| 1. |
- Andersson, Jens, et al.
(författare)
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Plastic reources polymorphism : effects or resource availability on Arctic char (Salvelinus alpinus) morphology
- 2005
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Ingår i: Biological Journal of the Linnean Society. - London : Acad. P.. - 0024-4066 .- 1095-8312. ; 85:3, s. 341-351
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Tidskriftsartikel (refereegranskat)abstract
- Resource polymorphism has been suggested to be a platform for speciation. In some cases resource polymorphism depends on phenotypic plasticity but in other cases on genetic differences between morphotypes, which in turn has been suggested to be the ongoing development of a species pair. Here we study environmentally induced morphological differences in two age classes of Arctic char (Salvelinus alpinus) influencing char performance and diet in relation to resource availability. We found that structurally complex habitats with relatively lower zooplankton densities gave rise to individuals with a deeper body, and a downward positioned tip of the snout compared with individuals from structurally simple habitats with relatively higher zooplankton densities for both age classes. Environment also had an effect on foraging efficiency on zooplankton, with fish from structurally simple habitats had a higher foraging rate than fish from structurally complex habitats. Diet analyses showed that resource use in char mainly depends on the relative abundance of different resources. Therefore, to gain further understanding of resource polymorphism we suggest that future studies must include population dynamic feedbacks by the resources on the consumers.
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| 2. |
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| 3. |
- Ask, Jenny, 1976-, et al.
(författare)
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Terrestrial organic matter and light penetration : Effects on bacterial and primary production in lakes
- 2009
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Ingår i: Limnology and Oceanography. - : American Society of Limnology and Oceanography, Inc.. - 0024-3590 .- 1939-5590. ; 54:6, s. 2034-2040
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Tidskriftsartikel (refereegranskat)abstract
- We investigated productivity at the basal trophic level in 15 unproductive lakes in a gradient ranging from clear-water to brown-water (humic) lakes in northern Sweden. Primary production and bacterial production in benthic and pelagic habitats were measured to estimate the variation in energy mobilization from external energy sources (primary production plus bacterial production on allochthonous organic carbon) along the gradient. Clear-water lakes were dominated by autotrophic energy mobilization in the benthic habitat, whereas humic lakes were dominated by heterotrophic energy mobilization in the pelagic habitat. Whole-lake (benthic + pelagic) energy mobilization was negatively correlated to the light-extinction coefficient, which was determined by colored terrestrial organic matter in the lake water. Thus, variation in the concentration of terrestrial organic matter and its light-absorbing characteristics exerts strong control on the magnitude, as well as on the processes and pathways, of energy mobilization in unproductive lakes. We suggest that unproductive lakes in general are sensitive to input of terrestrial organic matter because of its effects on basal energy mobilization in both benthic and pelagic habitats.
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| 4. |
- Ask, Jenny, 1976-, et al.
(författare)
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Whole-lake estimates of carbon flux through algae and bacteria in benthic and pelagic habitats of clear-water lakes
- 2009
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Ingår i: Ecology. - Washington, DC, USA : Ecological Society of America. - 0012-9658 .- 1939-9170. ; 90:7, s. 1923-1932
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Tidskriftsartikel (refereegranskat)abstract
- This study quantified new biomass production of algae and bacteria in both benthic and pelagic habitats of clear-water lakes to contrast how carbon from the atmosphere and terrestrial sources regulates whole-lake metabolism. We studied four small unproductive lakes in subarctic northern Sweden during one summer season. The production of new biomass in both benthic and pelagic habitats was calculated as the sum of autotrophic production by algae and heterotrophic production by bacteria using allochthonous organic carbon (OC). Whole-lake production of new biomass was dominated by the benthic habitat (86% +/- 4% [mean +/- SD]) and by primary production (77% +/- 9%). Still, heterotrophic bacteria fueled by allochthonous OC constituted a significant portion of the new biomass production in both benthic (19% +/- 11%) and pelagic habitats (51% +/- 24%). In addition, overall net production (primary production minus respiration) was close to zero in the benthic habitats but highly negative (-163 +/- 81 mg C.m(-2).d(-1)) in pelagic regions of all lakes. We conclude (1) that allochthonous OC supported a significant part of total production of new biomass in both pelagic and benthic habitats, (2) that benthic habitats dominated the whole-lake production of new biomass, and (3) that respiration and net CO2 production dominated the carbon flux of the pelagic habitats and biomass production dominated the benthic carbon flux. Taken together, these findings suggest that previous investigations have greatly underestimated the productivity of clear-water lakes when benthic autotrophic production and metabolism of allochthonous OC have not been measured.
