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Emergent properties of bio-physical self-organization in streams
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| Abstract |
Self-organization is increasingly recognized as an important regulating process inseveral ecosystem types. Many studies of self-organization in biology have focusedon the emergent effects of self-organized spatial patterns on biological properties,such as enhanced productivity or resilience to disturbances. Despite its prevalencein biological theory, self-organization is not yet considered extensively ingeophysical studies. Most studies do not fully incorporate the interactive biophysicalfeedbacks between biological and physical processes. For this reason, it isunknown if self-organization has emergent effects on both physical and biologicalproperties. In this thesis, using submerged aquatic macrophytes in streams as amodel system, I study the emergent properties of self-organization – resulting fromthe two-way interaction between plant growth and flow redistribution – for bothhydrological and ecological processes. Specifically, I study the role of selforganizationof aquatic macrophytes in terms of regulation or river flow (velocitiesand depth), biological interactions (inter-specific effects on growth and dispersal,and intra-specific effects on spatial patterning) and resource uptake. My studycombines field experiments and field observations, laboratory flume experimentsand mathematical models.In Chapter 2, I examine whether self-organization, resulting from the twowayinteraction between plant growth and flow redistribution, has emergentproperties for stream-level hydrodynamic conditions. The results of a combinedmathematical modelling and empirical study suggest that this self-organizationprocess creates heterogeneity in plant biomass and water flow. In turn, it stabilizesboth flow velocities and water levels under varying discharges, with multipleecosystem benefits. Therefore, my results reveal an important link between plantdrivenself-organization processes of streams and the ecosystem services theyprovide in terms of water flow regulation and habitat diversity.Summary182The regulation of water flow by submerged aquatic macrophytes studied inChapter 2 point to important implications of plant-driven hydrodynamicheterogeneity for species interactions and biodiversity. Consequently, in Chapter3 I explore the link between self-organization and facilitation. Model and field datasuggest that self-organized pattern formation promotes plant species coexistencein lotic communities by creating a ‘landscape of facilitation’. Here, multiple newniches arise for species adapted to a wide range of hydrodynamic conditions.Model predictions are confirmed by field observations of species coexistence andtransplantation experiments supporting the hypothesis of facilitation. This studytherefore highlights that understanding of the way in which competition andfacilitation interact in many ecosystems is crucial for successful management oftheir biodiversity.The self-organization process described in Chapter 2 and 3 is based on thedivergence of water flow around vegetation patches. Divergence of physical stressis a common mechanism underlying the patchy distribution of foundation speciesin many ecosystems. Yet, it is still unknown if the mechanisms underlying selforganizedspatial pattern formation are important for facilitation of speciesestablishment. Retention of vegetative propagules by existing vegetation is animportant bottleneck for macrophyte establishment in streams. Water flow is boththe dispersal vector of plant propagules and the stress factor that leads tovegetation patchiness. In Chapter 4, I study how this flow divergence mechanismaffects facilitation through propagule retention within existing macrophytepatches, using mesocosm, flume and field studies. My study suggests thatfeedbacks between patch reconfiguration and water movement, leading to selforganization,can facilitate the establishment of macrophyte species duringdispersal and primary colonization.In Chapter 5, I test if existing spatial patchiness of macrophytes, resultingfrom the two-way interaction between vegetation and hydrodynamics, affectsvegetation occurrence through intra-specific facilitation. Field manipulationexperiments reveal that vegetation patches in streams organize themselves in Vshapesto minimize hydrodynamic and drag forces, resembling the flight formationadopted by migratory birds. My findings highlight that bio-physical interactionsSummary183shape the way organisms position themselves in landscapes exposed to physicalflows.In Chapter 6, I explore the emergent effects of self-organized spatialpatchiness due to different species on resource uptake. Many studies of planthydrodynamicfeedbacks deal with monospecific canopies. However, naturallandscapes are a diverse community composed of patches of different species withcontrasting traits. These patches potentially influence each other through theirhydrodynamic effects, for instance affecting the uptake of resources that is crucialfor productivity. My findings suggest that patches of macrophyte species interactwith each other through facilitation of resource uptake, by influencing turbulence.This was tested in racetrack flume experiments combining hydrodynamicmeasurements and 15N labelled ammonium incubations. My study reveals theimportance of turbulence as an agent of interaction between different species.Moreover, the findings suggest that interactions between heterogeneous,multispecific patchy vegetation are crucial to understand aquatic ecosystemfunctioning and services of nutrient load reduction.In conclusion, my research highlights the crucial emergent effects of selforganizationfor a range of physical and biological properties in ecosystems. Thisstudy reveals a previously unexplored link between self-organized biologicalpatterns and ecosystem services such as flow regulation, habitat and speciesdiversity. Understanding the regulating functions of spatial self-organization isessential to maintain the valuable ecosystem services it supports. In manyecosystems, bio-physical interactions are still approached in a static way that doesnot fully incorporate the dynamic feedbacks. Future empirical and modellingstudies in other biogeomorphic landscapes should aim to further include thesereciprocal feedbacks. This will increase our understanding of the full range ofemergent properties of spatial patterning in ecosystems, and the widerapplicability of the conclusions presented here. The findings of this thesis alsosuggest how bio-physical interactions can be used to guide the management andrestoration of aquatic ecosystems. Hence, our fundamental research questions onself-organization can be closely linked to applied research. Such linkage is valuableto guide the management and conservation of ecosystems. |
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