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- Accomando, E., et al.
(författare)
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Physics with e + e - linear colliders
- 1998
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Ingår i: Physics Reports. - 0370-1573. ; 299:1, s. 1-78
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Tidskriftsartikel (refereegranskat)abstract
- The physics potential of e + e - linear colliders is summarized in this report. These machines are planned to operate in the first phase at a center-of-mass energy of 500 GeV, before being scaled up to about 1 TeV. In the second phase of the operation, a final energy of about 2 TeV is expected. The machines will allow us to perform precision tests of the heavy particles in the Standard Model, the top quark and the electroweak bosons. They are ideal facilities for exploring the properties of Higgs particles, in particular in the intermediate mass range. New vector bosons and novel matter particles in extended gauge theories can be searched for and studied thoroughly. The machines provide unique opportunities for the discovery of particles in supersymmetric extensions of the Standard Model, the spectrum of Higgs particles, the supersymmetric partners of the electroweak gauge and Higgs bosons, and of the matter particles. High precision analyses of their properties and interactions will allow for extrapolations to energy scales close to the Planck scale where gravity becomes significant. In alternative scenarios, i.e. compositeness models, novel matter particles and interactions can be discovered and investigated in the energy range above the existing colliders up to the TeV scale. Whatever scenario is realized in Nature, the discovery potential of e + e - linear colliders and the high precision with which the properties of particles and their interactions can be analyzed, define an exciting physics program complementary to hadron machines.
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| 2. |
- Jenkins, T. M., et al.
(författare)
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Effects of population density on individual growth of brown trout in streams
- 1999
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Ingår i: Ecology. ; 80:3, s. 941-956
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Tidskriftsartikel (refereegranskat)abstract
- Some studies suggest that lotic populations of brown trout (Salmo trutta) are regulated through density-dependent mortality and emigration to the extent that mean growth rates of resident survivors are unrelated to trout densities. To test this, we studied the relationship between density and growth, mortality, and emigration of brown trout in two alpine streams and a set of stream channels in eastern California. We sampled trout at the scale of "segments" (5-31 m long riffles, runs, and pools) and "sections" (340-500 m in length) of Convict Creek over a 3-yr period. Trout were also sampled during 6 yr in seven 90-m sections of Mammoth Creek. For 2 yr, we manipulated trout densities in Convict Creek by removing trout from two sections and adding trout to two other sections. We also manipulated densities in seven 50-m stream channels, using a natural size distribution of trout in one year and underyearlings only in a second year. In both streams, average size (body length or mass) of underyearlings in fall was negatively related to trout density and was furthermore affected by sampling location and year. The strong, negative relationship between individual mass and density of trout could be detected at the spatial scale of whole sections, but not at the scale of individual segments. The Convict Creek and stream channel experiments also revealed strong negative effects of density on average mass of underyearlings in fall, and on proportional mass increase of yearling and older trout from spring to fall. In contrast, mortality and emigration were unrelated to initial stocking densities in the channels. In all our data, the negative effects on growth were most pronounced at densities <1 trout/m(2) and the growth-density relationships were well described by negative power curves. Large individuals were always less affected by increasing trout density than were small individuals, suggesting a competitive advantage of large over small trout that increased with density. We conclude that individual growth of brown trout in streams can be affected by trout density to an extent that suggests a substantial influence on population regulation. Results from our multiyear, multiscale, and experimental study indicate that density dependence in the growth of stream salmonids will be difficult to detect in purely observational data, especially in systems with relatively high fish densities (where the growth-density relationship has a flat slope), when data are collected and analyzed at small spatial scales, and when insufficient information is collected to assess the contribution of interannual variation in growth.
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| 3. |
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| 4. |
- Nisbet, R. M., et al.
(författare)
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Primary-productivity gradients and short-term population dynamics in open systems
- 1997
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Ingår i: Ecological Monographs. ; 67:4, s. 535-553
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Tidskriftsartikel (refereegranskat)abstract
- We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer reproduction, and at the spatial scales typically employed in field experiments. These models incorporate flexible behavioral responses of organisms to their predators and resources in spatially heterogeneous environments that are open to immigration and emigration. The basic models include passive immigration at all trophic levels, producer growth rates and losses to grazer consumption, grazer emigration rate as a behavioral response to producer and predator densities, grazer losses to predator consumption, and predator emigration as a function of grazer density. We model this system as: (1) a set of ordinary differential equations (''well-mixed model''); (2) a set of partial differential equations describing a population of discrete grazers foraging on discrete patches of primary producers (''discrete-grazer model''); and (3) a set of simulation rules describing the movement and foraging of individual grazers and the growth of primary producers on discrete patches in explicit space (''individual-based model''). The ordinary differential-equation models produced similar results to individual-based models with well-mixed producers, and the discrete-grazer and individual-based models produced similar results when grazers possessed a long-term memory of patch reward rates. The well-mixed and discrete-grazer models thus represent specific, limiting cases of the general individual-based model. Multiple equilibria and sustained oscillations are possible but are less likely in the discrete-grazer and individual-based models than in the well-mixed model, because localized foraging of discrete grazers leads to the rapid development of spatial heterogeneity in producer biomass and, hence, to a decrease in overall primary production. All models predict that stable equilibrium densities of all trophic levels increase with enrichment, provided grazers increase their emigration rates as predator density increases. If increasing predator density leads to decreasing grazer-emigration rates, predator and grazer densities increase, but producer biomass may increase or decrease with enrichment. These results contrast with predictions from models that assume ideal free distributions of grazers and/or predators with respect to their resources. Our models also predict that densities at all trophic levels will increase with increasing producer immigration, and that producer density will decline with increasing grazer immigration and increase with increasing predator immigration. Our qualitative findings on enrichment are used to interpret an experiment dealing with the short-term dynamics of a stream community open to grazers and predators.
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