| 1. |
- Curtsdotter, Alva, 1983-
(författare)
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Extinctions in Ecological Communities : direct and indirect effects of perturbation on biodiversity
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the dawning of what may become Earth’s 6th mass extinction the topic of this thesis, understanding extinction processes and what determines the magnitude of species loss, has become only too relevant. The number of known extinctions (~850) during the last centuries translates to extinction rates elevated above the background rate, matching those of previous mass extinction events. The main drivers of these extinctions have been human land use, introduction of exotic species and overexploitation. Under continued anthropogenic pressure and climate change, the current extinction rates are predicted to increase tenfold.Large perturbations, such as the extinction drivers mentioned above, affects species directly, causing a change in their abundance. As species are not isolated, but connected to each other through a multitude of interactions, the change in abundance of one species can in turn affect others. Thus, in addition to the direct effect, a perturbation can affect a species indirectly through the ecological network in which the species is embedded. With this thesis, I wish to contribute to our basic understanding of these indirect effects and the role they play in determining the magnitude of species loss. All the studies included here are so called in silico experiments, using mathematical models to describe ecological communities and computer simulations to observe the response of these communities to perturbation.When a perturbation is severe enough, a species will be driven to extinction. The loss of a species from a system is in itself a large perturbation, and may result in further extinctions, so called secondary extinctions. The traits of the species initially lost, can be a potential predictor of the magnitude of secondary species loss. In Paper I of this thesis, I show that when making such predictions, it is important to incorporate temporally dynamic species interactions and abundances, in order not to underestimate the importance of certain species, such as top predators.I further show that species traits alone are not particularly good predictors of secondary extinction risk (Paper I), but that in combination with community level properties they are (Paper II). Indeed, there seems to be an interaction such that the specific property making a community prone to secondary species loss, depends on what kind of species was lost in the primary extinction. As different types of perturbation put different types of species at risk of (primary) extinction, this means that the specific property making a community prone to secondary species loss, will depend on the type of perturbation the community is subjected to.One of the predicted main drivers of future species extinction is climate change. If the local climate becomes adverse, a species can either migrate to new and better areas or stay and evolve. Both these processes will be important in determining the magnitude of species loss under climate change. However, migration and evolution do not occur in vacuum – the biotic community in which these processes play out may modulate their effect on biodiversity. In paper III, I show that the strength of competition between species modulates the effect of both dispersal and evolution on the magnitude of species loss under climate change. The three-way interaction between interspecific competition, evolution and dispersal, creates a complex pattern of biodiversity responses, in which both evolution and dispersal can either increase or decrease the magnitude of species loss. Thus, when species interactions are incorporated, it is clear that even though migration and evolution may alleviate the impact of climate change for some species, they may indirectly aggravate the situation for others.In Paper III, the aspect of climate change incorporated in the model is an increase in mean annual temperature. But climate change is also predicted to increase environmental variability. Paper IV shows that species-rich communities are more sensitive to high environmental variability than species-poor ones. The smaller population sizes in the species-rich communities increased the extinction risk connected to population fluctuations driven by the variable environment. Hence, systems such as tropical forests and coral reefs are predicted to be particularly sensitive to the increased variability that may follow with climate change.In Paper IV, primary extinctions of primary producers result in extinction cascades of consumer species, when they lose their prey. However, in reality a consumer species might be able to switch to another prey, and such flexibility has both been observed and suggested as a potential rescue mechanism. But what is beneficial for an individual predator in the short-term can become detrimental to the ecological community in the long-term. Paper V shows that consumer flexibility often led to consumers continuously overexploiting their new prey, in the worst case to the point of system collapse. Thus, the suggested rescue mechanism aggravated the effect of initial species loss, rather than ameliorating it.Overall, the research presented here, underscores the importance of including population dynamics and biotic interactions when studying the effects of perturbation on biodiversity. Many of the results are complex, hard to foresee or even counter-intuitive, arising from the indirect effects of the perturbation being translated through the living web of species interactions.
