| 1. |
- Jonsson, Anders, et al.
(författare)
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Limitations of acyclic causal graphs for planning
- 2014
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Ingår i: Artificial Intelligence. - : Elsevier. - 0004-3702 .- 1872-7921. ; 210, s. 36-55
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Tidskriftsartikel (refereegranskat)abstract
- Causal graphs are widely used in planning to capture the internal structure of planning instances. Researchers have paid special attention to the subclass of planning instances with acyclic causal graphs, which in the past have been exploited to generate hierarchical plans, to compute heuristics, and to identify classes of planning instances that are easy to solve. This naturally raises the question of whether planning is easier when the causal graph is acyclic. In this article we show that the answer to this question is no, proving that in the worst case, the problem of plan existence is PSPACE-complete even when the causal graph is acyclic. Since the variables of the planning instances in our reduction are propositional, this result applies to STRIPS planning with negative preconditions. We show that the reduction still holds if we restrict actions to have at most two preconditions. Having established that planning is hard for acyclic causal graphs, we study two subclasses of planning instances with acyclic causal graphs. One such subclass is described by propositional variables that are either irreversible or symmetrically reversible. Another subclass is described by variables with strongly connected domain transition graphs. In both cases, plan existence is bounded away from PSPACE, but in the latter case, the problem of bounded plan existence is hard, implying that optimal planning is significantly harder than satisficing planning for this class.
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| 2. |
- Jonsson, Anders, et al.
(författare)
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When Acyclicity is not Enough: Limitations of the Causal Graph
- 2013
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Ingår i: Proceedings of the Twenty-Third International Conference on Automated Planning and Scheduling. - : AAAI Press. - 9781577356097 ; , s. 117-125
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Konferensbidrag (refereegranskat)abstract
- Causal graphs are widely used in planning to capture the internal structure of planning instances. In the past, causal graphs have been exploited to generate hierarchical plans, to compute heuristics, and to identify classes of planning instances that are easy to solve. It is generally believed that planning is easier when the causal graph is acyclic. In this paper we show that this is not true in the worst case, proving that the problem of plan existence is PSPACE-complete even when the causal graph is acyclic. Since the variables of the planning instances in our reduction are propositional, this result applies to STRIPS planning with negative pre-conditions. Having established that planning is hard for acyclic causal graphs, we study a subclass of planning instances with acyclic causal graphs whose variables have strongly connected domain transition graphs. For this class, we show that plan existence is easy, but that bounded plan existence is hard, implying that optimal planning is significantly harder than satisficing planning for this class.
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| 3. |
- Jonsson, Tomas, et al.
(författare)
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Trophic interactions affect the population dynamics and risk of extinction of basal species in food webs
- 2010
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Ingår i: Ecological Complexity. - : Elsevier. - 1476-945X .- 1476-9840. ; 7:1, s. 60-68
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Tidskriftsartikel (refereegranskat)abstract
- This paper addresses effects of trophic complexity on basal species, in a Lotka–Volterra model with stochasticity. We use simple food web modules, with three trophic levels, and expose every species to random environmental stochasticity and analyze (1) the effect of the position of strong trophic interactions on temporal fluctuations in basal species’ abundances and (2) the relationship between fluctuation patterns and extinction risk. First, the numerical simulations showed that basal species do not simply track the environment, i.e. species dynamics do not simply mirror the characteristics of the applied environmental stochasticity. Second, the extinction risk of species was related to the fluctuation patterns of the species.More specifically, we show (i) that despite being forced by random stochasticity without temporal autocorrelation (i.e. white noise), there is significant temporal autocorrelation in the time series of all basal species’ abundances (i.e. the spectra of basal species are red-shifted), (ii) the degree of temporal autocorrelation in basal species time series is affected by food web structure and (iii) the degree of temporal autocorrelation tend to be correlated to the extinction risks of basal species.Our results emphasize the role of food web structure and species interactions in modifying the response of species to environmental variability. To shed some light on the mechanisms we compare the observed pattern in abundances of basal species with analytically predicted patterns and show that the change in the predicted pattern due to the addition of strong trophic interactions is correlated to the extinction risk of the basal species. We conclude that much remain to be understood about the mechanisms behind the interaction among environmental variability, species interactions, population dynamics and vulnerability before we quantitatively can predict, for example, effects of climate change on species and ecological communities. Here, however, we point out a new possible approach for identifying species that are vulnerable to environmental stochasticity by checking the degree of temporal autocorrelation in the time series of species. Increased autocorrelation in population fluctuations can be an indication of increased extinction risk.
