Source-sink migration of natural enemies drives local maladaptation of victim populations in edge habitats

Natural enemies are critical drivers of species biogeography. Local adaptation of victim populations in edge habitats is particularly likely to be limited by enemies. We experimentally tested this hypothesis using a model microbial system, bacterium Pseudomonas fluorescens (victim) and a lytic bacteriophage (enemy). When evolving alone, bacterial populations in a low temperature environment (10°C) showed obvious abiotic adaptation in terms of increased growth performance; and immigration of bacteria from an optimal environment (28°C) reduced such evolutionary adaptation. However, when phages were present, no significant abiotic adaptation was observed. Crucially, phage immigrants from source populations even caused maladaptation (decreased growth performance relative to the ancestral genotype), and bacterial adaptation was less affected when both bacteria and phages had joint migration. Our results demonstrate intraspecific apparent competition mediated by enemies with which prosperity in core habitats can exacerbate hardship in edge habitats.