Parsing the Particulars of Pollination: Ecological and Anthropogenic Drivers of Plant and Pollinator Dynamics

My research focuses on wild pollinating insects and the external influences on their population dynamics in both natural and human altered settings. Pollination from wild insects (e.g. wild bees, flies, butterflies, etc.) is critically important for both agricultural systems and the maintenance of wild/native plant biodiversity. Unfortunately, similarly to honey bees, numerous wild pollinating insects are experiencing global declines in abundance and diversity. Causes for the declines are varied and far reaching with mounting evidence showing these declines manifest in both, natural and human altered environments. Accordingly, the declines in pollinator health will have similarly widespread consequences, posing a precipitous threat to biodiversity, food production, and economic stability. The breadth and severity of the global pollinator decline highlights the need to develop a thorough understanding of how wild pollinators interface with their environments in both natural and human altered settings. Specifically, my research aims to help elucidate the drivers of natural plant and pollinator dynamics as well as the causes of wild pollinator decline utilizing comprehensive interwoven empirical and theory-based approaches. The first half of this thesis investigates the effects of urban development on wild bee communities using urban gardens as study sites in southeastern Michigan. My colleagues and I developed a large-scale multi-faceted research project sampling thousands of bees and numerous environmental variables across our sites. Results described in chapter two reveal that the negative effects of urban development on ground nesting bumble bees are driven entirely by declines in females while males show no response to urbanization. It also details a surprisingly abundant bumble bee population in the city of Detroit MI. Chapter three expands focus to the entire sampled set of bees and shows that the differential effect of urban development on females and males is apparent in all sampled ground nesting bees groupings. However, wild bees which nest in above-ground cavities have positive correlations with urban development. Chapter four uses US census data to investigate how socioeconomic conditions in urban settings can influence the location and floral quality of our study sites, urban gardens. The second half examines wild plant and pollinator dynamics in natural settings using theoretical models informed by empirical data and observations. Chapter five investigates the direct and indirect effects of insect herbivores on pollination in a community context. When attacked by herbivores, plants mount chemical defenses which deter herbivores but also deter pollinators and consequently reduce individual plant reproduction. Using empirically vetted mathematical representations of these interactions, I show that while this defense strategy has significant costs to individual reproduction it has stabilizing effects on the population and community level. Chapter six focuses on an often overlooked pollinator, predatory syrphid flies. These flies are pollinators when adults but predators of insect herbivores when in their larval stage. While this can be beneficial, I demonstrate how this dynamic can lead to a negative feedback loop in communities isolated from background biodiversity. Chapter seven expands the consideration of ecologically distinct developmental stages to plants. Incorporating independent stages of plant development into a model framework is shown to fundamentally alter the effects certain demographic rates on both population and community dynamics. This work presents novel findings regarding pollinator interactions with their environment in both anthropogenic and natural settings, contributing to foundational ecological information which will hopefully aid in managing and conserving pollinator biodiversity.PHDEcology and Evolutionary BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145954/1/prglaum_1.pd

