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The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic-feeding fish, which disturb bed material sediments during their search for food, has received little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. A series of experiments were conducted to investigate the impacts of benthic feeding fish on the structure and composition of gravel-bed river sediments, and the implications for bed material transport. An ex-situ experiment was conducted to investigate the impact of a benthic feeding fish (European Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume, changes in bed surface topography were measured and grain displacements examined when an imbricated, water-worked bed of 5.6-16 mm gravels was exposed to feeding juvenile Barbel. For substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four-hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes caused by fish increased bed load flux by 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. An ex-situ experiment utilising Barbel and Chub Leuciscus cephalus extended this initial study by considering the role of fish size and species as controls of sediment disturbance by foraging. Increasing the size of Barbel had a significant effect on measured disturbance and bedload transport. Specifically, the area of disturbed substrate, foraging depth, microtopographic roughness and sediment structure all increased as functions of fish size, as did bedload flux and total transported mass. In a comparison of the foraging effects of like-sized Barbel and Chub 8-10 in length, Barbel foraged a larger area of the riverbed and had a greater impact on microtopographic roughness and sediment structure. Foraging by both species was associated with increased sediment transport, but the bed load flux after foraging by Barbel was 150% higher than that following foraging by Chub and the total transported mass of sediment was 98% greater. An in-situ experiment quantified the effects of foraging fish, primarily Cyprinids (specifically Barbel and Chub), on gravel-river bed sediment structures, surface grain-size distributions, sediment transport fluxes and grain entrainment in the River Idle, Nottinghamshire, UK. This was achieved by installing large experimental sediment trays seeded with food at typical densities. The experiments yielded data about 1) topographic and structural differences between pre- and post-feeding substrates using DEMs interpolated from laser scans, 2) modifications to surface and sub-surface grain-size distributions as a function of fish foraging and 3) differences in sediment entrainment from water-worked substrates exposed to feeding fish and control substrates, without fish. Small sections of the substrate trays were recovered in tact from the field and for substrates that had been exposed to feeding fish and control substrates which had not, grain entrainment rates and bedload fluxes were measured under a moderate transport regime in the laboratory. On average, approximately 74% of the substrate, by area, was modified by foraging fish during a twelve-hour period, resulting in increased microtopographic roughness and substrate coarsening which had significant implications for bed material transport during the steady entrainment flow. Together, results from these experiments indicate that by increasing surface microtopography, modifying the composition of fluvial substrates and undoing the naturally stable structures produced by water working, foraging can influence sediment transport dynamics, predominately by increasing the mobility of river bed materials. The implication of this result is that by influencing the quantity of available, transportable sediment and entrainment thresholds, benthic feeding may affect sediment transport fluxes in gravel-bed rivers. In addition, three discrete studies were performed alongside the core experiments described above. A quantitative examination of habitat conditions favoured by feeding Barbel was conducted in the River Idle (Nottinghamshire, UK) which served to supplement existing literature pertaining to Barbel ecology, and inform experimental design during the core experiments. Two further studies considered the potential importance of foraging as a zoogeomorphic activity in terms of spatial extent, at a variety of scales, thereby extending core experiments to larger spatial scales in-situ.

The spread of non-native species is a global threat and the rate at which biological invasions occur is likely to increase in the future. This thesis examines the implications of the invasive signal crayfish, Pacifastacus leniusculus (Dana), for instream communities and abiotic processes within lowland rivers in England. The potential effects on lotic macroinvertebrate communities and fine sediment dynamics are investigated at a range of spatial and temporal scales, from the examination of national long-term datasets through to short-term detailed mesocosm experiments. Interrogation of macroinvertebrate community data from three English regions was undertaken to understand the temporal and spatial extent of signal crayfish effects. Invasive crayfish had significant long-term and persistent effects on resident macroinvertebrate communities regardless of the lithology or other environmental characteristics of the region. The resultant modifications to community composition had repercussions for several widely employed freshwater biomonitoring tools which employ faunal abundance in their derivation. A reach-scale field study was undertaken at two sites, one invaded by crayfish and one which did not support populations of crayfish (control), to assess the potential contribution of signal crayfish for fine sediment dynamics (ingress, fluxes and ultimately the overall implications for sediment budgets). Reach-scale field experiments examining the effect of crayfish invasion on resident macroinvertebrate communities over a 126-day period indicated that once crayfish populations are well-established their effects are persistent. However, separating the effects of crayfish from wider macroinvertebrate community dynamics and life-history characteristics of the invader and resident taxa means that attributing the effects is far from clear. The thesis presents the results of a series of novel mesocosm experiments which examined the dynamic and two-way interactions of predator-prey relationships and potential zoogeomorphological effects of signal crayfish and the freshwater shrimp, Gammarus pulex. Signal crayfish had a significant effect on the mass of fine sediment infiltrating into the gravel matrix associated with foraging and predatory activity; however this was strongly controlled by prey availability. Finally, through the development of conceptual models, the thesis illustrates the need for greater integration of ecological and geomorphological theories, at relevant environmental scales (temporal and spatial) to achieve truly interdisciplinary research.

Humans are modifying animal populations, indirectly accelerating or reducing the geomorphic alterations caused by animals. Species have been monitored and studied with focus on domesticated animals but little research has been undertaken on wild animals. This study analyses the geomorphic impact of elephants on Tembe Elephant Park, so that the changes they cause to the landscape may be quantified. To conduct this research four sites were chosen: an area where elephants had been excluded for twenty-five years, where excluded for five years, where elephants exist at present and where elephants mud wallow. Three of the four study sites were classed as sand forest (twenty-five-years exclusion, five-years exclusion and where elephants exist) and were analysed and compared to determine the similarities and differences in climate, microclimate, vegetation and the soil’s physical and chemical properties. The wallow site was not compared to any other study site, but was observed and mapped to quantify the geomorphic impact of elephants wallowing. When the sand forest sites were compared the climate, vegetation type and soil were found to be similar. Where elephants were present: the vegetation was inconsistent in basil cover, canopy height, structure and class. Soils were more compacted with a low infiltration rate, higher temperature, lower soil moisture, higher pH and a lower electric conductivity and air relative humidity was the highest. Where elephants have been excluded for twenty-five years, the opposite trends arose from the data analysis. The vegetation was consistent in basil cover, canopy height, structure and class, and the soils were less compacted with a high infiltration rate, low temperature, higher soil moisture, lower pH and a higher electric conductivity. The microclimate showed a trend where the air relative humidity was the lowest. At the elephant wallow site data showed that the wallows were in general circular in shape, 52.5m3 of soil was removed per month for the last nine months and the surface area of the wallows increased by 165.5m2 per month for nine months from April to December 2008. All the results from this study show that the elephant activity in Tembe Elephant Park has geomorphic consequences. From the results, it is possible to conclude that the geomorphic impacts of elephants on Tembe Elephant Park are contributing to a nutrient cycle shift in the sand forest biome, as they change aspects of the vegetation, microclimate, soil and landscape, which are the foundation of the cycle. / Dissertation (MA)--University of Pretoria, 2011. / Geography, Geoinformatics and Meteorology / MA / Unrestricted

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