Many studies have shown that plant taxa tend to have high species richness in urban areas. In fact, plant diversity is positively correlated with human population density at some spatial scales. However, there is usually a caveat in studies of urban plant diversity, which is that species richness is high, but many urban species are non-native. Implicit in many urban ecological studies are assumptions that this non-native diversity has lower ecological value than native diversity. What is the evidence for and against these assumptions? Clearly, horticultural imports have resulted in catastrophic releases of exotic invasive species that have had many negative consequences for natural ecosystems. However, most horticultural introductions are not invasive. For these species, how do we evaluate their contributions to ecosystem function, habitat provision, and human wellbeing for the purposes of planning and designing functional urban landscapes?
Results/Conclusions
Here we review what is known about the ecological performance of native and non-native species from a range of perspectives, including plant water use, cooling benefits, habitat provision, aesthetic properties, and human perceptions. We discuss cases in which native functional types are largely unavailable (trees in non-forested ecosystems, for example) and for which an interdisciplinary framework for selecting species and plantings is needed. We pose the broader question: how can ecologists, designers, and planners help guide non-native and cultivated plantings to provide needed ecological functions in urban environments? While recommendations from ecologists most often rely on planting native species, results to date show that non-native species may sometimes “outperform” native species based on their habitat of origin. For example, in dry environments non-native species may have lower water requirements than native species, even when native species are intentionally selected for lower water use. Non-native trees also provide significant urban benefits such as cooling and shade provision in non-forested regions. While ecology has yet to develop a comprehensive framework for guiding the creation of non-native ecosystems, how can we more closely link ecological science to planning and design to develop a shared vision and process for shaping these novel ecosystems? We propose a strawman framework for discussion that requires contributions from the range of disciplines invested in planning, designing, and managing urban landscapes.