The response of dependent communities to ontogenetic and genetic change in Eucalyptus : the case of the Eucalyptus globulus x nitens hybrid system

The abundance of 44 dependent taxa (insects and fungal pathogens) was censused on three year old trees of Eucalyptus globulus, E. nitens and their hybrids (F\\(_1\\), F\\(_2\\) and backcrosses) in a field trial, half of which had undergone the transition to adult foliage (heteroblastic) while the remainder still retained juvenile foliage (homoblastic). This ontogenetic change was shown to have an enormous impact on the composition of dependent communities, far exceeding the effect of genetic differences between the pure species and their hybrids. The dependent communities on the adult foliage was distinct from the community on the juvenile foliage, regardless of whether the juvenile foliage occurred in the upper or lower canopy. Significant differences in the communities was demonstrated on the upper and lower canopy of the homoblastic trees, clearly emphasising the importance of removing positional effects per se in studies of ontogenetic responses by dependent species. Eighty percent of the common taxa exhibited significant specialisation for either juvenile or adult foliage. Complex ontogenetic specialisation was shown by the Chrysophtharta agricola leaf eating beetle a significant pest species of eucalypt plantations in Tasmania. The adult beetle fed on the adult foliage of the eucalypts but the eggs were laid and the larvae predominantly fed on the juvenile foliage. This preferential response for adult foliage by the adult beetle was maintained in laboratory feeding trials clearly indicating a response to changing foliage characteristics. The dependent communities on E. globulus and E. nitens, the two most important plantation eucalypts in Tasmania, were shown to be significantly different on juvenile foliage but not adult foliage. Of the 20 common taxa studied 35% showed significant specialisation to one or other eucalypt species. Most species generalists exhibit either no response to F\\(_1\\) hybrids or tend to be more abundant on the hybrid (hybrid susceptibility). In contrast species specialists showed either dominant or additive responses to the F1 hybrids. On average trees of E. globulus and E. nitens supported equal numbers of dependent taxa, but all hybrid classes supported significantly greater numbers of dependent taxa than the pure species. This study is one of the few to remove confounding positional and genetic effects when studying the response of dependent taxa to ontogenetic change and is the first study to show a significant species response to ontogenetic variation at the community level. This is important in establishing a genetic basis to the increased dependent taxa reported .previously on hybrids as it has not confounded responses with differences in heteroblasty between pure species and hybrids hosts.