Beyond model systems in urban ecology: A multi-scale multi-species assessment of disruptions to insect-plant interactions along an urban-rural gradient
Loss of biodiversity is a major factor contributing to declines in the quality of remnant vegetation in urban landscapes. Disruptions to ecological processes driven by biodiversity shifts, such as regulation of herbivores by higher trophic levels, are often described in these novel ecosystems. However, seminal studies revealing these patterns typically focus on single species approaches to describe them. In addition to focusing on these model systems, work linking vegetation condition to landscape context often fails to consider finer scale mechanistic drivers of change. The aim of this work was to identify how landscape and plant traits assessed at multiple spatial scales could predict the patterns in insect herbivory in urban ecosystems, examining multiple plant species occurring in relatively undisturbed continuous forest, remnant urban forests and as isolated street trees. I sampled arboreal insect communities and assessed levels of herbivory for chewing, mining and galling along an urban-rural gradient in Sydney, Australia for four prominent plants, Banskia serrata, Angophora costata, Acacia longifolia and Pittosporum undulatum. I then assessed the extent to which landscape type, site characteristics and microhabitat characteristics could be used to predict patterns of change within and among the 4 focal species.
There was no coherent signal of herbivory across the urban-rural gradient owing to the extensive variability among plant species and sites. Isolated trees showed the greatest variation in both level of herbivore damage and turnover of insect herbivore species. Tree traits varied significantly among landscape types, with isolated trees being shorter and having denser canopies than in their natural settings. The damage caused by different functional groups of insect herbivores also varied significantly across plant species and landscape contexts. A. costata and B. serrata suffered high levels of chewing damage while P. undulatum showed highest levels of galling and mining. Herbivory increased on A. costata and B. serrata in urban areas while P. undulatum suffered reduced herbivory in urban areas. 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 urban remnants supported vegetation suffering significant levels of herbivory, the inability to identify a “holy grail” of a consistent vegetative and insect response to urbanization, so often predicted by single species studies, confirms the need to adopt multi-species approaches considering coarse and fine scale landscape traits to assess declines in remnant vegetation in urban ecosystems.