While studies have compared differences in processes and properties for terrestrial and marine habitats (May et al. 1994; Grosberget al. 2012; Webb 2012), comparisons are lacking for terrestrial and freshwater ecosystems, which are highly threatened (Carpenteret al. 2011; Belletti et al. 2020). Here, we aim to build a more complete understanding of how and why human impacts vary among terrestrial and freshwater ecosystems by identifying differences and similarities in the fundamental processes shaping communities (Figure 1). To do this, we build on theory that distinguishes four fundamental processes (Box 1) that comprehensively describe how species are gained and lost from communities (Vellend 2010, 2016): dispersal, speciation, ecological selection (hereafter selection) and ecological drift (hereafter drift). These processes capture the mechanisms by which community attributes such as species richness, species-abundance relationships and species turnover emerge, and are general enough to allow for comparisons across many ecosystem types. Importantly, the relative importance of these processes is likely modulated by ecosystem-specific physical and spatial attributes, for example the properties of media (e.g., air vs water), or geometric constraints of habitat (e.g., open vs dendritic, Table 1). Thus, our goal is to develop mechanistic bridges between global change drivers and their impacts by explicitly considering both the fundamental processes and properties shaping ecosystems (Figure 1, Table 1).