Ecospace is a spatially explicit model for policy evaluation which allows considering the impact of marine protected arease (MPAs) in an ecosystem (i.e., trophic) context, and which relies on the Ecopath mass-balance approach for most of its parameterization. Additional inputs are movement rates, used to compute exchanges between grid cells, the settings (top- down vs. bottom up control) also required for Ecosim, the dynamic simulation routine derived from the system of linear equations in Ecopath, and habitat preferences for each of the functional groups included in the model. `Cascade' effects, wherein prey organism are low where predators are abundant, e.g. in areas onto which high fishing costs have been mapped, or in MPAs are discussed. It is then shown that the potential benefits of local effort reductions can be easily negated by high movement rates, and especially by the concentration of fishing effort at the edge of the MPAs, where cascade effects generate prey gradients which attract predators out of the protected areas. Despite various limitations (e.g., no explicit consideration of seasonal changes or directed migration), the outward simplicity of Ecospace, and the information-rich graphs it generates, coupled with the increasingly global availability of the required Ecopath files, should ensure a wide use for this approach. This will be of use for both generating hypotheses about ecosystem function and for evaluating policy choices.