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| 4. |
- Lövdén, Martin, et al.
(författare)
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The dimensionality of between-person differences in white matter microstructure in old age
- 2013
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Ingår i: Human Brain Mapping. - : Wiley. - 1065-9471 .- 1097-0193. ; 34:6, s. 1386-1398
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Tidskriftsartikel (refereegranskat)abstract
- Between-person differences in white matter microstructure may partly generalize across the brain and partly play out differently for distinct tracts. We used diffusion-tensor imaging and structural equation modeling to investigate this issue in a sample of 260 adults aged 60–87 years. Mean fractional anisotropy and mean diffusivity of seven white matter tracts in each hemisphere were quantified. Results showed good fit of a model positing that individual differences in white matter microstructure are structured according to tracts. A general factor, although accounting for variance in the measures, did not adequately represent the individual differences. This indicates the presence of a substantial amount of tract-specific individual differences in white matter microstructure. In addition, individual differences are to a varying degree shared between tracts, indicating that general factors also affect white matter microstructure. Age-related differences in white matter microstructure were present for all tracts. Correlations among tract factors did not generally increase as a function of age, suggesting that aging is not a process with homogenous effects on white matter microstructure across the brain. These findings highlight the need for future research to examine whether relations between white matter microstructure and diverse outcomes are specific or general. Hum Brain Mapp, 2012. © 2012 Wiley Periodicals, Inc.
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| 5. |
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| 6. |
- Almgren, Magnus, 1972, et al.
(författare)
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Mapping Systems Security Research at Chalmers
- 2011
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Ingår i: First SysSec Workshop (SysSec 2011). - 9780769545301 ; , s. 67-70
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Konferensbidrag (refereegranskat)abstract
- The department of Computer Science and Engineering at Chalmers University has a long tradition of research in systems security, including security metrics, attack detection, and mitigation. We focus on security issues arising in four specific environments: (1) backbone links, (2) sensor networks, (3) the connected car, and (4) the smart grid. In this short summary we describe recent results as well as open research questions we are exploring.
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| 7. |
- ANGHOLT, JONAS, et al.
(författare)
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A First Security Analysis of a Secure Intermodal Goods Transport System
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The goods transport business involves a lot of money and is a big part of the infrastructure of any European country. There are often many different actors involved in each transport and the communication network is rather complex due to the point-topoint communication structure. It is easy to understand why there is a high demand for increased simplicity and effectiveness. With this in mind, the e-Freight project which is based on PEPPOL has moved towards a standardized solution by developing a communication system based on access points (APs). These APs act as the interface to the system and makes it easy to establish communication between any two connected actors.With PEPPOL and e-Freight as a foundation, VOLVO leads the SITS project in close cooperation with Stena Line and DSV. The goal is to develop a harmonized communication framework that promotes increased sharing of information between actors and enable new applications to increase effectiveness and security in the chain of transportation. This leads to simplified accessibility for actors to a set of services by being connected to an AP. At the same time service providers benefit from being able to easily set up cloud services available for all actors. In addition to the back-office communication between APs, external devices such as cellphones, in-vehicle computers and check-in terminals can communicate directly with each other over short distances. This type of communication is only partially specified and a mutual standard is yet to be decided upon.In this report we have analyzed the SITS project from an IT-security perspective. The back-end system derived from e-Freight is looked into and communication links, access points, protocols, certificate handling etc., are examined. Another concern in the SITS project is the short-range communication between trucks and terminals. Since RFID is a highly potential candidate for use in this area, we have studied the technology by categorizing a typical RFID system into three distinct layers and researched important security threats with the classic CIA approach. Based on the security issues found, countermeasures such as encryption, authentication and protection against man-in-themiddle attacks are reviewed.
