Understanding biodiversity responses to human impacts will first require an understanding of how the strength of ecological selection, defined as deterministic differences in fitness among individuals of different species (Box 1, Vellend 2016), differs between ecosystems (Figure 3, Table 1). This is because human impacts such as land use, warming and invasive species alter the selection regime by modifying either abiotic gradients and heterogeneity (e.g., climate change velocity, Loarieet al. 2009) or biotic interactions such as competition, predation and mutualism (e.g., novel species interactions, Alberti 2015; Alexander et al. 2015). Below we compare abiotic and biotic aspects of selection across terrestrial and freshwater ecosystems, and how humans alter the selection regime through various global change drivers.