Browsing by Author "Nuwagaba, Savannah"

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    The architecture of antagonistic networks
    (Stellenbosch : Stellenbosch University, 2013-03) Nuwagaba, Savannah; Hui, Cang; Stellenbosch University. Faculty of Science. Department of Mathematical Sciences.
    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.
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    The architecture of antagonistic networks : node degree distribution, compartmentalization and nestedness
    (International Academy of Ecology and Environmental Sciences, 2015-12-01) Nuwagaba, Savannah; Hui, Cang
    Describing complex ecosystems as networks of interacting components has proved fruitful – revealing many distinctive patterns and dynamics of ecological systems. Of these patterns, three have often been brought up in literature, including species degree distribution, compartmentalization and nestedness, due largely to their implications for the functionality and stability of communities. Here, using 61 empirical antagonistic networks, we aim to settle the inconsistency in literature by (i) fitting their node degree distributions to five different parametric models and identifying the one fits the best, (ii) measuring the levels of nestedness and compartmentalization of these 61 networks and testing their significance using different null models, and (iii) exploring how network connectance affects these three network architecture metrics. This research showed that most antagonistic networks do not display power law degree distributions and that resource species are generally uniformly distributed. We also clearly showed that the conclusion of whether a network is significantly compartmentalized or nested depends largely on the null model used.
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    The eco-evolutionary dynamics of complex adaptive food webs
    (Stellenbosch : Stellenbosch University., 2017-12) Nuwagaba, Savannah; Hui, Cang; Stellenbosch University. Faculty of Science. Dept. of Mathematical Sciences.
    ENGLISH ABSTRACT: Predator-prey interactions are ubiquitous since almost every species on earth participates in at least one predator-prey interaction. As a result, they shape the food web structure, the functioning of ecosystems and the response to perturbations. Predator-prey interactions have been studied extensively. However, the interplay between their ecological and evolutionary dynamics and how these contribute to regulate food web dynamics have received less attention. In this thesis, therefore, I developed a predator-prey model in which the predator exhibited type II functional response, its body size could evolve and its handling time was dependent on predator-prey body size ratio. Using adaptive dynamics, a mathematical tool which has been developed to study feedback between ecological and evolutionary processes, I investigated the influence of non-linear functional response on the evolution of predator’s body size. I found that increasing handling time reduces the predator’s body size. In fact, there exists a threshold beyond which an increase in handling time drastically reduces the body size such that evolutionary regime shifts occur. I concluded that predators’ feeding rates, as influenced by the current climate trends, coupled with evolution, could explain the observed regime shifts in species body sizes. I extended this model to allow for polymorphism and showed that starting with a single prey and predator, food webs emerge through the process of mutation and natural selection. I checked the density-body size relationship in the emergent food webs to investigate the generality of the energetic equivalence rule and found no support for it. Instead, my results showed a hump-shaped relationship, except for food webs that were generated from the predators which exhibited the linear functional response. I further allowed potential invaders into co-evolving food weds to test how invasion success depends on species body size, propagule pressure, native species diversity and introduction time. I found that whenever potential invaders have a bigger body size, they always have a higher invasion success. In addition, I found that although the propagule pressure plays an important role, it is irrelevant in a diverse food web in which most or all niches have been occupied, hence strongly supporting the diversity-invasibility hypothesis.
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    Robustness of rigid and adaptive networks to species loss
    (Public Library of Science, 2017) Nuwagaba, Savannah; Zhang, Feng; Hui, Cang
    Controversies in the complexity-stability debate have been attributed to the methodologies used such as topological vs. dynamical approaches or rigid vs. adaptive foraging behaviour of species. Here, we use a bipartite network model that incorporates both topological and population dynamics to investigate the robustness of 60 real ecological networks to the loss of generalist and specialist species. We compare the response in both adaptive and rigid networks. Our results show that the removal of generalists leads to the most secondary extinctions, implying that conservation strategies should aim to protect generalist species in the ecosystem. We also show that adaptive behaviour renders networks vulnerable to species loss at initial stages but enhances long term stability of the system. However, whether adaptive networks are more robust to species loss than rigid ones depends on the structure of the network. Specifically, adaptive networks with modularity < 0.3 are more robust than rigid networks of the same modularity. Interestingly, the more modular a network is, the less robust it is to external perturbations.