|dc.description.abstract||New Zealand’s isolation and periods of marine transgression have limited its biota to an
extent which can be considered depauperate, even by island standards. Endemic vertebrates
are rare and prominent invertebrate families, such as the renowned forest defoliators of the
Lymantriidae, are absent. The proven vulnerability of the flora to introduced vertebrates
reaffirms a belief in the invasiveness of islands and fuels the contingency plans aimed at
averting similar devastation from further alien invertebrate defoliators.
Nothofagus is a dominant element of the climax forests of New Zealand and the larger
landmasses bordering the South Pacific Ocean. Assessments of the resistance to defoliation of
continental and New Zealand species of Nothofagus, and a range of other forest genera
endemic to New Zealand, was undertaken using bioassays of naïve polyphagous defoliators.
The bioassays were undertaken in Europe, utilising gypsy moth, Lymantria dispar
(Lepidoptera: Lymantriidae) and the fall webworm, Hyphantria cunea (Lepidoptera:
Arctiidae) as defoliators, fed foliage plants growing in European arboreta. In New Zealand,
bioassays utilised Australian painted apple moth, Teia anartoides (Lepidoptera:
Lymantriidae) and tree species from local arboreta, gardens and natural populations.
Larval growth rate was the primary parameter recorded to assess plant resistance. The
relevance of growth rate was investigated by comparison with other recorded parameters and
resistance to a surrogate pathogen, in the form of commercially available bio-insecticide.
Larval growth rate was positively correlated with survivorship, potential fecundity, mating
success and resistance to disease. The growth rate of larvae fed Nothofagus was positively
correlated to the species-specific leaf nitrogen content.
The results of the bioassays showed that despite the accepted paradigms, New Zealand’s flora
was largely resistant to exotic defoliators. As an explanation of this apparent anomaly, the
Island Resource Allocation (IRA) hypothesis was developed and posits that ‘the palatability
of a plant to invertebrate herbivores is proportional to the geographic range of the plant’. The
basis for the IRA hypothesis proposes a redefinition of the fundamental ecological principle
of the species: area relationship. Islands, or similarly geographically constrained ecosystems,
which support lower biodiversity, have impoverished trophic levels and consequently have
weaker top-down regulation of herbivores by natural enemies. The IRA hypothesis argues that
island ecosystem stability is achieved through the bottom-up process of plant defence.
The IRA hypothesis was tested intra-specifically using bioassays using painted apple moth in
which larvae were offered foliage of specimens from naturally discontinuous populations of
Nothofagus truncata. The results supported the hypothesis in that the smallest populations of
N. truncata exhibited the greatest resistance to the defoliator. The IRA hypothesis and a
demonstrated mechanism for a differential resistance in Nothofagus species could resolve a
number of enduring debates in ecology. Habitat area appears to explain the relative strengths
of top-down and bottom-up regulation of herbivores. It also predicts the strengths of
reciprocal evolution within the geographic mosaic of co-evolution and highlights the
influence of biodiversity in invasive ecology. It may also help to resolve the contentious and
extremely relevant debate of the role of biodiversity in ecosystem function.||en