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| 5. |
- Byström, Mikael, 1963-
(författare)
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En broder, gäst och parasit : Uppfattningar och föreställningar om utlänningar, flyktingar och flyktingpolitik i svensk offentlig debatt 1942-1947
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Earlier studies have proposed that Swedish refugee policy started to change around 1942, when a restrictive refugee policy became more generous and humanitarian. From a quantitative point of view this statement is true: there were about ten thousand refugees in 1941, compared to almost two hundred thousand by the end of the war. However, this does not tell us whether the well-known discourses of Swedish inter-war anti-Semitism, nationalism and xenophobia underwent the same changes. The aim of this dissertation is to analyse the public debate concerning foreigners, refugees and refugee policy in 1942–1947.The dissertation puts forward the hypothesis of The Nordic prerogative. In brief, this prerogative meant that Sweden primarily held itself obliged to accept ethnical Northeners as refugees, and looked upon this obligation as more important than other considerations, such as the refugee’s ideological views, need of protection or humanitarian needs. Symptomatically, the groups which could not be entirely encompassed within the idea of a Nordic prerogative, particularly the Balts and the Danish Jews, were perceived as the most problematical refugee groups, both on a general level of the debates, and in specific issues.The idea of a Nordic prerogative did not derive from a sense of ethnical fraternity and humanitarian considerations alone, however. Several undertakings were also brought about by pragmatic considerations. Sweden sought goodwill, and reception of refugees was seen as one way of winning it.The dissertation also shows that the idea of a Nordic prerogative seems to become less important when the refugee comes closer to the everyday life of Sweden, where the Nordic refugees too were referred to as ”foreigners”, ”aliens” etc. As such, they had to put up with being spoken of in negatively loaded expressions, in the same way as other foreigners.
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| 6. |
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| 7. |
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| 9. |
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| 10. |
- Byström, Pär, et al.
(författare)
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Substitution of top predators : effects of pike invasion in a subarctic lake
- 2007
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Ingår i: Freshwater Biology. - : John Wiley & Sons, Inc. - 0046-5070 .- 1365-2427. ; 52:7, s. 1271–1280-
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Tidskriftsartikel (refereegranskat)abstract
- 1. Invasions of top predators may have strong cascading effects in ecosystems affecting both prey species abundance and lower trophic levels. A recently discussed factor that may enhance species invasion is climate change and in this context, we studied the effects of an invasion of northern pike into a subarctic lake ecosystem formerly inhabited by the native top predator Arctic char and its prey fish, ninespined stickleback. 2. Our study demonstrated a strong change in fish community composition from a system with Arctic char as top predator and high densities of sticklebacks to a system with northern pike as top predator and very low densities of sticklebacks. A combination of both predation and competition from pike is the likely cause of the extinction of char. 3. The change in top predator species also cascaded down to primary consumers as both zooplankton and predator-sensitive macroinvertebrates increased in abundance. 4. Although the pike invasion coincided with increasing summer temperatures in the study area we have no conclusive evidence that the temperature increase is the causal mechanism behind the pike invasion. But still, our study provides possible effects of future pike invasions in mountain lakes related to climate change. We suggest that future pike invasions will have strong effects in lake ecosystems, both by replacing native top consumers and through cascading effects on lower trophic levels.
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| 11. |
- Huss, Magnus, 1979-, et al.
(författare)
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Influence of growth history on the accumulation of energy reserves and winter mortality on young fish
- 2008
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Ingår i: Canadian Journal of Fisheries and Aquatic Sciences. - : EBSCO Host. - 0706-652X .- 1205-7533. ; 65:10, s. 2149-2156
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Tidskriftsartikel (refereegranskat)abstract
- In seasonal environments accumulated energy reserves are important to avoid starvation mortality during periods of low resource levels. Here we investigated patterns of energy accumulation and the importance of growth history for winter survival in young-of-the-year Eurasian perch (Perca fluviatilis). Under simulated winter conditions in aquaria’s we showed that high winter mortality most likely relate to the depletion of energy reserves in small perch. Correspondingly in a field study, using 4 lakes covering 3-6 lake years each, overwinter survival within cohorts was positively related to individual size. However, average size in autumn did not explain the variation in overwinter survival between cohorts. Instead we showed that seasonal growth history is an important factor. High growth rates late in season may increase cohort survival over winter irrespective of average size, related to a positive growth dependent increase in allocation to energy reserves when approaching winter. Mechanisms regulating within-season temporal dynamics of growth rates are therefore suggested to be important for overall cohort performance.
