Giovannoni et al. reply.

Replying to S. Våge, J. E. Storesund & T. F. Thingstad Nature 499, http://dx.doi.org/10.1038/nature12387 (2013). In the debate about top-down versus bottom-up control, most would argue that both are important. The most probable scenario is that in the absence of high rates of transport, the equilibrium between these competing processes determines bacterial abundance. In their Brief Communication Arising, Våge and co-workers respond to our Letter, arguing the case for defense specialism. ‘Defence specialism’ is not synonymous with ‘defence’. Because of the prevalence of viruses, it is probable that most bacteria have acquired defences against predation. ‘Defence specialism’ refers to success by defence, in the sense that a cell might allocate a large proportion of its resources to defence and thereby become successful.