List of symbols
1.3 Thesis roadmap
To answer the abovementioned research questions, several steps are taken, analysed and discussed throughout this thesis, separated in 10 different chapters. The content of most chapters and how they tackle a specific research question has already been shortly presented in Figure 1.2, Figure 1.3 and Figure 1.4, though merits a more elaborate description to reflect the overall structure and continuity of this work. Throughout the following paragraphs, the content of each chapter is introduced along with its relevance and contribution to the aforementioned research questions, which is ultimately summarised in a comprehensive scheme (see Figure 1.5).
Within this first chapter, the general background of the study is provided to introduce the societal relevance and scientific necessity of this work. Attention is given to the importance of water in society, as well as the value of aquatic ecosystems and the provided services. Due to the connection between aquatic and terrestrial systems, wetlands are considered to be a prominent starting point for combining water treatment, storage and purification along with a positive note towards the improvement of terrestrial biodiversity and river conservation. Based on this starting point, a series of research questions with underlying objectives are identified to steer and frame all subsequent chapters.
In Chapter 2, additional focus is given to the concept of Integrated Constructed Wetlands (ICWs) and the identification of interactions among several biotic groups within shallow eutrophic freshwater systems. Specific attention is given to pollutant removal within constructed wetlands and the structuring role of macrophytes in many aquatic systems. The chapter concludes with a summary of key issues that need further investigation to support the implementation of ICWs as a multi-purpose technique dealing with water pollution and biodiversity improvement. The majority of subsequent chapters elaborates on one (or more) of the key issues identified here.
Next, Chapter 3 dives into the world of data-driven habitat suitability and species distribution models by discussing the advantages and drawbacks of five modelling techniques. Application of these techniques within ecosystem management is illustrated by means of examples, while highlighting the different steps and approaches to be considered during model development. The chapter includes an introduction to decision trees, generalised linear models, artificial neural networks, fuzzy logic and Bayesian belief networks as well as a description on model conceptualisation, requirements, calibration and evaluation. Moreover, it provides an introduction to the potential of and criticism on ecological modelling to support environmental management. The chapter concludes with the endorsement of a promising data-driven modelling technique.
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Subsequently, Chapter 4 provides a general description of the data and modelling technique underlying the following chapters. An in-depth description of the data set is provided (spatiotemporal coverage, number of instances, number of explanatory variables, number of species, missing data, …), as well as a more detailed discussion of the selected model algorithm. Moreover, the methodology and experimental design behind the applied data cleaning are touched upon. Finally, the focus species of the autecological experiments are introduced.
In Chapter 5, the first step in cleaning the available data is performed. More specifically, different approaches to deal with missing data are introduced to avoid the traditional information loss caused by removing the incomplete instances from the data set. Four techniques replacing the missing value by a data-derived value (i.e. ‘data imputation’) are discussed in more detail, being mean value, least squares, k-nearest neighbours and missForest. The application of each technique to a range of differently-sized data sets provides a conclusion on the most accurate technique, while mentioning computation time as a side aspect during method evaluation.
Following data selection and preparation, Chapter 6 discusses the potential of further data pre-processing in concert with the selected algorithm, while identifying a lack of clear guidelines. The removal of redundant variables and potentially faulty instances is expected to reduce overall data complexity, to improve model performance and to decrease computation time. Throughout the chapter, four techniques are dealt with in more detail: (i) instance removal based on outliers, (ii) instance removal based on false absences, (iii) variable removal based on correlation score and (iv) variable removal based on variable importance. The chapter concludes with a statement on the effects of data pre-processing on model performance and provides a suggestion for further research.
Next, Chapter 7 builds further on the findings of Chapter 5 (imputation technique) and Chapter 6 (data pre-processing) and combines them in the development of species-specific abiotic habitat suitability models. For each species, model fit is improved by optimising hyperparameter settings and comparing final model performance with baseline and null model performance. Based on these models, species-specific variable importance is derived, while additionally providing the opportunity to assess the effect of different management scenarios. Finally, the chapter concludes with an overview of the identified steering variables and how management effects depend on the starting conditions within the system under consideration, while highlighting that controlled experiments can improve understanding of the dynamic interactions occurring within ecosystems.
INTRODUCTION
17 After the model-based approach, Chapter 8 considers the application of controlled laboratory experiments to define invasive behaviour of an alien species. Two highly similar species are exposed to a range of nutrient concentrations under optimal growing conditions, while a selection of functional traits is being monitored. Based on ecological theory, it is expected that an invasive species exhibits higher performance either at the level of resource intake or biomass production. Finally, a conclusion on the applicability of the selected traits for inferring invasive potential of alien species is provided.
In contrast, Chapter 9 elaborates on the post-establishment phase, where an invasive alien species is continuously introduced in a new environment and threatens native populations. Management of these native species by means of harvesting can disturb natural conditions and benefit the alien species. Meanwhile, biomass removal of an invasive alien species can help creating opportunities for the re-colonisation by (a) native species. Within this chapter, a dynamic interaction of management and introduction pressure is applied on the same species from Chapter 8 and is studied to infer the potential detrimental effects of management without prior study. Based on these observations, management suggestions are formulated to conclude this chapter.
Finally, Chapter 10 combines all the observations into a general discussion, followed by a conclusion for future freshwater management. Within this chapter, the answers to the research questions and objectives identified in the first chapter are summarised and re-framed in a bigger story. More specifically, the chapter discusses the application of the suggested modelling technique in combination with the appropriate data pre-processing and hyperparameter optimisation and subsequently couples back with the need for restoring and constructing wetlands. Moreover, attention is given to the threat posed by invasive alien species and how performed experiments help in identifying solutions and challenges for both proactive and reactive management. Ultimately, the chapter concludes with an introduction to the future perspectives of ecosystem modelling and invasive species management.
From this, it is clear that all chapters are linked and provide a linear story throughout the whole thesis. Each chapter tackles a specific research question and the underlying objectives (see Figure 1.2, Figure 1.3 and Figure 1.4), while often building on previous chapters. A complete overview of this work and how the chapters are linked, is provided in Figure 1.5.
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Figure 1.5: Roadmap of this thesis. Three main themes can be identified: (1) Summarising available information by means of literature review (Setting the scene); (2) Optimise the available data via imputation and pre-processing and develop habitat suitability models (Modelling) and (3) Perform controlled experiments to complement the models (Experiments).