Herbivory and the evolution of divaricate plants: Structural defences lost on an offshore island

Abstract

AbstractMany island plants are characterized by unique morphology. For example, the high branching angles and small leaves of divaricate plants are a common feature of the New Zealand flora. The divaricate growth form may be an adaptation to deter browsing by extinct avian herbivores (moa); alternatively aspects of the insular climate may be responsible. However, our understanding of the selective pressures responsible for the high branching angles and small leaves of divaricate plants is incomplete. Here, I tested for differences in traits associated with the divaricate growth form between plants from Chatham Island and the New Zealand mainland. Moa never reached the Chatham Islands and its flora is derived from plants on mainland New Zealand. Therefore, I predicted Chatham Island plants to have lost morphological adaptations that may have deterred moa herbivory. Traits were quantified on 316 individuals in the field, allowing for 12 island‐mainland taxonomic comparisons. Chatham Island plants consistently produced smaller branching angles, larger leaves, shorter internodes and larger stems than related mainland plants. Results are therefore consistent with the hypothesis that selection for small leaves and high angled branching may be relaxed on the Chatham Islands due to an absence of moa. Smaller branching angles and larger leaves may offer a competitive advantage to Chatham Island plants.