Reply to Brun et al.: Fingerprint of evolution revealed by shifts in realized phytoplankton niches in natural populations

Abstract

Irwin et al. (1) show that the realized niches of many phytoplankton species shifted over a 15-y time period at the CARIACO (Carbon Retention in a Colored Ocean) Ocean Time-Series Station. The realized niche is a restricted version of the fundamental niche arising from at least three factors: biotic interactions, dispersal limitation, and the absence of some of the fundamental niche from the environment. At CARIACO, changes in the realized niches of phytoplankton species tracked a decadal-scale rise in average temperature and an increase in average irradiance in the upper mixed layer related to the deepening of the nutricline. The magnitudes of the changes in realized niches were proportional to the change in the mean environmental conditions, revealing a fingerprint of evolution. In contrast, changes in nitrate niches divided species into two categories: A minority of species shifted to lower nitrate niches, and the majority of species exhibited fixed niches in response to decreasing nitrate concentration. It is likely more difficult for phytoplankton to adapt to decreasing nitrate concentrations than modest changes in temperature or irradiance. We hypothesize that the changes we observed in realized niches were a result of evolution because of the structure in the changes of the realized niches. Laboratory studies have shown that microalgae can adapt to new environments in a few hundred generations, demonstrating the potential for rapid evolution in natural communities (2).