The lack of any measurements of the secondary production of the prosobranch snail Sulcospira hainanensis (Bröt, 1872) (Pachychilidae), an abundant primary consumer in Hong Kong streams, represents a major gap in our knowledge of the magnitude of benthic productivity in these systems where S. hainanensis constitutes a substantial proportion of community biomass. This study investigated the population dynamics and production of S. hainanensis between February 2011 and February 2012 in 4 Hong Kong streams (2 unshaded and 2 shaded). Annual production by amphibiotic insects was also quantified by emergence traps to evaluate the importance of in-stream production by fully aquatic animals versus that leaving streams in the form of adult aquatic insects. In addition, snail grazing effects on algae and the potential for consequent competitive interactions with benthic insects was compared under different shading conditions and between wet and dry seasons. Sulcospira hainanensis was ovoviviparous and had balanced sex ratios. Brood size ranged from 52 to 1189, and 3 recruitment episodes were observed in both populations in the one-year study period. Hatchlings reached sexual maturity in 3 – 5 years whereas adults could live for 6 – 12 years. Mean population densities varied from 108.4 to 206.1 individuals m^(-2), while mean biomass was 1003.0 to 4430.2 mg ash-free dry mass (AFDM) m^(-2) and was generally higher in the dry season. Snail production was estimated using the size-frequency, instantaneous-growth (field- and computer-based) and increment-summation (IS) methods, with the IS estimates (1612.8 – 6123.9 mg AFDM m^(-2) 〖year〗^(-1)) considered to most accurately represent production by S. hainanensis. Annual turnover ratios were 1.36 – 2.24 〖year〗^(-1), and production was higher in unshaded streams where growth was more rapid, reflecting higher availability of algal food. The contribution of S. hainanensis to production by benthic animals as previously estimated in one study site was relatively low (15%) compared with its contribution to total standing biomass (26%). Annual insect emergence varied from 167.5 – 780.2 mg AFDM m-2 year-1, and constituted ~13% of total benthic production in one site, and this preliminary finding suggests that the water-to-land energy flux attributable to emerging insects along tropical Hong Kong streams is rather minor. The competition effects of S. hainanensis were studied during the dry season of 2012 and the wet season of 2011 by means of snail inclusion-exclusion manipulations. Snails exerted strong effects on algae and insects during the dry season, but did not significantly affect the structure of benthic assemblages. Algae in shaded streams were more severely depleted by snails, though the reduction in snail densities did not lead to increases in insect abundance or biomass. The effects of snails were not detectable during the wet season, when spate-induced disturbances were more frequent and intense. This generally agreed with the harsh-benign hypothesis, which predicts a reduction in the significance of biotic interactions under more disturbed conditions. Therefore, competition between S. hainanensis and insects was important only at base-flow conditions during the dry season. / published_or_final_version / Biological Sciences / Master / Master of Philosophy

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Thesis (M.Sc.(Environmental Biology))--Mahidol University, 1983.

Thesis (M.Sc. (Environmental Biology)) -- Mahidol University, 1979.

Thesis (M.S.)--University of Northern British Columbia, 2000. / Includes bibliographical references (leaves 99-105).

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Field populations of Margarites pupillus (Gould), a numerically abundant prosobranch mollusc occuring on rocky shores in Puget Sound, Washington, have previously received little autecological attention. Seawater table studios and SCUBA observations led to the following conclusions: Populations of Margarites are primarily subtidal. Population density and growth are directly related to algal canopy density. Algal canopy may function in reducing predator efficiency, Margarites populations have a 1:1 sex-reatio and no size dimorphism. Sexually mature individuals live for at least two years, and possibly more than four years.

Prestonella bowkeri and Prestonella nuptialis are montane specialists endemic to the southern Great Escarpment of South Africa. Phylogeographic analyses of these species based on mitochondrial markers CO1 and 16S reveal extremely high levels of divergence between populations indicating a lack of gene flow between populations. This is not surprising, because P. nuptialis and P. bowkeri have limited dispersal capacity, low vagility, a highly fragmented distribution and are habitat specialists that are restricted to isolated mesic refugia associated with waterfalls and montane seepages. A relaxed Bayesian clock estimate suggests that populations diverged from one another during the mid-late Miocene (12.5-7 MYA) which coincides with the modern trends of seasonal aridity which began during the Miocene. This result should be viewed with caution because the rates used are at best imprecise estimates of mutation rates in snails. There is no clear dichotomy between the two species and P. bowkeri is paraphyletic with respect to P. nuptialis, as a consequence the taxonomy is unclear. Due to the high levels of sequence divergence between populations they may be considered as evolutionary significant units (ESU’s). An assessment of haplotype diversity (h) and nucleotide diversity (π) reveals that populations in the western part of the Great Escarpment are more genetically depauperate than populations in the east. Correlations between genetic diversity and climatic variables show that genetically depauperate populations are found in areas that have lower annual rainfall, less reliable rainfall and higher potential evaporation, all factors associated with a drier, less mesic environment that increases the chances of a population bottleneck. This indicates that a shift towards a more arid environment may be a driver of genetic erosion. Historical climate change may thus have affected the amount and distribution of genetic diversity across the Great Escarpment since the Miocene. This has serious future implications for the survival of Prestonella. With predicted increase in global temperatures, climate change in South Africa is likely to result in range contraction and an eastward range shift for many species in the drier central and western areas (Erasmus et al. 2002) and regions along the Great Escarpment are likely to become more arid. Prestonella populations found living on inselbergs along the Great Escarpment are already restricted to site specific watercourses and seepages. An increase in the periods between stream flow, and increasing rainfall variability and mean annual potential evaporation are likely to have an adverse affect on species living in these habitats, resulting in further bottlenecks and possibly local extinction. An IUCN assessment of P. nuptialis and P. bowkeri suggests that these two species are probably endangered. The issue surrounding the conservation of Prestonella species is that they are threatened by global climate change, which cannot be simply restricted or prevented, which makes dealing with the threat of climate change difficult. Assisted migration (MA) may be considered as a method to prevent possible future extinctions of Prestonella populations, but will only be considered as a last resort. The thermal tolerance (Arrhenius breaking temperature and flat-line temperature) of individual snails from three Prestonella populations (one forest population and two thicket populations) were assessed using infrared sensors that detected changes in heart rate with increasing temperature. The forest population had a significantly lower Arrhenius breaking temperature (ABT) and flat-line temperature (FLT) than the two thicket population (p<0.05). Our results do not show a correlation between upper thermal limits and maximum habitat temperatures or other climatic variables in Prestonella populations. Although no correlation is found between ABT and maximum habitat temperature, it is likely that the differences seen between these populations are due to local micro-climate adaptation. The climatic variables used in this experiment are coarse estimates from GIS data and do not reflect actual microhabitat conditions. Forest environments are less heat stressed than thicket environments due to the forest canopy which may explain the lower ABT and FLT of the forest population.

No description available.