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| 12. |
- Huss, Magnus, 1979-
(författare)
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Ontogenetic scaling and the development of within-cohort size structure
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is increasingly recognized that individuals of the same species differ from each other and influence and respond to their environment in unique ways. This thesis deals with size variation among individuals that not only are of the same species but also of similar age. Such variation may develop even when individuals are born in the same environment, i.e. within a cohort. I have studied the sources and consequences of variation within and among cohorts from egg through early ontogeny using young-of-the-year (YOY) perch (Perca fluviatilis) as study organism. In agreement with predictions based on model results only taking exploitative interactions among individuals into account, I found that the broader the initial size distributions were, the more did the degree of size variation among individuals decrease over time. Still, with initially small size variation among individuals, in several experiments also size divergence was observed. Furthermore, size variation among individuals increased more under high compared to at low densities. Increased size variation over time may be explained by size-dependent diet shifts allowing for initially larger individuals to make an early diet shift when the first resource becomes limiting. However, as size divergence also was observed in situations with only shared resources available, it can be concluded that diet shifts are not a prerequisite for size divergence in young animal cohorts. Hence, I also suggest that mechanisms not related to competition for limiting resources, such as genetic variation, stochasticity and behavioural traits must be taken into account, especially when initial size differences are small. The importance of considering size variation among individuals within cohorts was demonstrated in a study of winter mortality in YOY perch cohorts. A large individual size in autumn was shown to increase overwinter survival within cohorts. However, late summer growth rather than average body size reached in autumn explained variation in overwinter survival between cohorts. Higher accumulation to lipid reserves and accordingly lower mortality over winter was observed in years with high growth rates late in the season. In another study I showed that apparent patterns of density-dependent growth can emerge among larval fish, but rather than a result of density-dependent resource limitation this was due to variation in size-selective predation pressure. Individuals in the right end of the size distributions grew in to a high predation pressure from cannibalistic perch when cannibal density was high, coinciding with high larval perch densities. Finally, as substantial size variation among individuals can develop within cohorts, also intra-cohort cannibalism can occur. Using a physiologically structured population model it was shown that the development of size bimodality within cohorts as a result of intra-cohort cannibalism is critically dependent on long hatching periods, high victim densities and density-dependent feedbacks on shared resources.
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| 13. |
- Huss, Magnus, 1979-, et al.
(författare)
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Resource heterogeneity, diet shifts and intra-cohort competition : effects on size divergence in YOY fish
- 2008
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Ingår i: Oecologia. - Berlin / Heidelberg : Springer. - 0029-8549 .- 1432-1939. ; 158:2, s. 249-257
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Tidskriftsartikel (refereegranskat)abstract
- Most organisms exhibit a substantial size variation among individuals due to individual differences in experienced biotic and abiotic environmental conditions and because individuals undergo growth and development during most of their life time. One important issue in this context is how size variation within cohorts may develop over time. Here we tested the hypothesis, in gape-limited animals such as fish, that size divergence among individuals within a cohort depends on the opportunity to undergo size-dependent diet shifts, by allowing initially larger individuals to make an early diet shift when the first resource becomes limiting. We used young-of-the-year perch (Perca fluviatilis) as our study organism. Competitive intensity and the opportunity to undergo a diet shift from zooplankton to macroinvertebrates affected both mean growth rates and the extent to which inter-individual variation in growth was manifested. As predicted, increased competition combined with the presence of both zooplankton and benthic macroinvertebrates increased the degree of size variation. However, size divergence was also observed among individuals when only the initial resource, zooplankton, was available. We argue that only non-exploitative interactions, such as dominance structures and social interactions could have caused this latter pattern, as exploitative competition is expected to lead to size convergence due to the superior competitive ability of smaller individuals. Our results suggest that diet shifts are not a prerequisite for size divergence in animal cohorts and that dominance and social interactions may have similar effects on size variation within cohorts. Finally, development of size variation is suggested to have strong implications for overall cohort performance.
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| 14. |
- Huss, Magnus, 1979-, et al.
