Single-species models are generally used for the design of bioprocess control laws. Nonetheless, most of the bioprocesses, if not all, involve an important biodiversity (different species or mutants). Here we propose to define and study the multispecies robustness of bioprocess control laws: given a control law designed for one species, what happens when two or more species are present? We illustrate our approach with a control law which regulates substrate concentration using measurement of growth activity. Depending on the properties of the additional species, the control law can lead to the correct objective, but also to an undesired monospecies equilibrium point, coexistence, or even a failure point.