here we explore a
model of the eco-evolutionary dynamics between two populations connected by
dispersal, where the productivity of each is defined by a trait complex that is
subject to local selection. Although general in nature, our model is inspired
by salmon metapopulations, where dispersal between populations is defined in
terms of the straying rate, which has been shown to be density-dependent. The
results of our model reveal that increased straying between evolving
populations leads to alternative stable states, which has large and nonlinear
effects on two measures of metapopulation robustness: the portfolio effect and
the time to recovery following an induced disturbance. We show that
intermediate levels of straying result in large gains in robustness, and that
increased habitat heterogeneity promotes robustness when straying rates are
low, and erodes robustness when straying rates are high. Finally, we show that
density-dependent straying promotes robustness, particularly when the aggregate
biomass is low and straying is correspondingly high, which has important
ramifications for the conservation of salmon metapopulations facing both
natural and anthropogenic disturbances.