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| 8. |
- Balk, Lennart, et al.
(författare)
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Biomarkers in Natural Fish Populations Indicate Adverse Biological Effects of Offshore Oil Production
- 2011
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Ingår i: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 6:5, s. e19735-
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Tidskriftsartikel (refereegranskat)abstract
- Background: Despite the growing awareness of the necessity of a sustainable development, the global economy continues to depend largely on the consumption of non-renewable energy resources. One such energy resource is fossil oil extracted from the seabed at offshore oil platforms. This type of oil production causes continuous environmental pollution from drilling waste, discharge of large amounts of produced water, and accidental spills. Methods and principal findings: Samples from natural populations of haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) in two North Sea areas with extensive oil production were investigated. Exposure to and uptake of polycyclic aromatic hydrocarbons (PAHs) were demonstrated, and biomarker analyses revealed adverse biological effects, including induction of biotransformation enzymes, oxidative stress, altered fatty acid composition, and genotoxicity. Genotoxicity was reflected by a hepatic DNA adduct pattern typical for exposure to a mixture of PAHs. Control material was collected from a North Sea area without oil production and from remote Icelandic waters. The difference between the two control areas indicates significant background pollution in the North Sea. Conclusion: It is most remarkable to obtain biomarker responses in natural fish populations in the open sea that are similar to the biomarker responses in fish from highly polluted areas close to a point source. Risk assessment of various threats to the marine fish populations in the North Sea, such as overfishing, global warming, and eutrophication, should also take into account the ecologically relevant impact of offshore oil production.
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| 9. |
- Berg, Sofia, 1980-
(författare)
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Community Robustness Analysis : Theoretical Approaches to Identifying Keystone Structures in Ecological Communities
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most of the world’s ecosystems suffer from stress caused by human activities such as habitat destruction, fragmentation, overexploitation of species and climate change. These factors affect the reproduction and/or survival of individual species as well as interactions between species in ecological communities. Forthcoming effects of this are altered abundances, direct species loss, and indirect cascading extinctions, with yet largely unknown consequences on community structure and functioning. Today, biodiversity loss is of global concern since human society and welfare depend upon resources and services provided by ecosystems. The importance of considering entire ecological communities as a target for conservation and management has been increasingly recognized due to the interdependencie of species. Our ability to make predictions of the response of ecological communities to stress and biodiversity loss is in need of a deeper understanding of how structure and dynamical processes contributes to the functioning and stability of a community. In this thesis I use mathematical theory and dynamical models to study the response of community structure and resilience to a variety of disturbances affecting species and species interactions, ranging from small perturbations (Papers I-II) to large perturbations (species extinctions, Papers IIIIV).In Paper I we develop Community Sensitivity Analysis (CSA) as an analytical tool to study how a small permanent perturbation to the intrinsic growth rate, or mortality rate, of species is expected to affect i) the resilience (return rate) and ii) the structure (distribution of species equilibrium abundances) of an ecological community. Species interactions are described using Lotka-Volterra predator-prey dynamics. We apply CSA on the pelagic food webs of Lake Vättern and the Baltic Sea, respectively, and find that a change in the mortality rate of large-bodied species has a higher impact on community resilience and structure, compared to a perturbation to small-bodied species. However, analyzing the effect of a proportional change to the growth or mortality rate of species (elasticity analysis) shows that smallbodied species have proportionally larger effects on species equilibrium abundances, but not on resilience. CSA can also be used to study the effect of permanent (absolute or proportional) changes to inter- and intraspecific interaction strengths. For the two pelagic systems used in this study, CSA reveal that changes in the effect of a prey on its consumer tend to affect community structure and resilience significantly more than changes in the effect of a predator on its prey.In Paper II we assess the importance of rare species for the structure and resilience of ecological communities. First we show analytically, for a two species predator-prey system, that a change in the intrinsic growth rate of the rare species affect resilience more than a change in the growth rate of the common species. To test the generality of these results we next apply CSA on complex model food webs. In the analysis we distinguish between four trophic groups, each including only species