Summary: How does environmental change reshape ecosystem functioning? Animals play a key role in ecosystem dynamics because their interactions structure ecological communities and influence ecosystem processes. Yet most biodiversity–ecosystem functioning research treats species as uniform entities with fixed ecological roles. In reality, most animals undergo profound shifts in ecological role throughout development, as physiology, resource use, and interactions change with growth. These shifts generate complex interaction networks that can drive nonlinear ecosystem responses, including trophic cascades and abrupt regime shifts. I will show how integrating organismal development and eco-evolutionary dynamics can reshape our understanding of ecosystem functioning, and how future research combining models, experiments, and field data can help us predict ecosystem responses to environmental change