Increased abstraction of water in the catchment results in a reduced or altered pattern of river flow and this holds serious consequences for the downstream estuarine ecosystem. In South Africa this is a serious concern because freshwater is in limited supply and the demand for freshwater can be expected to increase in the future. A large multi-disciplinary consortium of South African scientists are working on projects to determine the freshwater requirements of estuarine ecosystems. As part of this, this thesis reports on research undertaken to develop mathematical models to determine the freshwater requirements of estuarine macrophytes. Three key macrophytes are selected. The macrophytes are Zostera capensis Setchell, Ruppia cirrhosa Grande, and Phragmites australis. They are common macrophytes in South African estuaries. Zostera and Ruppia are submerged macrophytes and Phragmites is an emergent macrophyte. They have different freshwater environments and therefore respond differently to alterations in freshwater flow. A first order differential equation model is used to determine the effect of different combinations of open and closed mouth conditions of the estuary on Zostera and Ruppia. The scenarios are selected to determine whether achieving a switch in states from a Zostera-dominated estuary to a Ruppia-dominated estuary is possible. To predict encroachment rates and colonisation patterns, a cellular automaton of the vegetative spread of existing Zostera beds is developed. After analysing various scenarios accounting for both an increase and a decrease in freshwater supply, the cellular automaton is extended to include interactions between Ruppia and Phragmites. The multi-species model is applied to the Kromme estuary, South Africa and the Great Brak estuary, South Africa. Various freshwater scenarios are examined from the natural runoff condition to the situation of no freshwater inflow. A sensitivity analysis of the spatial model with Zostera, Ruppia and Phragmites is conducted. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1998.

Increased abstraction of water in the catchment results in a reduced or altered pattern of river flow and this holds serious consequences for the downstream estuarine ecosystem. In South Africa this is a serious concern because freshwater is in limited supply and the demand for freshwater can be expected to increase in the future. A large multi-disciplinary consortium of South African scientists are working on projects to determine the freshwater requirements of estuarine ecosystems. As part of this, this thesis reports on research undertaken to develop mathematical models to determine the freshwater requirements of estuarine macrophytes. Three key macrophytes are selected. The macrophytes are Zostera capensis Setchell, Ruppia cirrhosa Grande, and Phragmites australis. They are common macrophytes in South African estuaries. Zostera and Ruppia are submerged macrophytes and Phragmites is an emergent macrophyte. They have different freshwater environments and therefore respond differently to alterations in freshwater flow. A first order differential equation model is used to determine the effect of different combinations of open and closed mouth conditions of the estuary on Zostera and Ruppia. The scenarios are selected to determine whether achieving a switch in states from a Zostera-dominated estuary to a Ruppia-dominated estuary is possible. To predict encroachment rates and colonisation patterns, a cellular automaton of the vegetative spread of existing Zostera beds is developed. After analysing various scenarios accounting for both an increase and a decrease in freshwater supply, the cellular automaton is extended to include interactions between Ruppia and Phragmites. The multi-species model is applied to the Kromme estuary, South Africa and the Great Brak estuary, South Africa. Various freshwater scenarios are examined from the natural runoff condition to the situation of no freshwater inflow. A sensitivity analysis of the spatial model with Zostera, Ruppia and Phragmites is conducted. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1998.