Role of metabolism and ecology in the emergence of microbial communities

Polymicrobial communities often show complex patterns of metabolic and ecological interactions, yet our understanding of how the properties of communities emerge from the metabolic rules of species interactions is still limited. A central feature of metabolic interactions within microbial communities is ‘cross-feeding’, where one species or lineage consumes the metabolic by-products of another. Cross-feeding bacteria excrete and consume a wide range of metabolites and this sets the stage for diverse intra- and inter-specific metabolic interactions. In this thesis, I use ecological and evolutionary theory to address a number of critical questions posed by cross-feeding bacteria, with a particular focus on the role played by microbial metabolism in driving the emergence and dynamics of microbial interactions. First, I explore the conditions that favour the emergence and maintenance of cooperative cross-feeding and show that the evolutionary outcome depends strongly on the shape of the trade-off curves between the costs and benefits of cooperation. Second, I investigate the origins of cross-feeding interactions via single lineage diversification and derive new predictions on the physiological mechanisms that may explain the stable coexistence of a cross-feeding polymorphism that evolved from a single clone. Third, I investigate what are the ecological consequences of cross-feeding metabolic interactions and demonstrate theoretically that a simple mechanism of trade can generate a diverse array of ecological relationships. Furthermore, I show the importance of the metabolic by-product properties in determining the ecological outcome. Fourth, I investigate how metabolic constraints of individual species shape the emergent functional and structural relationships among species. I show that strong metabolic interdependence drives the emergence of mutualism, robust interspecific mixing, and increased community productivity. Furthermore, I show that these emergent community properties are driven by demographic feedbacks. In general, these findings support the idea that bridging microbial ecology and metabolism is a critical step toward a better understanding of the factors governing the emergence and dynamics of polymicrobial interactions

Putting ecological theories to the test : individual-based simulations of synthetic microbial community dynamics

Microbial communities are critical for the proper functioning of each and every ecosystem on Earth. The ability to understand the structure and functioning of these complex communities is crucial to manage and protect natural communities, as well as to rationally design engineered microbial communities for important applications ranging from medical and pharmaceutical uses to various bioindustrial processes. In recent years, synthetic microbial communities have gained increasing interest from microbiologists due to their reduced complexity and increased controllability, which favours them over more complex natural systems for examining ecological theories. In this thesis, the in silico counterpart of this approach was used to test ecological theories relating to biodiversity and functionality through the use of mathematical models. Models are abstractions of reality which allow for the testing of hypotheses in a controlled way. In this thesis, individual-based models of synthetic microbial communities were developed and used in simulation studies to answer research questions relating to community diversity, stability, productivity and functionality. The models are spatially explicit and track through time the characteristics, interactions and activities of every individual in the community. The modelling framework is flexible and thus also extendable to other avenues of research

