Natural systems are challenged by invasions, extinctions, urbanization, and disturbance. Some species (or genotypes) persist despite these challenges, whereas others are lost. This dissertation asks [1] which species' attributes predict their ability to respond to environmental change, [2] how do changes in the composition of plant communities affect system functioning, and [3] can we use information about how species interact in diverse communities to inform the design of urban systems? Chapter 1 addresses the first question with an examination of plasticity among native and exotic congeners in response to altered competition and fertilization. We found that weedy exotics were not more plastic than natives, but instead that plasticity was more similar within genus. Chapter 2 and Chapter 3 explore how changes in diversity impact natural systems. Species richness is generally increasing due to the introduction of exotic species, and Chapter 2 asks how changes in exotic versus native plant diversity impact plant productivity and arthropod community structure. We found that diverse exotic communities were equally, if not more, productive than native communities and that they recruited an equally abundant and diverse arthropod fauna. However, exotics diminished the relative fruit production of co-occurring native species and recruited fewer arthropod species than natives. Chapter 3 provides the first direct comparison of how changes in genotypic diversity compare to changes in species diversity. We show that increasing either genotypic diversity of common evening primrose (Oenothera biennis) or old-field species diversity resulted in nearly equivalent increases in aboveground primary production. Arthropod species richness also increased with both types of plant diversity. Finally, Chapter 4 integrates ecological principles from the biodiversity-ecosystem functioning literature into the design of vegetated rooftops. Most green roof plantings include only one or a few drought-tolerant species. We review the green roof and ecological literature to establish a clear research agenda for creating diverse and dynamic green roof ecosystems.