At steady state, the number of coexisting species in phytoplankton communities cannot exceed the number of limiting resources unless additional mechanisms are involved. One such mechanism is predation, which may increase the diversity of phytoplankton communities by facilitating the evolution of defence mechanisms, provided that the employment of defences has an associated cost. While empirical and theoretical studies have documented the importance of predation in maintaining and enhancing biodiversity, the function of many proposed defence mechanisms still remains elusive, and any trade-offs often remain unquantified or unknown. Many suggested defensive traits are inducible, that is, their intensity increases in the presence of grazers or grazer cues (e.g. toxin production, silica wall thickness, etc.). This suggests that these proposed traits indeed have a defensive function, whether or not the mechanism is understood. It also suggests the presence of a cost to such defences: if there was no cost, why would phytoplankton only use these defences when needed? We argue that the costs of defence mechanisms often only realize (i) under natural conditions, or (ii) become evident under resource-deficient conditions where a rivalry for limiting resources between growth and defence occurs.
Read more about plankton defense mechanisms in their paper here.
Figure above shows a mosaic of morphological traits in phytoplankton with scale bars.