Stochastic models for the ecology and population genetics of introduced species

The long-term success of an introduced population depends on the ecological conditions in its new environment, but is also influenced by stochasticity. This is particularly clear in the first stage of an invasion when the population is still small and either goes extinct quickly or establishes a self-sustaining population. Once established, some populations grow and spread spatially, with potential impacts on native communities and ecosystems. The role of stochasticity during these later invasion stages remains unclear. Furthermore, little is known about the population genetic and evolutionary consequences of stochastic invasion trajectories. With this dissertation, I would like to contribute to a stochastic eco-genetic theory of the entire invasion process—from the first introduction up to potential impacts. The overarching questions in this dissertation are: a) How does a population’s movement through the invasion process depend on ecological factors influencing its average growth rate? b) How does it depend on factors influencing the stochastic variability in the population dynamics? c) How much genetic diversity do introduced populations harbor on average upon reaching a certain point in the invasion process? d) To what extent can the population-genetic consequences of invasion trajectories feed back onto the population dynamics? Together with my advisors and coauthors, I have conducted four studies, each addressing two or more of these questions for specific ecological scenarios. We employ several types of stochastic models: Markov chains, Markov processes, their diffusion approximations, and coalescent-like genealogy simulations. In Chapter 1 (Wittmann et al., 2013a, appeared in Theoretical Population Biology), we focus on a factor influencing the introduced population’s average growth rate: the intensity of competition with an ecologically similar native species. Our results indicate that the expected time until the introduced species drives the native competitor to extinction is smallest for intermediate competition intensity. This phenomenon results from the opposing effects of competition intensity at different points of the invasion process: On the one hand, intense competition renders the establishment of the introduced population more difficult; on the other hand, it facilitates the later exclusion of the native species. In Chapter 1, we also investigate to what extent the native species’ extinction is accelerated if a reduction in population size entails a reduction in genetic diversity and thus a reduced ability to adapt to a changing environment. We find this eco-genetic feedback to be particularly strong at small competition intensities. In Chapter 2 (Wittmann et al., 2013b, in press at Oikos), we compare introduction regimes with the same average number of individuals introduced per time unit, but with a different temporal distribution. Relative to regimes with many small introduction events, regimes with few large introduction events generate more variability in population-size trajectories. We show that this variability helps introduced populations to overcome difficult stages in the invasion process (those with a negative average growth rate), but is disadvantageous during easy stages (those with a positive average growth rate). In the light of our results, we can reinterpret three published data sets on invasion success under different introduction regimes. In Chapters 3 and 4 (Wittmann et al., 2013c,d), we examine levels of genetic diversity in populations that have successfully overcome a strong demographic Allee effect. In this ecological scenario, the average population growth rate is negative below a certain critical population size and positive above, such that the first stage in the invasion process is difficult and the second one easy. In Chapter 3, we assume Poisson-distributed offspring numbers. We show that compared to successful populations without an Allee effect, successful Allee-effect populations are expected to harbor either more or less genetic diversity, depending on the magnitude of typical founder population sizes relative to the critical population size. Part of the explanation is that, counter-intuitively, successful Allee-effect populations escape particularly fast from the range of small population sizes where genetic drift is strongest. In Chapter 3, we also identify conditions under which the critical population size can be estimated from genetic data. In Chapter 4, we consider a range of offspring-number models leading to either more or less variability in population dynamics than the Poisson model. For a fixed founder population size, we observe that the Allee effect has a negative influence on genetic diversity for small amounts of variability, but a positive influence for large amounts of variability. We show that the differences between our various offspring-number models are so substantial that they cannot be resolved by rescaling