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Designing a mechanistic model that can give rise to realistic architecture of ecological networks is central to the understanding of how species assemble and function in ecosystems. As species are constantly adjusting their diets in an antagonistic network, we here incorporate this adaptive behaviour of diet choice into a bipartite network model, with the effect of antagonistic interactions between species depicted by Holling’s type II functional response. Predictions of this model fit extremely well with the observed levels of nestedness, modularity and node-degree distributions for 61 real host-parasitoid and plant-herbivore networks. We further examined two specific scenarios of our model (species with identical [neutral] demographic parameters and interactions with identical [neutral] benefit in the network) and found that the demography-neutral scenario overestimated observed modularity, whilst the benefit-neutral scenario over-estimate observed nestedness. Relationships between nestedness, modularity and connectance were found strong. Moreover, in contrast to the common belief of the high modularity in antagonistic networks, most real networks (> 80%) are significantly nested, whilst nearly 40% of the real networks are surprisingly less compartmentalized than random networks generated from null models. Regardless of the controversy on whether antagonistic networks are nested or compartmentalized, the proposed model captured the essence of the dynamic nature of structural emergence in antagonistic networks. Due to its predictive power, this model was further used to investigate robustness in antagonistic networks. Predictions showed that the robustness of a network is determined by many factors, such as connectance, resource degree distribution, resource-consumer ratio, diversity, nestedness and compartmentalisation. Surprisingly, the manner of network response to species loss was independent of the sequence followed while removing species from a network. Variations were only noticed in the intensity of the effect resulting from the removals. In addition, we also showed that species extinction procedures which ignore the interaction switch underestimate the effect of any loss of species in these networks. We must therefore value our knowledge of possible adaptive processes in the ecosystem as they may be important for resolving the diversity-stability debate. / AFRIKAANSE OPSOMMING: Die ontwerp van ’n meganistiese model wat aanleiding kan gee tot realistiese argitektuur van ekologiese netwerke is sentraal tot die begrip van hoe spesies bymekaar kom en funksioneer in ekosisteme. Soos spesies voortdurend hul dieet aanpas in ’n antagonistiese netwerk, het ons hierdie aanpasbare gedrag van dieet keuse in ’n bipartiet netwerk model ingewerk, met die effek van antagonistiese interaksies tussen spesies wat uitgebeeld word deur Holling se tipe II funksionele reaksie. Voorspellings van hierdie model pas baie goed met die waargenome vlakke van nestedness, modulariteit en node-graad uitkerings vir 61 ware gasheer-parasiet en plant-herbivoor netwerke. Verder het ons twee spesifieke gevalle van ons model (spesies met identiese [neutrale] demografiese parameters en interaksies met identiese [neutrale] voordeel in die netwerk) ondersoek en gevind dat die demografie-neutrale geval waargenome modulariteit oorskat, terwyl die voordeelneutraal geval waargenome nestedness oorskat. Verhoudings tussen nestedness, modulariteit en konnektiwiteit is sterk bevind. Verder, in teenstelling met die algemene verwagting van hoe modulariteit in antagonistiese netwerke, is oorhoofse werklike netwerke (> 80%) aansienlik geneste, terwyl byna 40% van die werklike netwerke is verbasend minder gekompartimenteerd as ewekansige netwerke gegenereer uit null modelle. Ongeag van die omstredenheid oor of antagonistiese netwerke geneste of gekompartimenteerd is, die voorgestelde model vang die essensie van die dinamiese aard van die strukturele opkoms in antagonistiese netwerke. As gevolg van sy voorspellende krag, is hierdie model verder gebruik om robuustheid te ondersoek in antagonistiese netwerke. Voorspellings het getoon dat die robuustheid van ’n netwerk word bepaal deur verskeie faktore, soos konnektiwiteit, hulpbron-graad verspreiding, hulpbron-verbruiker verhouding, diversiteit, nestedness en kompartementasie. Verrassend, die wyse van die netwerk reaksie op die verlies van spesies was onafhanklik van die reeks wat gevolg het toe die spesies verwyder is uit ’n netwerk. Variasies is slegs opgemerk in die intensiteit van die effek van die verskuiwings. Benewens, ons het ook aangetoon dat die prosedures van spesies se uitsterwing wat die interaksie skakelaar geignoreer het, onderskat die effek van ’n verlies van spesies in hierdie netwerke. Ons moet dus die waarde van ons kennis van die moontlike aanpassing prosesse in die ekosisteem in agneem, aangesien dit belangrik kan wees vir die oplossing van die diversiteit-stabiliteit debat.

