Journal of Vegetation Science && (2017) Biotic homogenization in an increasingly urbanized temperate grassland ecosystem Ben J. Zeeman, Mark J. McDonnell, Dave Kendal & John W. Morgan Keywords Alien species; Biotic differentiation; Disturbance; Extinction; Habitat fragmentation; Immigration; Invasion; Plant functional traits; Vegetation dynamics Nomenclature The International Plant Names Index: http://ipni.org/ Received 15 March 2016 Accepted 14 December 2016 Co-ordinating Editor: Sebastian Schmidtlein Zeeman, B.J. (corresponding author, BZeeman@ltu.edu.au) 1 , McDonnell, M.J. (markmc@unimelb.edu.au) 2 , Kendal, D. (dkendal@unimelb.edu.au) 2,3 , Morgan, J.W. (J.Morgan@latrobe.edu.au) 1 1 Department of Ecology, Environment and Evolution, La Trobe University, Bundoora, Vic. 3086, Australia; 2 School of BioSciences, The University of Melbourne, Burnley, Vic. 3010, Australia; 3 School of Ecosystem and Forest Sciences, The University of Melbourne, Burnley, Vic. 3121, Australia Abstract Question: How does urbanization and associated declines in fire frequency alter the floristic composition of native temperate grasslands? Does it lead to: (1) bio- tic homogenization, i.e. compositional similarity between remnants increases; (2) biotic differentiation, whereby similarity between remnants declines, or; (3) clustered differentiation, where similarity between remnants remains unchanged, but composition shifts from the historical state? Location: Victoria, Australia. Methods: Using site-level surveys, we examined changes in the floristic similar- ity of 29 urban grasslands from 1992 to 2013 and compared these changes to those of 63 rural grasslands from 1989 to 2014. Community-level changes in the representation of key functional traits were also examined in urban grasslands, with traits advantaged following disturbance regime change and urban fragmentation predicted to increase in frequency. Results: Our results supported the biotic homogenization hypothesis in urban grasslands. Compositional similarity between grasslands increased principally because of an increase in commonly shared non-native species, with change in native composition comparatively minor. However, no evidence of biotic homogenization was found in rural grasslands, with no significant change in overall composition identified. The most urbanized sites had the highest number of non-native species in both the current and historical data sets, yet non-native composition over the past two decades changed the most in sites on the urban fringe, becoming more similar to sites closer to the urban core. As expected, fol- lowing declines in fire frequency and increased urbanization, the overall compo- sition of urban grasslands shifted to taller plant species, while native species capable of vegetative reproduction and exotic species with an annual life span increased in frequency. Conclusion: Urbanization was an important driver of biodiversity change in the investigated system, with increasing competition intensity in response to distur- bance regime change a likely cause of biotic homogenization. Our results demon- strate that non-native species are a key driver of biotic homogenization, emphasizing the importance of managing non-native immigration and maintain- ing historical disturbance processes once native ecosystems become urbanized. Introduction Human activities are transforming native ecosystems across the globe (Vitousek et al. 1997; Steffen et al. 2007). Urbanization is a particularly threatening pro- cess, reducing the area of native vegetation and isolat- ing remnant patches through fragmentation (McDonald et al. 2008; Secretariat of the Convention on Biological Diversity 2012). Once bound within the urban matrix, resource supply between communities is disrupted and species dispersal is constrained (Ricketts 2001; Williams et al. 2006; Ibanez et al. 2014). Additionally, historic disturbance regimes are altered (Harrison & Bruna 1999), and environmental conditions are modified (Grimm et al. 2008), shifting the nature of species interactions, and creating novel ecosystems through 1 Journal of Vegetation Science Doi: 10.1111/jvs.12507 © 2017 International Association for Vegetation Science