the parameters of the Poisson model. Taken together, these results offer some general conclusions with respect to the four main questions raised above. a) How fast an introduced population completes the invasion process is mainly determined by the presence and severity of difficult stages. Therefore, an ecological change promotes invasion success if it lessens such difficult stages. b) From the perspective of the introduced population, variability is advantageous during difficult but not during easy stages of the invasion process. c) Because the strength of genetic drift depends on population size, a key to understanding the population genetic consequences of invasion trajectories is to consider how much time the population of interest spends in different population-size ranges. d) Feedbacks between a reduction in population size and a loss of genetic diversity are strongest in ecological scenarios where the population of interest spends considerable time at small population sizes. Some of the most striking results in this dissertation cannot be understood from a deterministic point of view, but only when considering stochasticity. Thus, stochasticity does not just add “noise” to some average outcome, but can qualitatively change the behavior of biological systems.Der langfristige Erfolg einer eingeführten Population hängt von den ökologischen Bedingungen in ihrer neuen Umgebung ab, aber auch vom Zufall. Besonders offensichtlich ist die wichtige Rolle des Zufalls für kleine Populationen im Anfangsstadium einer Invasion. In diesem Stadium entscheidet sich, ob die eingeführte Population nach kurzer Zeit ausstirbt oder sich dauerhaft etablieren kann. Manche etablierten Populationen wachsen dann weiter und breiten sich räumlich aus, zum Teil mit schwerwiegenden Folgen für einheimische Gemeinschaften und Ökosysteme. Bislang ist nicht klar, welche Rolle der Zufall in diesen späteren Invasionsstadien spielt und welche populationsgenetischen und evolutionären Auswirkungen vom Zufall geprägte Invasionsverläufe haben. Mit dieser Dissertation möchte ich beitragen zu einer stochastischen öko-genetischen Theorie des gesamten Invasionsprozesses – von der Einführung bis hin zu möglichen Auswirkungen. Meine übergreifenden Fragen sind: a) Welche Rolle für den Invasionsverlauf spielen ökologische Faktoren, die die durchschnittliche Wachstumsrate der eingeführten Population beeinflussen? b) Und welche Rolle spielen Faktoren, die die stochastische Variabilität der Populationsdynamik beeinflussen? c) Wie viel genetische Diversität weisen eingeführte Populationen im Durchschnitt auf, wenn sie einen bestimmten Punkt im Invasionsprozess erreichen? d) Inwiefern können die populationsgenetischen Auswirkungen von Invasionsverläufen wiederum die Populationsdynamik beeinflussen und so zu einer Rückkopplung führen? Zusammen mit meinen Betreuern und Koautoren habe ich vier Studien durchgeführt, die sich für bestimmte ökologische Szenarien jeweils mit mindestens zwei dieser Fragen befassen. Dazu kommen im Verlauf der Dissertation verschiedene Typen von stochastischen Modellen zum Einsatz: Markov-Ketten, Markov- und Diffusionsprozesse sowie Coalescent-artige Genealogie-Simulationen. In Kapitel 1 (Wittmann et al., 2013a, erschienen in Theoretical Population Biology) konzentrieren wir uns auf einen Faktor, der die durchschnittliche Wachstumsrate der Population beeinflusst: die Stärke der Konkurrenz mit einer ökologisch ähnlichen einheimischen Art. Unsere Ergebnisse deuten darauf hin, dass die erwartete Zeit bis zum Aussterben des einheimischen Konkurrenten für mittlere Konkurrenzstärken am kleinsten ist. Das können wir dadurch erklären, dass die Konkurrenzstärke gegensätzliche Auswirkungen in verschiedenen Stadien des Invasionsprozesses hat: Einerseits erschwert eine hohe Konkurrenzstärke die Etablierung der eingeführten Art, andererseits führt eine hohe Konkurrenzstärke aber auch dazu, dass die einheimische Art schnell verdrängt werden kann. Zusätzlich untersuchen wir in Kapitel 1, wie stark eine öko-genetische Rückkopplung das Aussterben der einheimischen Population beschleunigen würde. Dazu berücksichtigen wir, dass ein Rückgang der einheimischen Populationsgröße zu einem Verlust an genetischer Diversität führt, und das wiederum zu schlechterer Anpassung an veränderte Umweltbedingungen und darum weiterem Schrumpfen der Population. Unsere Ergebnisse legen nahe, dass diese öko-genetische Rückkopplung dann besonders stark ist, wenn die Konkurrenz zwischen einheimischer und eingeführter Art eher schwach ist. In Kapitel 2 (Wittmann et al., 2013b, im Druck bei Oikos) untersuchen wir für feste durchschnittliche Einführungsraten (Individuen pro Zeiteinheit), welche Rolle die zeitliche Verteilung der Individuen spielt. Besonders wichtig ist hierbei die Beziehung zwischen zeitlicher Verteilung und der Variabilität in der Größenentwicklung der Population. Wir zeigen, dass Fälle mit wenigen großen Einführungsereignissen zu mehr Variabilität führen als Fälle mit vielen kleinen Einführungsereignissen. Diese