The suggestion that spatial gradients in species richness are influenced by geometric constraints resulting in the mid-domain effect has been investigated by null models. The technical aspects of making such null models are well explored, but the implicit ecological assumptions behind these models are less explored. Four ecological models that all assume that species ranges are constrained by hard boundaries are made: evolutionary model, source-sink model, dynamic-environment model, and range-size model. These models give different predictions that make it possible to separate the models from each other, and from a model that assumes that hard boundaries are not important.