Simultaneous ongoing changes to earth's ecosystems, including climate change and species invasions, are reshuffling ecological communities in space and time. Spatially, species distributions are shifting, often in species-specific ways, leading to novel communities. Changing climate is also altering species’ phenologies – i.e., the seasonal timing of life cycle events such as flowering, bird migration, or insect emergence. Phenological responses to climate have served as useful indicators of climate change (e.g. spring is now arriving significantly earlier than a century ago), but the large variability among species may alter how species interact. In this talk, I will discuss how ecologists are using a variety of large datasets to understand how these global changes affect ecological communities. After a broad overview of how 'big data' are being used for ecological questions, I will present a few examples from our own work using forest inventory data, tree ring data, continental-scale citizen science observations, herbarium records, and next-generation sequencing data. Our overall aim is to use global changes as unique, large-scale “natural experiments” to test basic ideas about population dynamics and community assembly, while also building capacity to forecast ecological responses to ongoing change.