(författare)
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The origin and development of individual size variation in early pelagic stages of fish
- 2007
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Ingår i: Oecologia. - Berlin / Heidelberg : Springer. - 0029-8549 .- 1432-1939. ; 153:1, s. 57-67
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Tidskriftsartikel (refereegranskat)abstract
- Size variation among individuals born at the same time in a common environment (within cohorts) is a common phenomenon in natural populations. Still, the mechanisms behind the development of such variation and its consequences for population processes are far from clear. We experimentally investigated the development of early within-cohort size variation in larval perch (Perca fluviatilis). Specifically we tested the influence of initial variation, resulting from variation in egg strand size, and intraspecific density for the development of size variation. Variation in egg strand size translated into variation in initial larval size and time of hatching, which, in turn, had effects on growth and development. Perch from the smallest egg strands performed on average equally well independent of density, whereas larvae originating from larger egg strands performed less well under high densities. We related this difference in density dependence to size asymmetries in competitive abilities leading to higher growth rates of groups consisting of initially small individuals under high resource limitation. In contrast, within a single group of larvae, smaller individuals grew substantially slower under high densities whereas large individuals performed equally well independent of density. As a result, size variation among individuals within groups (i.e. originating from the same clutch) increased under high densities. This result may be explained by social interactions or differential timing of diet shifts and a depressed resource base for the initially smaller individuals. It is concluded that to fully appreciate the effects of density-dependent processes on individual size variation and size-dependent growth, consumer feedbacks on resources need to be considered.
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| 15. |
- Karlsson, Jan, et al.
(författare)
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Light limitation of nutrient-poor lake ecosystems
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 460, s. 506-509
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Tidskriftsartikel (refereegranskat)abstract
- Productivity denotes the rate of biomass synthesis in ecosystems and is a fundamental characteristic that frames ecosystem function and management. Limitation of productivity by nutrient availability is an established paradigm for lake ecosystems1, 2, 3. Here, we assess the relevance of this paradigm for a majority of the world's small, nutrient-poor lakes, with different concentrations of coloured organic matter4, 5. By comparing small unproductive lakes along a water colour gradient, we show that coloured terrestrial organic matter controls the key process for new biomass synthesis (the benthic primary production) through its effects on light attenuation. We also show that this translates into effects on production and biomass of higher trophic levels (benthic invertebrates and fish). These results are inconsistent with the idea that nutrient supply primarily controls lake productivity, and we propose that a large share of the world's unproductive lakes, within natural variations of organic carbon and nutrient input, are limited by light and not by nutrients. We anticipate that our result will have implications for understanding lake ecosystem function and responses to environmental change. Catchment export of coloured organic matter is sensitive to short-term natural variability and long-term, large-scale changes, driven by climate and different anthropogenic influences6, 7. Consequently, changes in terrestrial carbon cycling will have pronounced effects on most lake ecosystems by mediating changes in light climate and productivity of lakes.
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| 16. |
- Persson, Lennart, et al.
(författare)
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State-dependent invasion windows for prey in size-structured predator–prey systems: whole lake experiments
- 2007
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Ingår i: Journal of Animal Ecology. - : Wiley. - 0021-8790 .- 1365-2656. ; 76:1, s. 94-104
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Tidskriftsartikel (refereegranskat)abstract
- 1.In size-structured communities where individuals grow in size over their life cycle, interactions between species will shift between competitive and predatory interactions depending on size relationships. The outcome of interactions will subsequently depend on the strength of competitive and predatory interactions, respectively.2.In a whole lake experiment including four experimental lakes, it was tested under which conditions the competing prey, roach Rutilus rutilus, could successfully recruit into systems previously occupied by the predator, perch Perca fluviatilis. Two replicated introduction experiments were carried out 3 years apart.3.Roach were able to successfully recruit into three of the four experimental lakes of which two were also inhabited by the top predator pike Esox lucius. Resource levels were unrelated to whether roach could successfully recruit into the systems as recruiting roach in all years were feeding close to their maximum rate.4.High population fecundity of roach and low predation pressure by perch combined were necessary ingredients for successful recruitment and the presence of only one of these conditions did not result in successful recruitment.5.It is hypothesized that, although roach were able to successfully recruit into one lake with only perch present in addition to the two lakes that also inhabited pike, long-term coexistence of roach and perch depends on the presence of another top predator (e.g. pike) selectively preying on perch. This hypothesis was supported by data on co-occurrence of perch and roach in different lakes.6.Overall, the results are in accordance with expectation of size-structured life-history omnivory theory suggesting that coexistence between top predator and intermediate consumer is fragile.
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| 17. |
- Svensson, Kerstin, 1977-, et al.
(författare)
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Evolution of subspecies of Francisella tularensis
- 2005
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Ingår i: Journal of Bacteriology. - 0021-9193 .- 1098-5530. ; 187:11, s. 3903-3908
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Tidskriftsartikel (refereegranskat)abstract
- Analysis of unidirectional genomic deletion events and single nucleotide variations suggested that the four subspecies of Francisella tularensis have evolved by vertical descent. The analysis indicated an evolutionary scenario where the highly virulent F. tularensis subsp. tularensis (type A) appeared before the less virulent F. tularensis subsp. holarctica (type B). Compared to their virulent progenitors, attenuated strains of F. tularensis exhibited specific unidirectional gene losses.
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