Ecology is a branch of biology that studies the interactions between living organisms and their environment, including other living organisms and non-living physical and chemical factors. Ecosystems are dynamic and complex systems, where energy and matter flow through the food web and biogeochemical cycles. The structure and function of ecosystems depend on the multiple processes that occur at different levels of organization, from individual organisms to populations, communities, and landscapes. Ecologists use a variety of tools and methods, such as observation, experimentation, modeling, and remote sensing, to understand the patterns and processes of ecosystems and to address environmental issues, such as climate change, habitat fragmentation, and biodiversity loss.