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| 2. |
- Kaneryd, Linda
(författare)
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Dynamics of ecological communities in variable environments : local and spatial processes
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ecosystems of the world are currently facing a variety of anthropogenic perturbations, such as climate change, fragmentation and destruction of habitat, overexploitation of natural resources and invasions of alien species. How the ecosystems will be affected is not only dependent on the direct effects of the perturbations on individual species but also on the trophic structure and interaction patterns of the ecological community. Of particular current concern is the response of ecological communities to climate change. Increased global temperature is expected to cause an increased intensity and frequency of weather extremes. A more unpredictable and more variable environment will have important consequences not only for individual species but also for the dynamics of the entire community. If we are to fully understand the joint effects of a changing climate and habitat fragmentation, there is also a need to understand the spatial aspects of community dynamics. In the present work we use dynamic models to theoretically explore the importance of local (Paper I and II) and spatial processes (Paper III-V) for the response of multi-trophic communities to different kinds of perturbations.In paper I we investigate how species richness and correlation in species responses to a highly variable environment affect the risk of extinction cascades. We find that the risk of extinction cascades increases with increasing species richness especially when the correlation among species is low. Initial stochastic extinctions of primary producer species unleash bottomup extinction cascades, where specialist consumers are especially vulnerable. Although the risks of extinction cascades were higher in the species-rich systems, we found that the temporal stability of aggregate abundance of primary producers increased with increasing richness. Thus, species richness had a two-sided effect on community stability. Also during the extinction cascades it is possible that more robust species and interaction patterns will be selected which would further act to stabilize the post-extinction communities. In paper II we explore how the process of disassembly affects the structure of the interaction network and the robustness of the community to additional disturbances. We find that the disassembled communities are structurally different and more resistant to disturbances than equally sized communities that have not gone through a phase of disassembly. The disassembled communities are topologically as well as dynamically more stable than non-disassembled communities.In paper III, IV and V we expand the analysis to incorporate the spatial dimension. In paper III we analyze how metacommunities (a set of local communities coupled by species dispersal) in spatially explicit landscapes respond to environmental variation. We examine how this response is affected by varying 1) species richness in the local communities, 2) the degree of correlation in species response to the environmental variation, between species within patches (species correlation) and among patches (spatial correlation) and 3) dispersal pattern of species. First we can confirm that our previous findings from paper I regarding local species richness and correlation among species within a patch are robust to the inclusion of a spatial dimension. However our results also show that the spatial dynamics are of great importance: first we find that the risk of global extinctions increases with increasing spatial correlation. Second we find that the pattern and rate of dispersal are important; a high migration rate in combination with localized dispersal decrease the risk of global extinctions whereas a global dispersal pattern increases the risk of global extinctions. When dispersal is global the subpopulations of a species become more synchronized which reduces the potential for a patch to become recolonized following extinctions. We also demonstrate the importance of both local and spatial processes when examining the temporal stability of primary production at the scale of metapopulations, local communities and metacommunities.In paper IV we investigate how the spatial structure of the landscape (number of patches) and dispersal pattern of species affect a metacommunities response to increased mortality during dispersal and local loss of species. We find a two-sided effect of dispersal on metacommunity persistence; on the one hand, high migration rate significantly reduces the risk of bottom-up extinction cascades following the removal of a species when dispersal involves no risk. On the other hand, high migration rate increases extinction risks when dispersal imposes a risk to the dispersing individuals, especially when dispersal is global. Species with long generation times at the highest trophic level are particularly vulnerable to extinction when dispersal involves a risk. These results suggest that decreasing the mortality risk of dispersing individuals by constructing habitat corridors or by improving the quality of the habitat matrix might greatly increase the robustness of metacommunities to local loss of species by enhancing recolonisations and rescue effects.In paper V we use network theory to identify keystone patches in the landscape, patches that are of critical importance for the local and global persistence of species in the metacommunity. By deleting patches one at a time and investigating the risk of local and global extinctions we quantified the importance of a patch’s position in the landscape for the persistence of species within the metacommunity. A selection of indices were used including some local indices that measure the connectedness of a patch in the intact network and some indices which measure the decrease in a global index after the deletion of the patch from the network. Global indices are those that give an impression of the connectivity of the entire patch network. We find that deletion of patches contributing strongly to the connectivity of the entire patch network had the most negative effect on species persistence.
