Greenspace in cities retaining many “natural” attributes, supporting indigenous species and maintaining the integrity of ecological processes is highly valued. Conservation focused interventions typically focus on maintaining these values, using well-intentioned strategies such as removal of pervasive weeds. However, the complexity of ecological systems sometimes throws up curve balls for managers, where these intuitively sensible strategies result in perverse outcomes for biodiversity.
The aim of our work was to identify the effect of establishment of lantana (Lantana camara), a dominant, invasive, understorey weed, on insectivorous birds. We assessed how the presence of lantana affected bird assemblages, predation on insect models, and herbivory on dominant canopy trees.
We surveyed 18 bushland remnants in Sydney, Australia, half of which were dominated by lantana. Bird assemblages were surveyed multiple times over a 4 month period. At each site we also placed modelling clay caterpillars on dominant canopy trees, and assessed foliage on these trees for herbivore damage. We also assessed habitat complexity to teased out the effects of structure and vegetation identify in these systems.
Results/Conclusions
We found that lantana thickets were vital habitats for insectivorous birds in remnant habitat in urban contexts. These birds, central to regulating insect herbivores in urban ecosystems, were absent from almost all sites without lantana, most likely due to a combination of the pressures imposed by aggressive native honeyeaters and invasive predators such as cats and foxes. Elevated attack rates on model herbivores and decreased rates of herbivory on dominant trees were also associated with the presence of lantana, suggesting a surprisingly important ecological role of this invasive weed. While lantana increased overall habitat complexity, habitat complexity alone was insufficient to explain patterns in bird assemblages, herbivory and predation risk for insects.
These results support the hypothesis that the loss of insectivorous birds from high value areas of nature in cities contributes to declines in vegetation health, through increases in insect attack. It also points to one of the great challenges of managing nature in cities; the unpredictability of the array of interactions in these novel ecosystems. Our work supports suggestions that management for conservation in urban contexts requires novel thinking to accommodate the ecological pressures driven by the diverse and emerging ecological interactions in these systems.