This study explores the possible role of mathematical system theory in integrating existing ecological knowledge within the existing concepts of the structure of the biosphere. The objective of this integration is a theory of ecosystems which must include interactions. The basic unit of the biosphere is the biogeocoenose; similar to the ecosystem, but homogeneous with respect to topographic, microclimatic, vegetation, animal, pedalogical, hydrological and geochemical conditions. The role of the biogeocoenose in a theory of ecosystems based on system theory is discussed. The biogeocoenose may serve as the building block for modeling watersheds as ecosystems. The fundamentals of system theory are reviewed. As an example, an analysis and synthesis of the arid zone water balance follows. The water balance is resolved into twenty components which represent the water balance of (1) the canopy, (2) the mulch, (3) the soil surface, (4) the soil, and (5) the plant, including interactions. The twenty components were modeled as separate systems which were later coupled into one overall, complex, well defined ecosystem water balance system. The example illustrates the role of system theory in integrating ecological knowledge. Further discussion indicates the need for explicitly including plant behavior in the water balance model.

published_or_final_version / abstract / Statistics and Actuarial Science / Doctoral / Doctor of Philosophy

In natural systems, it has been observed that plankton exist in patches rather than in an even distribution across a body of water. However, the mechanisms behind this patchiness are not fully understood. Several previous modeling studies have examined the effects of abiotic and biotic factors on patch structure. Yet these models ignore a key point: zooplankton often undergo diel vertical migration. I have formulated a model that incorporates vertical movement into the Rosezweig-MacArthur (R-M) predator-prey model. The R-M model is stable only at a carrying capacity below a critical value. I found that adding vertical movement stabilizes the system even at a high carrying capacity. By analyzing temporal stability and spatial structure, my results show that vertical movement interacts with carrying capacity to determine patch structure. / by Simantha Ather. / Thesis (M.S.)--Florida Atlantic University, 2009. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2009. Mode of access: World Wide Web.

In order to understand how bunchgrasses achieve dominance over other plant growth forms and how they achieve dominance over one another in different environments, it is first necessary to develop a detailed understanding of how their growth strategy interacts with the resource limits of their environment. Two properties which have been studied separately in limited detail are architecture and disproportionate resource allocation. Architecture is the structural layout of organs and objects at different hierarchical levels. Disproportionate resource allocation is the manner in which resources are allocated across objects at each level of hierarchy. Clonal architecture and disproportionate resource allocation may interact significantly to determine the growth ability of clonal plants. These interactions have not been researched in bunchgrasses. This thesis employs a novel simulation technique, functional-structural plant modelling, to investigate how bunchgrasses interact with the resource constraints imposed in humid grasslands. An appropriate functional-structural plant model, the TILLERTREE model, is developed that integrates the architectural growth of bunchgrasses with environmental resource capture and disproportionate resource allocation. Simulations are conducted using a chosen model species Themeda triandra, and the environment is parameterised using characteristics of the Southern Tall Grassveld, a humid grassland type found in South Africa. Behaviour is considered at two levels, namely growth of single ramets and growth of multiple ramets on single bunchgrass clones. In environments with distinct growing and non-growing seasons, bunchgrasses are subjected to severe light depletion during regrowth at the start of each growing season because of the accumulation of dead material in canopy caused by the upright, densely packed manner in which they grow. Simulations conducted here indicate that bunchgrass tillers overcome this resource bottleneck through structural adaptations (etiolation, nonlinear blade mass accretion, residual live photosynthetic surface) and disproportionate resource allocation between roots and shoots of individual ramets that together increase the temporal resource efficiency of ramets by directing more resources to shoot growth and promoting extension of new leaves through the overlying dead canopy. The architectural arrangement of bunchgrasses as collections of tillers and ramets directly leads to consideration of a critical property of clonal bunchgrasses: tiller recruitment. Tiller recruitment is a fundamental discrete process limiting the vegetative growth of bunchgrass clones. Tiller recruitment occurs when lateral buds on parent tillers are activated to grow. The mechanism that controls bud outgrowth has not been elucidated. Based on a literature review, it is here proposed that lateral bud outgrowth requires suitable signals for both carbohydrate and nitrogen sufficiency. Subsequent simulations with the model provide corroborative evidence, in that greatest clonal productivity is achieved when both signals are present. Resource allocation between live structures on clones may be distributed proportionately in response to sink demand or disproportionately in response to relative photosynthetic productivity. Model simulations indicate that there is a trade-off between total clonal growth and individual tiller growth as the level of disproportionate allocation between ramets on ramet groups and between tillers on ramets increases, because disproportionate allocation reduces tiller population size and clonal biomass, but increases individual tiller performance. Consequently it is proposed that different life strategies employed by bunchgrasses, especially annual versus perennial life strategies, may follow more proportionate and less proportionate allocation strategies respectively, because the former favours maximal resource capture and seed production while the latter favours individual competitive ability. Structural disintegration of clones into smaller physiologically integrated units (here termed ramet groups) that compete with one another for resources is a documented property of bunchgrasses. Model simulations in which complete clonal integration is enforced are unable to survive for long periods because resource bottlenecks compromise all structures equally, preventing them from effectively overcoming resource deficits during periods when light is restrictive to growth. Productivity during the period of survival is also reduced on bunchgrass clones with full integration relative to clones that disintegrate because of the inefficient allocation of resources that arises from clonal integration. This evidence indicates that clonal disintegration allows bunchgrass clones both to increase growth efficiency and pre-empt potential death, by promoting the survival of larger ramet groups and removing smaller ramet groups from the system. The discrete nature of growth in bunchgrasses and the complex population dynamics that arise from the architectural growth and the temporal resource dynamics of the environment, may explain why different bunchgrass species dominate under different environments. In the final section this idea is explored by manipulating two species tiller traits that have been shown to be associated with species distributions across non-selective in defoliation regimes, namely leaf organ growth rate and tiller size (mass or height). Simulations with these properties indicate that organ growth rate affects daily nutrient demands and therefore the rate at which tillers are terminated, but had only a small effect on seasonal resource capture. Tiller mass size affects the size of the live tiller population where smaller tiller clones maintain greater numbers of live tillers, which allows them to them to sustain greater biomass over winter and therefore to store more reserves for spring regrowth, suggesting that size may affect seasonal nitrogen capture. The greatest differences in clonal behaviour are caused by tiller height, where clones with shorter tillers accumulate substantially more resources than clones with taller tillers. This provides strong evidence there is trade-off for bunchgrasses between the ability to compete for light and the ability to compete for nitrogen, which arises from their growth architecture. Using this evidence it is proposed that bunchgrass species will be distributed across environments in response to the nitrogen productivity. Shorter species will dominate at low nitrogen productivity, while taller species dominate at high nitrogen productivity. Empirical evidence is provided in support of this proposal. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

