The mid-domain effect and diversity gradients: Is there anything to learn?

The mid-domain effect (MDE) has been proposed as a null model for diversity gradients and an explanation for observed patterns. Here we respond to a recent defense of the concept, explaining that it cannot represent a viable model in either real or null worlds. First, the MDE misrepresents the nature of species ranges. There is also an internal logical inconsistency underlying the MDE because the range size frequency distribution, necessary to generate a hump-shaped pattern under randomization, cannot exist in the absence of environmental gradients and is generated by the ecological and historical processes that the MDE claims to exclude.