New Zealand divaricate plant species: Tensile strength and Remote Island occurrence

Abstract

Divaricate plant species account for more than 10% of New Zealand/Aotearoa's woody flora, a higher proportion for that life form than anywhere else on earth. Two main hypotheses have been proposed to account for the prevalence of this phenotype. The first suggests herbivorous birds, particularly large flightless moa, exerted selective pressure on many plants to adopt the form. The second proposes climate as the main driver since insular New Zealand has many exposed habitats and, historically, experienced stressing conditions during the Pleistocene. Our study investigated two questions to shed further light on the evolution of divaricates, in particular related to the potential influence of browsing by large avian herbivores. First, divaricate plants have been posited to have a higher tensile strength than non-divaricates as a defence mechanism against moa browsing. We tested the great majority of New Zealand plant genera within which divaricates occur, contrasting the stem tensile strength of these plants against their closest non-divaricate counterparts using accurate testing technology. The results indicate divaricate species have tensile strength that approaches twice that of non-divaricates across a broad and disparate phylogenetic range that includes gymnosperms and many clades of angiosperms. Second, we tested the species level distributions of widely dispersed woody plant genera across the New Zealand archipelago on islands where moa had been present or always absent. We found that no endemic divaricate species occurred on any islands from which moa had been permanently absent. In contrast, their non-divaricate counterparts were commonly endemic to those same islands.