Abstract:
New Zealand separated from the super continent Gondwana 85 million years ago and was isolated until 800y BP. The country has no endemic mammals except for three bats, and this led to the radiation of a unique avian fauna to fill available niches. Moa (Dinornithiformes) were large flightless herbivores and until recently were present on all three main islands together with many close satellite islands.
Divaricating plants in New Zealand have received a great deal of attention over the last 50 years, due to their unique form and the evolutionary puzzle they represent. Divaricate species account for over 10% of New Zealand’s woody flora, a higher proportion for this life form than anywhere else on earth. Two main hypotheses have been suggested to account for the divaricate phenotype. The first suggests herbivorous birds, particularly moa, produced to a high selective pressure on many plants to adopt the form. The second theory suggests climate as the main cause for the divaricate form, as New Zealand has many highly exposed regions, and historically has had harsh and stressing conditions during the Pleistocene. Many divaricate species are found on edges and in open regions where exposure could be a significant evolutionary influence.
This study investigated three questions to shed further light on the evolution of the divaricate form. First, divaricate plants have been hypothesised to have a higher tensile strength than non-divaricates under the influence of moa browse. However, prior studies which have investigated this have used small sample sizes, manual forms of testing and nursery specimens. In this study I tested every genus within which divaricates occur, contrasting the tensile strength of these plants against their closest non-divaricate counterparts using modern tensile strength testing technology. The results are statistically significant and show divaricate species have a considerably higher tensile strength than non-divaricates. These results favour the moa-hypothesis as species with high tensile strengths would have been more difficult for ratites to break and consume with a clamp and pull motion.
Second, polyploid species which have multiple sets of chromosomes above a diploid state often indicate probable speciation events and have been found to occur in climatically stressing conditions. In this study I compared divaricate species against non-divaricate species in terms of ploidy, to investigate whether divaricates have undergone more events of elevated ploidy than non-divaricates in recent history. The results show some evidence of a difference between divaricate and non-divaricate ploidy levels, with which divaricates have a greater ratio of polyploid species. My results also show in the genus Melicytus, only divaricate species are polyploid. Underlying causes for polyploidy may not, however, necessarily be closely tied with divarication. Rather, the ecological elasticity provided by elevated ploidy states may be a mechanism for plants to rapidly evolve and survive in harsh unstable climates.
Third, I tested the distribution of genera exhibiting both divaricate and non-divaricate species relative to island groups in the New Zealand archipelago where moa have been present or always absent. The plant genera for this study were selected based on their having both divaricates and non-divaricates within them and also having at least one species present on a remote island group that had never had land bridge contact with the New Zealand mainland (i.e. a genera that had an overwater dispersal capability). The results were statistically significant and showed no divaricate species are endemic to any of the remote islands where moa were always absent. This indicated that moa presence on the mainland islands of New Zealand was a possible evolutionary driver for the divaricate form.