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

Natural enemies are critical drivers of species biogeography, and they may often limit the evolutionary adaptation and persistence of victim populations in sink habitats. Source-sink migration is also a major determinant of adaptation in sink habitats. Here, we specifically suggest that source-sink migration of enemies reduces evolutionary adaptation of victim populations in sink habitats. The underlying mechanisms may include depressed population size (which limits the supply of genetic variation) and enforced resistance evolution in victims (which shows a trade-off with growth performance). We experimentally tested this hypothesis using a model microbial system, bacterium Pseudomonas fluorescens (victim) and its lytic bacteriophage (enemy). The ancestral bacterial strain had lower growth performance at a cold temperature (10 °C, considered as sink habitat) than at its optimal temperature (28 °C, source habitat). Evolutionary adaptation took place in bacterial populations that evolved alone in the cold environment. When phages were present, no significant abiotic adaptation was observed. Crucially, phage immigration from source populations caused maladaptation, i.e., decreased growth performance relative to the ancestral genotype, although this was not the case when there was simultaneous immigration of phage and bacteria. Therefore, enemy-mediated intraspecific apparent competition could lead to prosperity in core habitats causing hardship in edge habitats.