| 51. |
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| 52. |
- Berg, Sofia, et al.
(författare)
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Ecological communities are vulnerable to realistic extinction sequences
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Loss of species will directly change the structure of ecological communities, which in turn may cause additional species loss (secondary extinctions) due to indirect effects (e.g. loss of resources or altered population dynamics). The vulnerability of food webs to repeated species loss is expected to be affected by food web topology, species interactions and the order in which species go extinct. Species traits such as body size, abundance and connectivity probably determine the vulnerability to extinction of species and, thus, the order in which species go primarily extinct. However, how different sequences of primary extinctions affect the vulnerability of food webs to secondary extinctions, when species abundances are allowed to respond dynamically, is not well understood. So far, only one study has incorporated species dynamics when assessing the effect of different extinction sequences on community structure, and only a limited number of extinction sequences have been evaluated. Here, using complex model food webs and including population dynamics, we analyze the effect of 33 extinction sequences on community structure using R50 (the proportion of primarily removed species needed to cause a 50% reduction in species richness) as a measure of community robustness to secondary extinctions. As expected, we find community structure to be highly vulnerable to removal of primary producers. More surprisingly, removing species based on traits that are strongly linked to the trophic position of species (such as large-bodied species, rare species, species with a high net effect, species with a high trophic position) are found to be as destructive as removing only primary producers. Such top-down oriented removal of species are often considered to correspond to realistic primary extinctions of species, but earlier studies, based on topological approaches, have not found such realistic extinction sequences to have any drastic effect on the remaining community. Thus, our result suggests that ecological communities could be more vulnerable to realistic extinction sequences than previously believed.
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| 53. |
- Berg, Sofia, et al.
(författare)
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Rare but important: perturbations to uncommon species have disproportionately large impact on ecological communities
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The majority of species in the ecosystems of the world are rare. Because the contributions to community biomass and productivity of many of these species are small it has been suggested that loss of rare species should have relatively small ecological consequences. However, the extent to which rare species affect the structure and stability of ecosystems is largely unknown. Using a theoretical approach, based on analytical methods, we here investigate how perturbations to rare as well as common species affect the structure (distribution of equilibrium abundances of species) and resilience (recovery rate) of complex ecological communities. We show that, contrary to expectation, resilience and structure of ecological communities are generally more sensitive to perturbations to rare than to common species. We find the explanation for this to lie in the cause of rarity: rare species tend to interact strongly, on a per capita basis, with other species. Our results suggest that many rare species are likely to fill important ecological roles in ecosystems.
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| 54. |
- 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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| 55. |
- Berg, Sofia, et al.
(författare)
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Using species traits to predict secondary extinctions during food web disassembly
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Global change keeps pushing species towards extinction which results in altered structures of ecological communities. Consequently, the loss of certain species can trigger a cascade of secondary extinctions resulting in further degradation of the system. The importance of species for upholding the structure of communities may be linked to the traits of species. However, due to the altered structure of communities following species loss, the importance of species (and species traits) may change as the structure of the food web change. Using a dynamical approach and simulating species loss in complex model communities we analyze the potential importance of 11 species traits. We find that the most important trait varies for different degree of food web collapse and food web connectance. Though, as the most important traits of species usually are correlated we conclude that the importance of species traits is rather robust against structural changes in the communities (especially when only consumer species are targets of primarily extinctions). Interestingly, food webs display a collapse threshold (after the initial loss of approximately 25% of all species) from which secondary extinctions increases. Finally, consider only the loss of consumer species, the effect (number of secondary extinctions) on community structure caused by a large perturbation (species loss) is positively correlated to the response of food webs resulting from a small perturbation to the same species.
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| 56. |
- Bergfeldt, Ulla, et al.
(författare)
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Cortical activation changes and improved motor function in stroke patients after focal spasticity therapy- an interventional study applying repeated fMRI.
- 2015
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Ingår i: BMC neurology. - : Springer Science and Business Media LLC. - 1471-2377. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- Impaired dominant hand function in stroke patients is a common clinical problem. Functional improvement after focal spasticity therapy is well documented but knowledge about central correlates is sparse. Brain activity was therefore followed during therapy with repeated functional magnetic resonance imaging (fMRI). The purpose was to analyse motor function and central nervous system (CNS) correlates in response to a standardized motor task in stroke patients after a comprehensive focal spasticity therapy.
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| 57. |
- Berglund, Tomas, et al.
(författare)
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An obstacle-avoiding minimum variation B-spline problem
- 2003
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Ingår i: Proceedings. - Los Alamitos, Calif : IEEE Communications Society. - 0769519857 ; , s. 156-161
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Konferensbidrag (refereegranskat)abstract
- We study the problem of computing a planar curve, restricted to lie between two given polygonal chains, such that the integral of the square of arc-length derivative of curvature along the curve is minimized. We introduce the minimum variation B-spline problem, which is a linearly constrained optimization problem over curves, defined by B-spline functions only. An empirical investigation indicates that this problem has one unique solution among all uniform quartic B-spline functions. Furthermore, we prove that, for any B-spline function, the convexity properties of the problem are preserved subject to a scaling and translation of the knot sequence defining the B-spline.
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| 58. |
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| 59. |
- Berglund, Tomas, et al.
(författare)
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Epi-convergence of minimum curvature variation B-splines
- 2003
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We study the curvature variation functional, i.e., the integral over the square of arc-length derivative of curvature, along a planar curve. With no other constraints than prescribed position, slope angle, and curvature at the endpoints of the curve, the minimizer of this functional is known as a cubic spiral. It remains a challenge to effectively compute minimizers or approximations to minimizers of this functional subject to additional constraints such as, for example, for the curve to avoid obstacles such as other curves. In this paper, we consider the set of smooth curves that can be written as graphs of three times continuously differentiable functions on an interval, and, in particular, we consider approximations using quartic uniform B- spline functions. We show that if quartic uniform B-spline minimizers of the curvature variation functional converge to a curve, as the number of B-spline basis functions tends to infinity, then this curve is in fact a minimizer of the curvature variation functional. In order to illustrate this result, we present an example of sequences of B-spline minimizers that converge to a cubic spiral.
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| 60. |
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