Elevated levels of insect herbivory on an urban-rural gradient reveal species-specific signals of landscape degradation in novel ecosystems

Loss of biodiversity is a major factor contributing to declines in the quality of remnant vegetation in urban landscapes.  These novel ecosystems often support depauperate assemblages, with losses from higher trophic levels contributing to the decline of vital ecosystem processes.  The aim of this work was to identify the extent to which levels of insect herbivory in urban remnants could be predicted from landscape and plant traits, ultimately identifying benchmarks for ecological function. I sampled arboreal insects and assessed levels of herbivory for chewing, mining and galling along an urban-rural gradient in Sydney for four prominent plants, Banskia serrata, Angophora costata, Acacia longifolia and Pittosporum undulatum.  The damage caused by different functional groups of insect herbivores varied significantly across plant species, with A. costata and B. serrata suffering the highest levels of chewing damage and  P. undulatum showing highest levels of galling and mining. Herbivory increased on A. costata and B. serrata in non-urban areas while P. undulatum suffered reduced herbivory in urban areas.  There was no coherent signal of herbivory for all 4 plant species across the urban-rural gradient owing to the extensive variability among sites.  The results revealed idiosyncratic associations with degree of urbanization across plant species, with fine scale habitat traits consistently being more informative in predicting levels of herbivory than coarse habitat traits.  While these results showed that urban remnants supported vegetation suffering significant levels of herbivory, the goal of identifying benchmarks of ecological function in urban landscapes required species-specific approaches accommodating coarse and fine scale landscape traits.