The conservation genetics of the Clanwilliam cedar (Widdringtonia cedarbergensis)

Master Thesis


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University of Cape Town

Widdringtonia cedarbergensis is an endangered conifer species in the western Cape, South Africa. This species is under serious threat of extinction and is being actively managed by Cape Nature Conservation in a small section of the Cedarberg mountains in an attempt to boost population numbers with a seedling replanting scheme and preservation of adults from fires. This study set out to determine levels of genetic diversity and fitness within and among populations of the Clanwilliam cedar for the following reasons: (1) to assess the level of genetic diversity; (2) to screen the seed source for the replanting programme; (3) to locate vigorous seedling sources for replanting and (4) to determine the effect of population size on genetic diversity and fitness. Starch gel electrophoresis was employed to assess levels of genetic variation within and among seven populations of W. cedarbergensis. W. nodiflora and W. schwarzii, two congeneric species, were incorporated into this section of the thesis as benchmarks against which to compare levels of genetic variation in W. cedarbergensis. The three species are different with regard to biology and distribution and predictions as to their population genetic structures were set up accordingly. A major difference in their biology is the resprouting behaviour of W. nodiflora in response to fire. Fitness components in populations of W. cedarbergensis were divided into reproductive and "ecological" traits, and seedling growth traits. Reproductive and ecological traits were measured in the field and seedling growth traits were obtained from a seedling growth experiment carried out in the glasshouse. Overall, 15 potential fitness traits were measured. The same seven populations were tested for differences in these fitness traits. This enabled an assessment of the seed source used for replanting, as well as alternative seed sources. All fitness variables were tested for a significant relationship with genetic variation measured as heterozygosity. The populations were rated according to population size, density and isolation and correlated with heterozygosity to determine whether there was any relationship. Seventeen enzyme loci were resolved for each species. Estimates of genetic diversity showed that W. cedarbergensis and W. schwarzii had low allelic variation. This was attributed to the effect of bottlenecks. High levels of inbreeding and population substructuring were found in W. cedarbergensis which suggested that trees were selfing possibly due to limited pollen movement between trees as a result of tree isolation and fine-scale fragmentation incurred by fires. Although allelic diversity in W. nodiflora was extremely high, high levels of inbreeding were found within populations which was attributed to selfing among resprouted ramets of the same genet. Tests for differences in fitness traits between populations revealed no population as the most consistently fit for reproductive and ecological traits for the fittest population, although two populations, DG and CPS, were consistently found to have the most vigorous seedlings. The replanting seed source, MB, showed adequately vigorous seedlings. Four out of fifteen measures of fitness were found to co-vary with heterozygosity. These were embryo abortion fraction, germination rate, shoot biomass and total biomass. Seeds and seedlings were more vulnerable to the effects of inbreeding than traits related to fecundity. These four traits, in tum, co-varied with other traits. The relationship between germination rate and seedling:parent ratio, in particular, indicated that genetic phenomena are impacting the demography of populations. Reductions in fitness occurred in several traits below 30% heterozygosity and became critical below 25% heterozygosity. 30% heterozygosity occurred below a population size scale of 4 (4000 individuals) and 25% heterozygosity occurred below a population size scale of 1 (250 individuals). Population size, therefore, seemed important in maintaining the genetic diversity of populations. The effect of population density on fitness was not effectively examined in this thesis and deserves further attention. The results of this thesis had several implications for conservation and active management. My recommendations were the following: (1) two populations, DG and CPS, are the best seed sources for the replanting programme and should supplement the current and most accessible seed source (MB); (2) Replanting should be aimed at boosting seedling:parent ratios in small populations such as WB and KK, as well as closing gaps between trees and clumps of trees as far as possible to facilitate pollen movement and therefore outcrossing; (3) the bottleneck is at a critical stage where adult tree survival is of profound importance in ensuring seedlings are outbred and every effort should be made to reduce mortality due to fire.