Variabilität hilft den eingeführten Populationen dabei, schwierige Stadien im Invasionsprozess (also solche mit einer negativen durchschnittlichen Wachstumsrate) zu bewältigen, ist aber anderseits in einfachen Stadien mit positiver durchschnittlicher Wachstumsrate von Nachteil. Im Lichte unserer Ergebnisse können wir aus der Literatur bekannte Daten zu Invasionsprozessen neu interpretieren. In den Kapiteln 3 und 4 (Wittmann et al., 2013c,d) untersuchen wir die genetische Diversität von Populationen, die einen starken demografischen Allee-Effekt erfolgreich überwunden haben. Laut Definition ist dabei die durchschnittliche Wachstumsrate bei Populationsgrößen unterhalb einer gewissen kritischen Größe negativ und in größeren Populationen positiv, so dass das erste Stadium des Invasionsprozesses schwierig ist und das zweite einfach. In Kapitel 3 zeigen wir unter der Annahme Poisson-verteilter Nachkommenzahlen, dass erfolgreiche Allee-Effekt-Populationen je nach Startgröße entweder eine höhere oder eine niedrigere durchschnittliche genetische Diversität aufweisen als erfolgreiche Populationen ohne Allee-Effekt. Das kommt zum Teil daher, dass erfolgreiche Allee-Effekt-Populationen besonders schnell das schwierige erste Stadium des Invasionsprozesses verlassen, wo genetische Drift am stärksten ist. Außerdem untersuchen wir in Kapitel 3, unter welchen Bedingungen sich die kritische Populationsgröße aus genetischen Daten schätzen lässt. In Kapitel 4 betrachten wir eine Reihe von Modellen für die Anzahl an Nachkommen von Individuen oder Paaren in der Population. Manche dieser Modelle führen zu mehr stochastischer Variabilität in der Populationsdynamik, andere zu weniger Variabilität als das in Kapitel 3 betrachtete Poisson-Modell. Für feste Startgröße beobachten wir, dass der Allee-Effekt bei kleiner Variabilität einen negativen Einfluss auf die genetische Diversität hat und bei großer Variabilität einen positiven Einfluss. Wir zeigen weiterhin, dass die Unterschiede zwischen unseren Nachkommenzahl-Modellen so substanziell sind, dass sie sich nicht durch eine Umskalierung der Parameter des Poisson-Modells erklären lassen. Zusammen genommen erlauben uns diese Ergebnisse einige allgemeine Schlussfolgerungen bezüglich der vier oben aufgeführten übergreifenden Fragen. a) Wie schnell eine eingeführte Population den Invasionsprozess durchläuft, hängt hauptsächlich davon ab, ob es schwierige Stadien gibt, und wie schwierig diese sind. Deshalb begünstigt eine ökologische Veränderung den Invasionserfolg dann, wenn sie schwierige Stadien im Invasionsprozess mindert. b) Aus der Perspektive der eingeführten Population ist Variabilität in schwierigen Stadien des Invasionsprozesses von Vorteil, aber in einfachen Stadien von Nachteil. c) Da die Stärke der genetischen Drift von der Populationsgröße abhängt, können wir die populationsgenetischen Auswirkungen von Invasionsverläufen verstehen, indem wir analysieren, wie viel Zeit die betrachtete Population in verschiedenen Populationsgrößenbereichen verbringt. d) Rückkopplungen zwischen einem Rückgang der Populationsgröße und einem Verlust genetischer Diversität sind am stärksten, wenn die Population viel Zeit im Bereich kleiner Populationsgrößen verbringt. Einige der wesentlichsten Ergebnisse dieser Dissertation können aus einer deterministischen Perspektive nicht verstanden werden, sondern sind ein direktes Produkt von Stochastizität. Dies macht deutlich, dass Stochastizität nicht einfach einem gewissen Durchschnitts- ergebnis etwas Rauschen hinzufügt, sondern das Verhalten biologischer Systeme qualitativ verändern kann

A complex systems approach to education in Switzerland

The insights gained from the study of complex systems in biological, social, and engineered systems enables us not only to observe and understand, but also to actively design systems which will be capable of successfully coping with complex and dynamically changing situations. The methods and mindset required for this approach have been applied to educational systems with their diverse levels of scale and complexity. Based on the general case made by Yaneer Bar-Yam, this paper applies the complex systems approach to the educational system in Switzerland. It confirms that the complex systems approach is valid. Indeed, many recommendations made for the general case have already been implemented in the Swiss education system. To address existing problems and difficulties, further steps are recommended. This paper contributes to the further establishment complex systems approach by shedding light on an area which concerns us all, which is a frequent topic of discussion and dispute among politicians and the public, where billions of dollars have been spent without achieving the desired results, and where it is difficult to directly derive consequences from actions taken. The analysis of the education system's different levels, their complexity and scale will clarify how such a dynamic system should be approached, and how it can be guided towards the desired performance