Mobile genetic elements (MGEs) are integral members of microbial communities, interacting with cellular microbes as well as other mobile genetic elements (MGEs), and affecting the dynamics and evolution of cells, populations, and microbiomes. Conjugative plasmids are MGEs that can transfer themselves between bacteria, forming a major route for the transmission of diverse adaptive traits in microbial communities. Often MGEs are considered as features of the microbes that harbour them, but in carrying genes for their own replication and transmission, plasmids have their own fitness 'interests', generating conflicts with other genes in the cell. I use experimental approaches to understand these conflicts and their consequences for plasmid evolutionary ecology, focusing on a family of megaplasmids (>150 kb) in Pseudomonas spp. I will describe how ecological and evolutionary factors can sustain plasmids in bacterial communities.