| 1. |
- Krebs, C. J., et al.
(författare)
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Terrestrial trophic dynamics in the Canadian Arctic
- 2003
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Ingår i: Canadian Journal of Zoology-Revue Canadienne De Zoologie. ; 81, s. 827-843
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Tidskriftsartikel (refereegranskat)abstract
- The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.
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| 2. |
- Krebs, C. J., et al.
(författare)
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Synchrony in lemming and vole populations in the Canadian Arctic
- 2002
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Ingår i: Canadian Journal of Zoology-Revue Canadienne De Zoologie. ; 80:8, s. 1323-1333
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Tidskriftsartikel (refereegranskat)abstract
- Population fluctuations may occur in synchrony among several rodent species at a given site, and they may occur in synchrony over large geographical areas. We summarize information on synchrony in lemmings and voles from the Canadian Arctic for the past 20 years. The most detailed available information is from the central Canadian Arctic, where snap-trap samples have been taken annually at several sites for periods of up to 15 years. Geographical synchrony in the same species among different sites was strong, especially for the central and eastern Canadian Arctic. Synchrony among different species at a given site was also generally high. When one species is at high density, densities of all species at that site tend to be high. These results do not easily fit the mobile-predator hypothesis proposed to explain regional synchrony, and are more consistent with the weather hypothesis, which we suggest both entrains synchrony among sites and enforces synchrony among species within a site. We tentatively support the weather hypothesis for geographical synchrony in lemmings, and recommend the establishment of a circumpolar program to monitor lemming cycles and predator movements that would advance our understanding of these large-scale patterns of cyclic synchrony.
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| 3. |
- Fuhse, C, et al.
(författare)
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Interface quality and thermal stability of laser-deposited metal/MgO multilayers
- 2004
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Ingår i: Applied Optics. - 1559-128X .- 2155-3165. ; 43:34, s. 6265-6269
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Tidskriftsartikel (refereegranskat)abstract
- Metal/MgO multilayers (metal of Fe80Ni80Nb20, and Ti) with bilayer periods in the range 1.2-3.0 nm have been prepared by pulsed laser deposition and characterized by both hard and soft-x-ray reflectometry. The interface roughness is found to be less than or equal to 0.5 nm in all the samples and is nearly independent of the total number of deposited bilayers. The interface roughness, however, depends on the absolute thickness of the individual layers and increases from approximate to0.3 nm for a 3.0-nm period to approximate to0.5 nm for a bilayer period of 1.2 nm. The multilayers are found to be highly stable up to temperatures as high as 550 degreesC. The hard-x-ray reflectivity of the multilayers decreases for T > 300 degreesC, whereas the layered structure is stable up to 550 degreesC. The reflectivity in the water window region of soft x rays, lambda = 3.374 nm, was found to be 0.4% at an angle of incidence of approximate to54degrees for multilayers with 60 bilayers at a period of approximate to2.1 nm. (C) 2004 Optical Society of America.
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| 4. |
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| 5. |
- Sinclair, A R E, et al.
(författare)
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Testing hypotheses of trophic level interactions: a boreal forest ecosystem
- 2000
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 89:2, s. 313-328
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Tidskriftsartikel (refereegranskat)abstract
- Models of community organization involve variations of the top-down (predator control) or bottom-up (nutrient limitation) hypotheses. Verbal models, however, can be interpreted in different ways leading to confusion. Therefore, we predict from first principles the range of possible trophic level interactions, and define mathematically the instantaneous effects of experimental perturbations. Some of these interactions are logically and biologically unfeasible. The remaining set of 27 feasible models is based on an initial assumption, for simplicity, of linear interactions between trophic levels. Many more complex and non-linear models are logically feasible but, for parsimony, simple ones are tested first. We use an experiment in the boreal forest of Canada to test predictions of instantaneous changes to trophic levels and distinguish between competing models. Seven different perturbations systematically removed each trophic level or, for some levels, supplemented them. The predictions resulting from the perturbations were concerned with the direction of change in biomass in the other levels. The direct effects of each perturbation produced strong top-down and bottom-up changes in biomass. At both the vegetation and herbivore levels top-down was stronger than bottom-up despite some compensatory growth stimulated by herbivory. The combination of experiments produced results consistent with two-way (reciprocal) interactions at each level. Indirect effects on one or two levels removed from the perturbation were either very weak or undetectable. Top-down effects were strong when direct but attenuated quickly. Bottom-up effects were less strong but persisted as indirect effects to higher levels. Although the 'pure reciprocal' model best fits our results for the boreal forest system different models may apply to different ecosystems around the world.
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