The world we inhabit is constantly increasing in complexity and with this comes a need for new ways of understanding and thinking that can manage this complexity. A key part of managing this complexity involves constructing models of ecosystems of systems and reasoning about their properties. One approach to modelling systems has been based on a process-algebraic formulation of an abstract view of distributed systems. Here, building on this view, we concentrate on modelling ecosystems of systems by developing a rather general concept of interface between systems. Interfaces between models describe how they can be composed together to construct a model of an ecosystem. By introducing the idea of local reasoning, building on work in program analysis and verification, we develop tools for analysing the behaviour and performance of ecosystem models in an efficient way.