The influence of Microhodotermes viator nest density and dispersion on ecological processes

Swanepoel, Juan

The influence of Microhodotermes viator nest density and dispersion on ecological processes

Doctoral Thesis

2021

Abstract

Heuweltjies are typically described as raised (± 2 m in height) circular earth mounds (5 – 30 m diameter) ubiquitous with the southern harvester termite (Microhodotermes viator), that occur in regular hexagonal patterns, predominantly in the north-western Succulent Karoo of South Africa, and are associated with distinct floral communities and faunal interactions. Most of the current literature on heuweltjies centres around their origins, of which two dominant theories persist. The termite origin theory is based on the strong association of heuweltjies with M. viator and their activities as ecosystem engineers that generate “islands of fertility” capable of initiating a cascade of events resulting in heuweltjie formation. Alternatively, the bush-clump theory posits that such “islands of fertility” necessary to initiate these events were generated by decomposing plant detritus. This study explores the ecosystem services (nutrient cycling, hydraulic soil properties, and biodiversity) provided by heuweltjies in a spatial-patterning context to assess these theories through remote sensing satellite and fieldwork of several sites across a rainfall gradient, in situ experimentation in Van Riebeeck Park (Worcester), and greenhouse experiments at the University of Cape Town. Novel findings include the quantification of termite faecal matter (frass) on heuweltjies and the associated enrichment in Worcester, and identifying a relationship between the distinct plant communities associated with heuweltjies and invertebrate density and richness from several sites. This study marks the first comparison of the spatial and productivity properties of heuweltjies between areas of high and low-densities, and found that these landscapes are similar, despite differences in density. This study also contributes to the current knowledge on the relationship between heuweltjie density and environmental conditions across a rainfall gradient and different biomes, as well as selective pressures that result in the distinct plant communities observed on heuweltjies. Experimental approach and specific aims of each data chapter are outlined here: Chapter 2 - The spatial properties and productivity of plots with high and low-densities of M. viator mounds were compared, and the relationships between their spatial properties and productivity values, as a function of density, investigated. Using Google Earth images paired plots of high and low densities of M. viator mounds were identified and their nearest neighbour distances and dispersion measures measured. High-resolution productivity data for these sites were obtained from the South African National Space Agency, and lowresolution productivity data from EarthExplorer for both autumn and spring to account for respective perennial and annual productivity. Although no clear relationship between spatial measures and productivity of the matrix could be determined, the spatial properties of heuweltjies measured conform with the mechanism of regular pattern formation through termite foraging and competitive interaction. Chapter 3 – The enrichment of M. viator faecal matter (frass) was explored. Frass heaps deposited on superficial mounds were quantified (476 kg) and the mean enrichment of calcium (0.64 kg), carbon (64.4 kg) and nitrogen (3.65 kg) from these heaps calculated. These results were used to determine the enrichment levels of these nutrients. The nutrient profiles of frass, heuweltjie, and matrix soils were compared, and heuweltjie soils found to be more similar to frass (60 %) than matrix soils (58 %). Chapter 4 – Frass contributes to the enrichment of heuweltjie soils, however, construction and maintenance of nests by M. viator result in physical modification of soils. These physical and chemical differences between heuweltjie and matrix soils were quantified and compared and used for reciprocal greenhouse germination and growth experiments, which were supplemented with reciprocal in situ transplant and growth experiments. Germination trials tracked the germination and growth success of seeds from three plant species that occur predominantly on heuweltjies, and three that occur predominantly in the matrix. Germination was higher in heuweltjie soils for all seeds, however, two heuweltjie species had greater survival in heuweltjie soils than matrix soils. The reciprocal transplant experiment demonstrated significant transplant shock in seedlings and instead of growth, tolerance in different soil habitats was determined and species-specific tolerance responses were found for different organs of plants using multivariate regression models. Chapter 5 – Heuweltjies are typically associated with distinct plant communities. In this chapter I describe these distinct plant communities along with their matrix communities across a rainfall gradient and explore these relationships. In several field sites line transects were used to investigate plant communities by measuring species diversity and cover. Distinct plant communities on heuweltjies are often denser, attracting invertebrates as sites of dense food resources or refugia. In addition this chapter explores ant communities, as a representative taxon for invertebrates, and explores the relationship between community composition, diversity and biomass of ants with that of plant communities. Ants were collected via pitfall traps along the same transects used to measure plant communities. I found that heuweltjie plant communities are markedly distinct from that of their associated matrix communities. Ant communities on heuweltjies were not as distinct compared to ant communities in the matrix, however, plant biomass and species richness had a positive relationship with abundance and richness of ant species.

Keywords

Biological Sciences

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Collections

PhD / Doctoral

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