Bacterial resistance to phages
For evolution lines with phages (BP, BP+IP and BP+IBP), population-level bacterial resistance against within-microcosm phages did not differ among treatments, and did not differ between transfer 8 and 20 (linear mixed effect model, time, \(\chi_{1}^{2}\) = 2.267, P = 0.132; treatment, \(\chi_{2}^{2}\) = 4.712, P = 0.095; time × treatment,\(\chi_{2}^{2}\) = 1.561, P = 0.458; Figure 4a, 4b). Bacterial resistance against phages from the core habitat (SBP microcosms) increased from transfer 8 to 20, and did not differ among treatments (linear mixed effect model, time, \(\chi_{1}^{2}\) = 8.154, P = 0.004; treatment, \(\chi_{2}^{2}\) = 1.257, P = 0.534; time × treatment, \(\chi_{2}^{2}\) = 0.503, P = 0.778; Figure 4c, 4d). Note that reciprocal challenges between the core and edge habitats (SBP versus BP microcosms) showed signals of more intense coevolution at transfer 8 but not at transfer 20 (Supplementary Text; Figure S2).