with a similar trophic position, to separate the effect of abundance from the trophic position of species. Using mixed effect models we find support for our analytical predictions. More precisely, we find a strong negative relationship between the importance (sensitivity) of a species and its equilibrium abundance within all consumer groups and a weaker, but significant, relationship for producer species. The results from this study suggest that rare species can act as keystones through their effect on both community resilience and community structure, regardless of its trophic position.In Paper III we evaluate the risk of food web collapse caused by different trait-based extinction scenarios. In previous studies, groups of species, e.g. rare species, large-bodied species and top predators, have been identified to be relatively more prone to extinctions and other studies have found that extinctions of such species have comparably small effects on the remaining community. Using mathematical models of species dynamics we study the response of ecological communities to species removal (i.e. the proportion of species needed to be primarily removed to cause a 50% reduction in species richness, R50) when species are sequentially removed from the food web based on eight different traits. We show, contrary to some previous studies of sequential extinction simulations, that communities can be very vulnerable to realistic species loss. We furthermore find that the response of communities seems to depend on whether the extinction sequence follows a bottom-up or top-down direction, making it difficult to identify one particular extinction sequence as the most important/severe sequence.Finally, in Paper IV we aim to identify traits of species that can be used to identify keystone species, in terms of causing the highest proportion of secondary extinctions following their loss, in food webs with different degree of disassembly. Moreover, we analyze if the loss of a species that triggers a cascade of many secondary extinctions are the same species being identified as a keystones using Community Sensitivity Analysis. To answer these questions we randomly remove species from a set of 100 model communities. We analyze the relationship between the number of secondary extinctions following the randomly removed species and a range of species traits in communities where i) 75-100% of the initial number of species remain, ii) 50-75% of all species remain, iii) 25-50% of all species remain and iv) only 0-25% of all species remain. We find that the variation in secondary extinctions explained using species traits increases when the degree of food web disassembly and food web connectance are taken into account. The most important trait varies for different degrees of food web disassembly and also depends on whether basal species can go primarily extinct or not. However, due to correlation between most important traits, we conclude that the key status of different traits is rather robust against structural changes in the model food webs. Interestingly, food webs seem to be most sensitive to a random species loss after the loss of more than 25% of all initial species, suggesting that there is a threshold from which secondary extinctions increases. We also conclude that species being identified as keystones, based on the effect of their loss, are to some extent the same species being identified as having the largest effect on community structure and resilience, respectively, following a small perturbation.
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| 10. |
- Berg, Sofia, et al.
(författare)
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Using sensitivity analysis to identify keystone species and keystone links in size-based food webs
- 2011
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 120:4, s. 510-519
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Tidskriftsartikel (refereegranskat)abstract
- Human-induced alterations in the birth and mortality rates of species and in the strength of interactions within and between species can lead to changes in the structure and resilience of ecological communities. Recent research points to the importance of considering the distribution of body sizes of species when exploring the response of communities to such perturbations. Here, we present a new size-based approach for assessing the sensitivity and elasticity of community structure (species equilibrium abundances) and resilience (rate of return to equilibrium) to changes in the intrinsic growth rate of species and in the strengths of species interactions. We apply this approach on two natural systems, the pelagic communities of the Baltic Sea and Lake Vättern, to illustrate how it can be used to identify potential keystone species and keystone links. We find that the keystone status of a species is closely linked to its body size. The analysis also suggests that communities are structurally and dynamically more sensitive to changes in the effects of prey on their consumers than in the effects of consumers on their prey. Moreover, we discuss how community sensitivity analysis can be used to study and compare the fragility of communities with different body size distributions by measuring the mean sensitivity or elasticity over all species or all interaction links in a community. We believe that the community sensitivity analysis developed here holds some promise for identifying species and links that are critical for the structural and dynamic robustness of ecological communities.
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