Population dynamics of translocated Frithia humilis : an endangered sandstone endemic
Jansen, Peter Gerald
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Frithia humilis Burgoyne is an endangered succulent and edaphic specialist that is endemic to the Rand Highveld Grassland between the towns of Middelburg, eMalahleni (Witbank) and Bronkhorstspruit. The species is restricted to sedimentary Dwyka and Ecca sandstone rock plates (Karoo Supergroup). Underlying these rock plates are valuable coal deposits and as such coal mining has become the greatest threat to this species. In 2008 a population of F. humilis was discovered at the Inyanda Coal mine (Exxaro mining group), north of eMalahleni, before mining activities commenced. In situ conservation was impossible due to open cast mining practices. Thus an alternative solution was required to save the population. Consequently Exxaro, in cooperation with the South African Biodiversity Institute and the Mpumalanga Tourism and Parks Agency, translocated the population to three receptor sites within the species‟ natural distribution range. The first receptor site‟s substrate consisted of the typical Ecca sandstone habitat and received the majority of plants. Two smaller groups of plants were also translocated to two different receptor sites for experimentation. The substrates of these atypical habitat types consisted of outcrops of the sedimentary Wilge River Formation (Waterberg Group) and the igneous felsite outcrops of the Rooiberg Group (Transvaal Supergroup). Translocation is still a controversial method of conservation due to its numerous challenges and varied results. However it is useful for species preservation, population augmentation and research purposes. Furthermore, as pressure on natural habitats and endangered species increases, translocation may inevitably become a vital part of conservation methodology. Since no similar conservation effort has been made for a succulent plant species in South Africa, a monitoring program was initiated in 2010 to establish whether translocation is a feasible method of conservation for F. humilis. Subsequently, a repeatable monitoring programme was established to gather baseline data and to monitor post-translocation progress. In addition, a population at Ezemvelo Nature Reserve also occurring on Dwyka and Ecca sandstone, was chosen as control for comparison with translocated populations. In this study, population monitoring was continued with the purpose of supplementing existing knowledge of pollinators, investigating fecundity at the various receptor sites and determining the health of the population structure for the translocated populations compared to a control population. Observations for pollinators were made at two of the receptors sites and captured using hand nets. Insect identification was done with the help of international experts while examination for pollen was done using a stereomicroscope and scanning electron microscope. The fecundity of translocated populations was established based on count data which was analysed using a linear mixed model. The number of flowers, fruits, seedling, sub-adults and adults were compared to a control population and between populations. Habitat variables influencing the health of translocated plants were also investigated using non-metric multidimensional scaling and principal component analysis. Population structure at various receptor sites was analysed in terms of size-class distribution using linear regression analysis, Permutation Index, Simpson‟s index of dominance and quotient analysis. Results were compared to a control population and between translocation populations. Initial qualitative results revealed that pollinators were generalists consisting of bees and flies (Apidae, Megachilidae (Hymenoptera) and Bombyliidae (Diptera)). Observations for pollinators reinforced previous findings that pollination is not a limiting factor to population reproduction. Carriers of F. humilis pollen included Notolomatia sp. (Bombyliidae), Paragus sp. (Syrphidae), Ammophila sp. (Sphecidae), Lipotriches sp. and Seladonea sp. (Halictidae) and Quartinia sp. (Masarina). These species extended the list of visitors to F. humilis flowers and support the standing Mellitophilous pollination syndrome, while also presenting the possibility of an alternative syndrome. Several primary pollinators were suggested, mostly bees, based on the distribution and number of observations, while reserve pollinators were identified from several different genera. Fruit production as a percentage of adult plants indicated that pollination was more successful at receptor sites than the control population, though this may be density related. Furthermore, the high percentage of fruit production suggested that one or more of the observed insect species is likely an effective pollinator and may be confirmed from among those observed in this study. Population analysis was based on new and previously collected data. In addition to investigating the fecundity of translocated populations, specific habitat conditions were identified which influenced the health and recruitment of plants, particularly in the typical receptor site. Reproduction was found to be stable and functional for all populations. Results in terms of numbers of seedlings, sub-adults and adults varied between translocated populations. Generally, populations showed declines for all life stages, though these were not as severe for some as for others. Habitat quality was found to have the greatest influence on population performance. Although flowering and fruiting was occurring at high percentages for the translocated populations seedling and sub-adult numbers have been declining over time. This is ascribed to lack of suitable micro-habitats for seedling establishment, and patch deterioration and competition limiting the establishment of sub-adults. These problems occurred at both a-typical and typical geologies, though less so at the latter, in patches which maintained habitat conditions similar to that of the control population. The health of translocated populations was determined by examining size class distribution, population slope, stability and evenness, and comparing it to a control population. It was determined that some instability and variation within even the natural populations may be normal. Two of the translocated populations showed relatively healthy size class distributions, population slope, stability and evenness, while for the other two these variables did not fall within the limits set by the control population, indicating unhealthy population structures. The healthiest populations occurred on the typical F. humilis geologies while the unhealthiest populations occurred on a-typical geologies. Despite past indicators supporting translocation as a feasible conservation tool, results from this study suggest that translocation is not a long term conservation solution for F. humilis since current trends suggest continued habitat and consequential population deterioration. Further studies have to be conducted on the species‟ habitat requirements and the selection of suitable receptor sites. Until our understanding of the habitat preferences of the species is sufficiently increased the translocation of this species is strongly discouraged in favour of in situ conservation.