Browsing by Author "Theron, Kasselman Jurie"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- ItemBiodiversity assessments using remote sensing and integrative modelling for improved conservation management(Stellenbosch : Stellenbosch University, 2022-04) Theron, Kasselman Jurie; Pryke, James Stephen; Samways, Michael John; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Insect diversity is declining globally, threatening ecosystem collapse and human well-being. Accurate information on biodiversity dynamics is needed to guide landscape management. This is important within actively managed production landscapes (especially forestry and agriculture), where land use change and habitat degradation impacts biodiversity, which in turn influences ecological resilience. Satellite remote sensing allows for collection of data over time, which enables a greater understanding of species distributions and drivers of change. Combining satellite-derived variables with advanced integrative modelling approaches, such as time series analysis, habitat suitability modelling, functional connectivity analysis and deep learning image fusion, allows for better understanding of biodiversity dynamics to guide landscape management. In this dissertation, I focus on grasshoppers, indicators of high-quality grasslands, and study their diversity within grassland corridors between plantation forests, to gain a greater understanding of how plantation landscapes can be managed (e.g. prescribed burning, invasive plant management, tree harvesting) to ensure ecological resilience. Satellite time series showed that changes in grassland structure can have a lasting positive impact on grasshoppers for up to three years. Prescribed burning, measured using historic Normalised Burn Ratio index, helps maintain high-quality grasshopper habitat, while also influencing functional connectivity of grassland corridors. Normalised Difference Vegetation Index (NDVI) was a useful tool to monitor grassland corridor condition and to assist with planning of prescribed burning. Large corridors with an elevation gradient support diverse assemblages and should be prioritised for conservation. Deep learning image fusion classified American bramble (Rubus cuneifolius), an aggressive invasive species in South African grasslands, with high accuracy compared to conventional satellite imagery. Bramble had negative impact on plant species richness, larger-sized grasshopper species, as well as species of conservation concern. Bramble invaded riparian areas more frequently, threatening Ensifera grasshopper species habitat. Harvesting timber compartments and prescribed burning increased bramble cover within the landscape. Active management of plantation tree saplings after harvesting can lead to enhanced functional landscape connectivity and reduced bramble cover. Grassland corridors have high vegetation heterogeneity caused by prescribed burning, which contribute to high species turnover. Corridors should be burned every 2-3 years. Using NDVI will help foresters to identify where and when to burn corridors, thereby maintaining high-quality grasshopper habitat and functional connectivity. Rotational harvesting of timber compartments is recommended, as logging only some compartments will have positive impact on functional connectivity as well as helping prevent bramble invasion through the landscape. I recommend combining prescribed burning with grazing by domestic cattle to control bramble invasion. Special attention is needed to control bramble two years after harvesting, as this is when bramble cover is highest. This dissertation demonstrated that grassland corridors maintain biodiversity within actively managed production landscapes when management incorporates biodiversity response to disturbance. This motivates for the maintenance and protection of grassland corridors in other parts of the world to conserve biodiversity and help address insect decline. Lastly, the techniques used here provide valuable insights into biodiversity response to actively manage landscapes, and can therefore be applied to other vegetation types for protecting biodiversity.
- ItemConservation of spider diversity within an agricultural mosaic : insights from the Greater Cape Floristic Region, biodiversity hotspot.(Stellenbosch : Stellenbosch University, 2017-12) Theron, Kasselman Jurie; Gaigher, Rene; Pryke, James S.; Samways, Michael J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Agricultural expansion is one of the main drivers of habitat fragmentation and land use change which negatively impacts biological diversity. The Greater Cape Floristic Region (GCFR), a biodiverse hotspot, has been recognised as a priority for conservation as its unique endemic diversity is threatened by historic land transformation and habitat fragmentation. Private nature reserves and proclaimed protected areas alone cannot conserve all biodiversity, especially with >80% of land not formally protected. Thus we must conserve biodiversity within production landscapes. Remnant patches of natural vegetation supports a wide variety of arthropod taxa. However, little information is available on spider diversity in remnant fynbos and even less on which environmental parameters drive this diversity. Furthermore, research on how the matrix impacts adjacent remnant patches, and how spiders respond to different matrix types, are needed for protecting spider diversity and the services they provide within the GCFR mosaic. This study aims to identify environmental parameters that shape spider diversity within fynbos remnant patches, and how spiders respond to different matrix types. Here, I sampled spider diversity within remnant fynbos patches of the GCFR mosaic to identify which landscape and patch variables are important for maintaining spider diversity. Fifteen environmental variables (at landscape and patch scales) were collected at each site and analysed to determine their influence on spider species richness and assemblage structure of the whole spider assemblage, and for different functional guilds. Local patch variables best predict spider diversity, particularly soil compaction and topographic complexity which negatively influenced overall and plant dwelling spider richness. This pattern of complexity is mainly driven by common spider species. Tree species richness (mostly alien trees) negatively influenced free-living spider richness. Lastly, level of site invasion by alien trees influenced overall and epigaeic spider assemblage structure. Spider diversity was more influenced by patch scale variables, which reflects local patch management, than the landscape context. I also assess how spider diversity responds to different land-use types, the magnitude of associated edge effects on spider diversity, and identify complementary habitat elements for enhancing spider diversity within agricultural mosaics of the GCFR. Spider diversity was sampled along replicated transects covering remnant fynbos vegetation into three different matrix types: old fields, vineyards and invasive alien tree stands. Fynbos remnants had significantly higher overall spider diversity than matrix sites with higher diversity in edge locations than at patch cores. Old fields had the highest spider diversity between all land-use types, as well as the greatest assemblage similarity to remnant vegetation assemblages. Lowest diversity was recorded within vineyards. Lastly, vegetation complexity enhanced spider diversity across all land-uses. In conclusion, I demonstrate that remnant vegetation is a critical landscape element for conserving spider biodiversity in GCFR mosaics, but that old fields can play an important role in increasing functional connectivity within the landscape mosaic. Increasing native vegetation diversity within the matrix helps improve spider diversity. Additionally, this work recommends alien tree removal from fynbos remnant patches within the GCFR for biodiversity conservation. Preserving remnant patches of all sizes in production landscapes, and softening the matrix, can increase heterogeneity which benefits spider diversity within the GCFR mosaic.