Arising from Y. Zhao et al. Nature 494, 357–360 (2013). The recent findings of abundant SAR11 viruses by Zhao et al. are intriguing, and add new insight into the on-going discussion of why SAR11 bacteria are highly successful in the pelagic ocean. On the basis of high SAR11 virus abundance, Zhao et al. claim that SAR11 bacteria are competition specialists. Alternatively, we show here how their findings could be consistent with a dominance of defensive SAR11 strains. Considering their high abundance, understanding why SAR11 bacteria are so successful has important implications for the study of the pelagic ecosystem. There is a Reply to this Brief Communication Arising by Giovannoni, S., Temperton, B. & Zhao, Y. Nature 499, http://dx.doi.org/10.1038/nature12388 (2013).