Spider ecology in southwestern Zimbabwe, with emphasis on the impact of holistic planned grazing practices
The current information on Zimbabwean spiders is fairly poor and is mostly restricted to taxonomic descriptions, while their ecology remains largely unknown. While taxonomic studies are very important, as many species are becoming extinct before they are described, a focus on the ecology of spiders is also essential, as it helps with addressing vital questions such as the effect of anthropogenic activities on spider fauna. Therefore, in order to address this research gap, assessment of the response of spiders to holistic management practises within Debshan Ranch, Shangani, Zimbabwe was done. Additionally, in order to establish baseline data on spider fauna, the standardised South African National Survey of Arachnida sampling protocol was utilised to assess its efficacy within the Khami World Heritage Site. Spider sampling was done in three sub-projects: the first included sampling in several geographic distances around previously kraaled inclusions and control sites within the ranch, using sweep nets and pitfall traps, in six sampling periods from July 2017 until April 2018; the second included sampling that was done inside the previously kraaled inclusions and their surrounding areas dating back to at least ten months since cattle occupation in two sampling intervals early summer (November 2017) and late summer (March 2018), using pitfall traps; the third entailed sampling within Khami in three sampling periods (summer, winter and spring 2018) using six sampling methods, namely pitfall traps, beating sheets, litter sifting, sweeping, day hand collecting and night hand collecting. The model that best explained changes in mean grass height (cm), as well as percentage grass cover around previously kraaled inclusions and the control sites, was that which included time since kraal removal, whereas inside the inclusions and their surroundings was that which included season and short duration kraaling. At the functional group level, only the web builder‟s genera richness responded negatively to short duration kraaling around the previously kraaled inclusions and their control sites. On the other hand, inside the previously kraaled inclusions and their surroundings only ground dwelling abundance responded negatively and significantly to short duration kraaling. The most important predictor amongst the vegetation structure variables around the previously kraaled inclusions and control sites was mean grass height (cm), which impacted genera richness and abundance of both ground dwellers and web builders. In contrast, genera richness and abundance of plant wanderers were positively associated with mean grass height (cm). However, inside the previously kraaled inclusions and their surroundings the most important predictor was time since kraal removal and the vegetation structure variable percentage coarse woody debris cover, which responded positively to the ground dwellers. In addition, the late sampling season had significantly lower ground dwelling abundance compared to the early sampling season. Within the Khami World Heritage Site the riparian woodland had the highest species richness compared to the other biotopes. Similarly, the summer period also produced the highest diversity, with winter recording the lowest species richness. Night and day hand collecting had the highest observed species richness with adult individuals. In order to sample 50% of the spider assemblages, 15 samples were required to be collected in the mixed woodland, which represented the biotope requiring the fewest samples. Seasonality effects explained a significant amount of variation in changes of mean grass height (cm) and percentage cover around previously kraaled sites and their control sites. However, when inter-seasonal variation was excluded by sampling previously kraaled sites within one season, short duration kraaling explained a significant amount of variation. Standardised sampling protocols aid in establishing databases of spider fauna which will in the long run ensure inclusion of spiders in biodiversity reports in Zimbabwe, which has historically not been the case, due to limited information.