  Although networks provide a powerful approach to study a large variety of
ecological systems, their formulation does not typically account for multiple
interaction types, interactions that vary in space and time, and interconnected
systems such as networks of networks. The emergent field of `multilayer
networks' provides a natural framework for extending analyses of ecological
systems to include such multiple layers of complexity, as it specifically
allows one to differentiate and model `intralayer' and `interlayer'
connectivity. The framework provides a set of concepts and tools that can be
adapted and applied to ecology, facilitating research on high-dimensional,
heterogeneous systems in nature. Here, we formally define ecological multilayer
networks based on a review of previous and related approaches, illustrate their
application and potential with analyses of existing data, and discuss
limitations, challenges, and future applications. The integration of multilayer
network theory into ecology offers largely untapped potential to further
address ecological complexity, to ultimately provide new theoretical and
empirical insights into the architecture and dynamics of ecological systems.