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| 3. |
- Eklöf, Anna, 1976-
(författare)
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Species extinctions in food webs : local and regional processes
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Loss of biodiversity is one of the most severe threats to the ecosystems of the world. The major causes behind the high population and species extinction rates are anthropogenic activities such as overharvesting of natural populations, pollution, climate change and destruction and fragmentation of natural habitats. There is an urgent need of understanding how these species losses affect the ecological structure and functioning of our ecosystems. Ecological communities exist in a landscape but the spatial aspects of community dynamics have until recently to large extent been ignored. However, the community’s response to species losses is likely to depend on both the structure of the local community as well as its interactions with surrounding communities. Also the characteristics of the species going extinct do affect how the community can cope with species loss. The overall goal of the present work has been to investigate how both local and regional processes affect ecosystem stability, in the context of preserved biodiversity and maintained ecosystem functioning. The focus is particularly on how these processes effects ecosystem’s response to species loss. To accomplish this goal I have formulated and analyzed mathematical models of ecological communities. We start by analyzing the local processes (Paper I and II) and continue by adding the regional processes (Paper III, IV and V).In Paper I we analyze dynamical models of ecological communities of different complexity (connectance) to investigate how the structure of the communities affects their resistance to species loss. We also investigate how the resistance is affected by the characteristics, like trophic level and connectivity, of the initially lost species. We find that complex communities are more resistant to species loss than simple communities. The loss of species at low trophic levels and/or with high connectivity (many links to other species) triggers, on average, the highest number of secondary extinctions. We also investigate the structure of the post-extinction community. Moreover, we compare our dynamical analysis with results from topological analysis to evaluate the importance of incorporating dynamics when assessing the risk and extent of cascading extinctions.The characteristics of a species, like its trophic position and connectivity (number of ingoing and outgoing trophic links) will affect the consequences of its loss as well as its own vulnerability to secondary extinction. In Paper II we characterize the species according to their trophic/ecological uniqueness, a new measure of species characteristic we develop in this paper. A species that has no prey or predators in common with any other species in the community will have a high tropic uniqueness. Here we examine the effect of secondary extinctions on an ecological community’s trophic diversity, the range of different trophic roles played by the species in a community. We find that secondary extinctions cause loss of trophic diversity greater than expected from chance. This occurs because more tropically unique species are more vulnerable to secondary extinctions.In Paper III, IV and V we expand the analysis to also include the spatial dimension. Paper III is a book chapter discussing spatial aspects of food webs. In Paper IV we analyze how metacommunities (a set of local communities in the landscape connected by species dispersal) respond to species loss and how this response is affected by the structure of the local communities and the number of patches in the metacommunity. We find that the inclusion of space reduces the risk of global and local extinctions and that lowly connected communities are more sensitive to species loss.In Paper V we investigate how the trophic structure of the local communities, the spatial structure of the landscape and the dispersal patterns of species affect the risk of local extinctions in the metacommunity. We find that the pattern of dispersal can have large effects on local diversity. Dispersal rate as well as dispersal distance are important: low dispersal rates and localized dispersal decrease the risk of local and global extinctions while high dispersal rates and global dispersal increase the risk. We also show that the structure of the local communities plays a significant role for the effects of dispersal on the dynamics of the metacommunity. The species that are most affected by the introduction of the spatial dimension are the top predators.
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| 4. |
- Gilljam, David, 1978-
(författare)
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Structure and Stability of Ecological Networks : The role of dynamic dimensionality and species variability in resource use
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main focus of this thesis is on the response of ecological communities to environmental variability and species loss. My approach is theoretical; I use mathematical models of networks where species population dynamics are described by ordinary differential equations. A common theme of the papers in my thesis is variation – variable link structure (Paper I) and within-species variation in resource use (Paper III and IV). To explore how such variation affect the stability of ecological communities in variable environments, I use numerical methods evaluating for example community persistence (the proportion of species surviving over time; Paper I, II and IV). I also develop a new method for quantifying the dynamical dimensionality of an ecological community and investigate its effect on community persistence in stochastic environments (Paper II). Moreover, if we are to gain trustworthy model output, it is of course of major importance to create study systems that reflect the structures of natural systems. To this end, I also study highly resolved, individual based empirical food web data sets (Paper III, IV).In Paper I, the effects of adaptive rewiring induced by resource loss on the persistence of ecological networks is investigated. Loss of one species in an ecosystem can trigger extinctions of other dependent species. For instance, specialist predators will go extinct following the loss of their only prey unless they can change their diet. It has therefore been suggested that an ability of consumers to rewire to novel prey should mitigate the consequences of species loss by reducing the risk of cascading extinction. Using a new modelling approach on natural and computer-generated food webs I find that, on the contrary, rewiring often aggravates the effects of species loss. This is because rewiring can lead to overexploitation of resources, which eventually causes extinction cascades. Such a scenario is particularly likely if prey species cannot escape predation when rare and if predators are efficient in exploiting novel prey. Indeed, rewiring is a two-edged sword; it might be advantageous for individual predators in the short term, yet harmful for long-term system persistence.The persistence of an ecological community in a variable world depends on the strength of environmental variation pushing the community away