The design of control laws for bioprocesses are generally based on simplified single-species models. Biodiversity (made of a mixture of different species or strains) is nonetheless inherent in any artificial ecosystem in a bioreactor. Here we propose to define and study the robustness to biodiversity of bioprocess control laws: given a control law designed for one species, what happens when additional species are present? We illustrate our approach with a well used 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 required objective, but also to an undesired monospecies equilibrium point, coexistence, or even a failure point. Finally, we show that, for this case, the robustness can be improved by a saturation of the control. Robustness to biodiversity is a difficult issue which should be better accounted for in the control design.