Priority effects are weakened by a short, but not long, history of sympatric evolution

Priority effects, or the effects of species arrival history on local species abundances, have been documented in a range of taxa. However, factors determining the extent to which priority effects affect community assembly remain unclear. Using laboratory populations of the bacterium Pseudomonas fluorescens, we examined whether shared evolutionary history affected the strength of priority effects. We hypothesized that sympatric evolution of populations belonging to the same guild would lead to niche differentiation, resulting in phenotypic complementarity that weakens priority effects. Consistent with this hypothesis, we found that priority effects tended to be weaker in sympatrically evolved pairs of immigrating populations than in allopatrically evolved pairs. Furthermore, priority effects were weaker under higher phenotypic complementarity. However, these patterns were observed only in populations with a relatively short history of sympatric evolution, and disappeared when populations had evolved together for a long time. Together, our results suggest that the evolutionary history of organismal traits may dictate the strength of priority effects and, consequently, the extent of historical contingency in the assembly of ecological communities.

优先效应（Priority effects），即物种到达历史对本地物种种群丰度的影响，已在多个类群中被记录报道。然而，决定优先效应影响群落组装（community assembly）程度的关键机制仍未明确。本研究以实验室培养的荧光假单胞菌（Pseudomonas fluorescens）种群为实验材料，探究共享进化历史是否会影响优先效应的强度。我们提出假说：同一功能群（guild）的种群若经历同域进化（sympatric evolution），将引发生态位分化（niche differentiation），进而形成可削弱优先效应的表型互补性（phenotypic complementarity）。实验结果与该假说一致：相较于异域进化（allopatric evolution）的迁入种群对，同域进化的迁入种群对所呈现的优先效应强度普遍更低。此外，表型互补性越高，优先效应的强度越弱。但该规律仅在同域进化历史相对较短的种群中被观测到，当种群长期共同进化时，该规律便不再显著。综上，本研究结果表明，生物性状的进化历史或可决定优先效应的强度，进而影响生态群落组装过程中历史偶然性（historical contingency）的程度。