The couplings between members of an ecosystem are far more flexible and transient than the couplings between members of an organism. In the long view, ecologies are temporary networks. Although some links become hardwired and nearly symbiotic, most species are promiscuous in evolutionary time, shacking up with a different partners as the partners themselves evolve. In this light of evolutionary time, ecology can be seen as one long dress rehearsal. It's an identity workshop for biological forms.
Species try out different roles with one another and explore partnerships.
Over time, roles and performance are assimilated by an organism's genes. In poetic language, the gene is reluctant to assimilate into its code any interactions and functions directly based upon its neighbors' ways because the neighborhood can shift at any evolutionary moment. It pays to stay flexible, unattached, and uncommitted. At the same time Clements was right. There is a basin of efficiency that, all things being equal, will draw down a certain mix of parts into a stable harmony.