No evolution of increased competitive ability or decreased allocation to defense in Melaleuca quinquenervia since release from natural enemies

Abstract

If invasive plants are released from natural enemies in their introduced range, they may evolve decreased allocation to defense and increased growth, as predicted by the evolution of increased competitive ability (EICA) hypothesis. A field experiment using the invasive tree Melaleuca quinquenervia was conducted to test this hypothesis. Seeds were collected from 120 maternal trees: 60 in Florida (introduced range) and 60 in Australia (home range). Plants grown from these seeds were either subjected to herbivory by two insects from Australia that have recently been released as biological control agents or protected from herbivores using insecticides. Genotypes from the introduced range were initially more attractive to herbivores than genotypes from the home range, supporting EICA. However, genotypes from the introduced and home range did not differ in resistance to insects or in competitive ability, which does not support EICA. Plants from the introduced range had a lower leaf hair density, lower leaf: stem mass ratio, and a higher ratio of nerolidol: viridifloral chemotypes compared to plants from the native range. Plants with an intermediate density of leaf hairs and with high specific leaf area were more susceptible to herbivory damage, but there were no effects of leaf toughness or chemotype on presence of and damage by insects. Herbivory had a negative impact on performance of Melaleuca. Other than an initial preference by insects for introduced genotypes, there was no evidence for the evolution of decreased defense or increased competitive ability, as predicted by the EICA hypothesis. It does not appear from this study that the EICA hypothesis explains patterns of recent trait evolution in Melaleuca.