Enemy dispersal drives the diversity loss of bacterial victim populations at diversity cold spots.

Dispersal between heterogenous habitats is a major determinant of population diversification, and may often introduce new morphotypes in habitats where population diversity is low. Natural enemies are also key factors affecting the diversification of victim populations. Co-dispersal of enemies may induce local diversity loss at diversity cold spots as enemies from diversity hots pots are often more efficient in predation. Here, we experimentally tested this hypothesis using a model microbial system: Pseudomonas fluorescens and its lytic phage. The ancestral bacterium diversified at three resource levels across eight temperature gradients in the presence and absence of phages. Bacteria diversified into more morphotypes at higher temperatures and higher resource levels when phages are absent, and dispersal increased population local diversity at low-diversity habitats. The presence of phages removed the differences in morphological diversity among different temperatures or resource levels. In addition, the co-dispersal of enemies caused higher morphotype loss at lower-quality habitats where the local bacteria are of lower resistance. The simultaneous dispersal of enemies and victims may have crucial consequences for population persistence in edge habitats.