No description available.

Bibliography: pages 163-183. / A generic, size-based simulation model is developed to investigate the dynamics of carbon and nitrogen flows in plankton communities. All parameters in the model are determined by body size using empirically-determined relationships calculated from published data. The model is robust with respect to most parameters and assumptions. Because the model is based on general ecological principles, it can be used to simulate microplankton community interactions in any planktonic ecosystem. Two coastal ecosystems from the southern Benguela region in South Africa are simulated; one typical of the relatively stable surface waters on the Agulhas Bank and one typical of upwelling plumes, usually found off the west coast of South Africa. Simulated communities compare well with field observations in terms of standing stocks and size composition, and simulation results indicate that the small-scale structure of the two ecosystems and the processes occurring within them are relatively well understood. Consequently, the dynamic functioning of the two systems is investigated at the ecosystem level, using the simulation results. Hypothetical carbon flow networks are constructed, and the average importance of different flow pathways at different times is assessed. In both ecosystems, the vast majority of carbon flows pass through short, efficient-transfer pathways, although longer pathways are potentially possible. Simulation analyses are extended from coastal to oceanic food webs, and the model results are consistent with the hypothesis that oceanic phytoplankton have rapid rates of primary production. At-sea sampling of a phytoplankton bloom is mimicked by "sampling" from simulation output, and interpretation of the data using standard techniques is compared with the model output. The dangers of extrapolating from snapshot measurements is highlighted, and the experiment emphasizes the importance of size-fractionated sampling of phytoplankton. A hypothetical pelagic food web is described, consisting of at least five different trophic pathways from phytoplankton to pelagic fish. It is suggested that coastal waters probably have all the different pathways, and the relative importance and efficiency of the different pathways will determine the total fish production in an ecosystem.

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