Complex effects of fragmentation on remnant woodland plant communities of a rapidly urbanizing biodiversity hotspot

In many cities worldwide, urbanization is leading to the rapid and extensive fragmentation of native vegetation into small and scattered urban remnants. We investigated the effects of fragmentation on plant species richness and abundance in 30 remnant Banksia woodlands in the rapidly expanding city of Perth, located in the southwestern Australian global biodiversity hotspot. We considered a comprehensive set of factors characterizing landscape fragmentation dynamics (current and historical remnant area and connectivity, time since isolation, and trajectories of landscape change), disturbance regimes (fire frequency, grazing, and intensity of human activities), and local environmental conditions (soil nutrient status and litter depth). We used generalized linear mixed models to determine the interactive effects of time since remnant isolation and remnant area on plant species richness, and structural equation models to disentangle the direct and indirect effects of landscape and local factors on plant species richness and abundance. Fragmentation impacts were most rapid in smaller remnants. Indeed, in the small remnants, native plant species richness was halved in only a few decades after isolation, suggesting an underlying rapid loss of habitat quality. We found that richness and abundance of woody species were higher in historically large remnants and lower in the rural areas, despite these rural remnants showing greater connectivity. Richness of native herbaceous species declined with time since isolation, mainly in the smaller remnants, and this was associated with altered soil properties. Furthermore, increased litter depth (possibly indicating higher productivity) and increased abundance of nonnative herbaceous species in the older and smaller remnants was associated with a decline in the abundance of native herbaceous species. Our study suggests that in rapidly expanding cities, landscape fragmentation can have major and complex effects on remnant vegetation. Yet these impacts might take several decades to manifest themselves. Hence, understanding the long-term conservation capacity of newly formed remnants, which is key to setting conservation priorities, requires consideration of landscape fragmentation and land use history. Moreover, the smaller and older remnants may already display changes due to fragmentation, providing clues for urban planning and ecosystem management that help to protect urban remnant plant diversity.