Distribution of the mean beta (a, with explanatory schema) and wGRR values b) and IS density c) of PTUs encoding (AMR+) or lacking (AMR−) antibiotic resistance genes. The beta parameters of the Heaps’ law were extracted from the fit of the rarefaction curve for each PTU. Boxplots are represented as in Fig. 1. Statistical tests (Mann–Whitney U): ***P ≤ 0.001; ****P ≤ 0.0001.