Trophic-level responses differ at plant, plot, and fragment levels in urban native forest fragments: a hierarchical analysis

Abstract

1. Using a hierarchical generalised linear model we examined effects of landscape fragmentation (fragment area, isolation, and urbanisation of the surrounding matrix), within-fragment habitat quality (host plant abundance and plant species richness), and properties of the individual trees, on plant herbivore and parasitoid abundance. 2. The present study was conducted in 10 present-day fragments of native forest near Wellington, New Zealand ranging in size from 12 to 1068 ha. The tri-trophic study system consisted of the plant Macropiper excelsum, its main insect herbivore Cleora scriptaria, and two parasitoid wasps: the braconids Aleiodes declanae (endemic specialist) and Meteorus pulchricornis (exotic generalist). 3. Parasitism by Aleiodes declanae was significantly higher in more isolated fragments and was negatively correlated with parasitism rates by other species, and positively correlated with tree size. 4. Parasitism by Meteorus pulchricornis was not related to landscape fragmentation, but was positively related to host larval densities measured at the individual plant-level, and declined with increasing plant species richness at the plot-level. 5. Herbivory was positively related to the local abundance of host plants, but was unrelated to the fragment-level properties. 6. Species showed individualistic associations to habitat fragmentation, with properties of within-patch quality often proving to be more important to the abundance and composition of this tri-trophic community. Parasitoids appear to display a variety of associations to fragmentation, rather than having a typical or general relationship as predicted by the trophic level hypothesis.