Fishing can undermine species persistence and disrupt species’ interactions via mechanisms such as selective pressure, alteration of key habitat, and/or by sheer mortality.  By reducing population densities of native species, fishing can also increase a community’s susceptibility to invasion by exotics. 
     Marine protected areas (MPAs) or reserves are becoming more prominent in fisheries management and have been proposed as an alternative or complementary tool which may help counteract multiple fishing impacts including invasion of exotics.  Single-species models indicate that there are important differences between shifting effort (MPAs) and reducing fishing pressure through conventional forms of fisheries regulations such as catch quotas, size limits, and gear regulations.  MPAs are likely to lead to population abundances which are much more spatially heterogeneous than conventional forms of fisheries regulations.  While large population sizes inside MPA boundaries may be able to sustain regional populations through spillover of larvae and adults, compacting the remaining fishing effort into a smaller area may result in lower population sizes outside MPA boundaries which may increase the local and therefore regional population’s susceptibility to invasion by exotics. 
     There is a lack of clarity as to how the trade-offs between spillover and compressed fishing effort will affect overall species interactions.  Using a simple model of two competing populations, we demonstrate when MPAs can be effective tools in warding off invasion by exotics, and when MPAs could increase the likelihood of exotic invasion due to higher localized fishing pressure on natives outside their boundaries.