The conservation biology of Pittosporum obcordatum: conservation genetics and habitat specificity
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Abstract
Pittosporum obcordatum (Pittosporaceae) is a threatened lowland shrub, primarily found in alluvial
sites with widely separated populations throughout the North and South Islands of New
Zealand. Threats to P. obcordatum include small population size, habitat loss, competition with exotic
plants and grazing from introduced mammals. The primary goal of this thesis was to was to better
understand genetic and ecological factors that could assist with the conservation of P. obcordatum. I
used Inter\Simple Sequence Repeat (ISSR) markers to study the genetic diversity of 128 individuals
from 10 populations of P. obcordatum from Kaitaia to Fiordland. Populations of P. obcordatum had
low to moderate genetic diversity, and smaller populations contained less genetic diversity than larger
populations. A high degree of geographic genetic structure was found, suggesting little or no recent
gene flow between populations. Small population sizes and geographic isolation of the populations
likely have an effect on this. Based on this data, recommendations are made for site\specific
restoration planting designed to maximise population size and genetic diversity, accompanied by
conservation strategies targeting threats from land use. Further, a newly discovered population of P.
obcordatum on Banks Peninsula has been found growing on a hillside, and this thesis investigated
why the species was growing in an unexpected habitat. When pot\grown seedlings from Banks
Peninsula and a Fiordland alluvial flat population were subjected to drought, mortality was lower for
Banks Peninsula seedlings than for Fiordland but not significantly so. When radishes (Raphanus
sativus) were used as a phytometer, Banks Peninsula soil produced significantly less biomass than
soil from three alluvial flats. Taken together, these findings indicate that slope and soil fertility have
less of an impact on the success of P. obdorcatum than would previously have been assumed based
on its alluvial sites, indicating that further populations may yet be found on other hillsides or in less
fertile areas. Search and further genetic study of such populations are therefore recommended in
order to elaborate on the habitat and genetic profile advanced by this research, allowing for a well
rounded approach to conservation planning.