from equilibrium compared to the strength of the deterministic feedbacks, caused by interactions among and within species, pulling the community towards the equilibrium. However, it is not clear which characteristics of a community that promote its persistence in a variable world. In Paper II, using a modelling approach on natural and computer-generated food webs, I show that community persistence is strongly and positively related to its dynamic dimensionality (DD), as measured by the inverse participation ratio (IPR) of the real part of the eigenvalues of the community matrix. A high DD means that the real parts of the eigenvalues are of similar magnitude and the system will therefore approach equilibrium from all directions at a similar rate. On the other hand, when DD is low, one of the eigenvalues has a large magnitude of the real part compared to the others and the deterministic forces pulling the system towards equilibrium is therefore weak in many directions compared to the stochastic forces pushing the system away from the equilibrium. As a consequence the risk of crossing extinction thresholds and boundaries separating basins of attractions increases, and hence persistence decreases, as DD decreases. Given the forecasted increase in climate variability caused by global warming, Paper II suggests that the dynamic dimensionality of ecological systems is likely to become an increasingly important property for their persistence.In Paper III, I investigate patterns in the size structure of one marine and six running freshwater food webs: that is, how the trophic structure of such ecological networks is governed by the body size of its interacting entities. The data for these food webs are interactions between individuals, including the taxonomic identity and body mass of the prey and the predator. Using these detailed data, I describe how patterns in diet variation and predator variation scales with the body mass of predators or prey, using both a species- and a size-class-based approach. I also compare patterns of size structure derived from analysis of individual-based data with those patterns that result when data are aggregated into species (or size class-based) averages. This comparison shows that analysis based on species averaging can obscure interesting patterns in the size structure of ecological communities. For example, I find that the strength of the relationship between prey body mass and predator body mass is consistently underestimated when species averages are used instead of the individual level data. In some cases, no relationship is found when species averages are used, but when individual-level data are used instead, clear and significant patterns are revealed. These results have potentially important implications for parameterisation of models of ecological communities and hence for predictions concerning their dynamics and response to different kinds of disturbances.Paper IV continues the analysis of the highly resolved individual-based empirical data set used in Paper III and investigates patterns and effects of within- and between species resource specialisation in ecological communities. Within-species size variation can be considerable. For instance, in fishes and reptiles, where growth is continuous, individuals pass through a wide spectrum of sizes, possibly more than four orders of magnitude, during the independent part of their life cycle. Given that the size of an organism is correlated with many of its fundamental ecological properties, it should come as no surprise that an individual’s size affects the type of prey it can consume and what predators will attack it (Paper III). In Paper IV, I quantify within- and between species differences in predator species’ prey preferences in natural food webs and subsequently explore its consequences for dynamical dimensionality (Paper II) and community stability in stage structured food web models. Among the natural food webs there are webs where species overlap widely in their resource use while the resource use of size-classes within species differs. There are also webs where differences in resource use among species is relatively large and the niches of sizeclasses within species are more similar. Model systems with the former structure are found to have low dynamical dimensionality and to be less stable compared to systems with the latter structure. Thus, although differential resource use among individuals within a species is likely to decrease the intensity of intraspecific competition and favor individuals specializing on less exploited resources it can destabilize the community in which the individuals are embedded.
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| 5. |
- Lü, Bo
(författare)
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Dynamics of the Early Stages in Metal-on-Insulator Thin Film Deposition
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thin films consist of nanoscale layers of material that are used in many technological applications to either functionalize a surface or serve as parts in miniaturized devices. The properties of a film are closely related to its microstructure, which in turn can be tuned during film preparation. Thin film growth involves a multitude of atomic-scale processes that cannot always be easily studied experimentally. Therefore, different types of computer simulations have been developed in order to test theoretical models of thin film growth in a highly controlled way. To be able to compare simulation and experimental results, the simulations must be able to model events on experimental time-scales, i.e. several seconds or minutes. This is achievable with the kinetic Monte Carlo method.In this work, kinetic Monte Carlo simulations are used to model the initial growth stages of metal films on insulating, amorphous substrates. This includes the processes of island nucleation, three-dimensional island growth and island coalescence. Both continuous and pulsed vapor fluxes are investigated as deposition sources, and relations between deposition parameters and film morphology are formulated. Specifically, the film thickness at what is known as the “elongation transition” is studied as a function of the temporal profile of the vapor flux, adatom diffusivity and the coalescence rate. Since the elongation transition occurs due to hindrance of coalescence completion, two separate scaling behaviors of the elongation transition film thickness are found: one where coalescence occurs frequently and one where coalescence occurs infrequently. In the latter case, known nucleation behaviors can be used favorably to control the morphology of thin films, as these behaviors are not erased by island coalescence. Experimental results of Ag growth on amorphous SiO2 that confirm the existence of these two “growth regimes” are also presented for both pulsed and continuous deposition by magnetron sputtering. Knowledge of how to avoid coalescence for different deposition conditions allows nucleation for metal-on-insulator material systems to be studied and relevant physical quantities to be determined in a way not previously possible. This work also aids understanding of the growth evolution of polycrystalline films, which in conjunction with advanced deposition techniques allows thin films to be tailored to specific applications.
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