Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Effectiveness of a Diet and Resistance Exercise Intervention on Muscle Health in Older Adults: ProMuscle in Practice
    Dongen, Ellen J.I. van; Haveman-Nies, Annemien ; Doets, Esmée L. ; Dorhout, Berber G. ; Groot, Lisette C.P.G.M. de - \ 2020
    Journal of the American Medical Directors Association (2020). - ISSN 1525-8610 - p. 1065 - 1072.e3.
    dietary protein - lean body mass - Physical functioning - resistance exercise - strength

    Objectives: Clinical studies show that resistance exercise and a protein-rich diet can counteract the age-related decline of muscle mass, strength, and physical performance. The aim of the ProMuscle in Practice study was to test effectiveness of a resistance exercise and dietary protein intervention for older adults implemented in a real-life setting. Design: A randomized controlled multicenter intervention study. Setting and Participants: One hundred sixty-eight community-dwelling older adults were included (age 75 ± 6 years). A 12-week intensive support intervention including progressive resistance exercise supervised by a physiotherapist and dietitian guidance on increasing protein intake was followed by a voluntary 12-week moderate support intervention to continue the adapted lifestyle pattern. The control group received no intervention. Methods: Compliance was measured through attendance lists and 3-day food records. Physical functioning, leg strength (3-repetition maximum, knee extension strength), lean body mass [(LBM) dual-energy X-ray absorptiometry], and quality of life (5-level EQ-5D) were measured at baseline, and after 12 and 24 weeks. Differences in change between groups were assessed with linear mixed model analysis. Results: The intervention group increased protein intake and attended 83.6% of the training sessions. Short Physical Performance Battery score slightly increased in intervention participants [from 10.1 (95% confidence interval 9.7–10.5) to 10.4 (10.0–10.8) at week 12 and 10.6 (10.2–10.9) at week 24], where control participants decreased (time × treatment interactions, P < .05). Improvements in intervention group compared with controls were also observed for Timed Up-and-Go, strength and LBM at both time points (time × treatment interactions, P < .05). No difference between groups was found for the 6-Minute Walking Test, activities of daily living, and quality of life. Conclusions and Implications: ProMuscle in Practice was effective on improving muscle strength and LBM, with small changes in the composite function score in community-dwelling older adults in a real-life setting. Further research should explore feasibility of real-life implementation, as well as improving long-term compliance.

    Quantifying the source-sink balance and carbohydrate content in three tomato cultivars
    Li, T. ; Heuvelink, E. ; Marcelis, L.F.M. - \ 2015
    Frontiers in Plant Science 6 (2015). - ISSN 1664-462X
    dry-matter production - leaf photosynthesis - plant-growth - leaves - strength - yield - metabolism - simulation - storage - light
    Supplementary lighting is frequently applied in the winter season for crop production in greenhouses. The effect of supplementary lighting on plant growth depends on the balance between assimilate production in source leaves and the overall capacity of the plants to use assimilates. This study aims at quantifying the source-sink balance and carbohydrate content of three tomato cultivars differing in fruit size, and to investigate to what extent the source/sink ratio correlates with the potential fruit size. Cultivars Komeet (large size), Capricia (medium size), and Sunstream (small size, cherry tomato) were grown from 16 August to 21 November, at similar crop management as in commercial practice. Supplementary lighting (High Pressure Sodium lamps, photosynthetic active radiation at 1 m below lamps was 162 mu mol photons m(-2) s(-1); maximum 10 h per day depending on solar irradiance level) was applied from 19 September onward. Source strength was estimated from total plant growth rate using periodic destructive plant harvests in combination with the crop growth model TOMSIM. Sink strength was estimated from potential fruit growth rate which was determined from non-destructively measuring the fruit growth rate at non-limiting assimilate supply, growing only one fruit on each truss. Carbohydrate content in leaves and stems were periodically determined. During the early growth stage, Komeet' and Capricia' showed sink limitation and 'Sunstream' was close to sink limitation. During this stage reproductive organs had hardly formed or were still small and natural irradiance was high (early September) compared to winter months. Subsequently, during the fully fruiting stage all three cultivars were strongly source-limited as indicated by the low source/sink ratio (average source/sink ratio from 50 days after planting onward was 0.17, 0.22, and 0.33 for 'Komeet, 'Capricia,' and 'Sunstream,' respectively). This was further confirmed by the fact that pruning half of the fruits hardly influenced net leaf photosynthesis rates. Carbohydrate content in leaves and stems increased linearly with the source/sink ratio. We conclude that during the early growth stage under high irradiance, tomato plants are sink-limited and that the level of sink limitation differs between cultivars but it is not correlated with their potential fruit size. During the fully fruiting stage tomato plants are source-limited and the extent of source limitation of a cultivar is positively correlated with its potential fruit size.
    There are no nonresponders to resistance-type exercise training in older men and women
    Churchward-Venne, T.A. ; Tieland, C.A.B. ; Verdijk, L. ; Leenders, M. ; Dirks, M.L. ; Groot, C.P.G.M. de; Loon, L.J.C. van - \ 2015
    Journal of the American Medical Directors Association 16 (2015)5. - ISSN 1525-8610 - p. 400 - 411.
    fiber contractile function - protein supplementation - myofiber hypertrophy - physical-activity - cluster-analysis - elderly-people - muscle size - sarcopenia - strength - humans
    Objective To assess the proposed prevalence of unresponsiveness of older men and women to augment lean body mass, muscle fiber size, muscle strength, and/or physical function following prolonged resistance-type exercise training. Design/Setting/Participants A retrospective analysis of the adaptive response to 12 (n = 110) and 24 (n = 85) weeks of supervised resistance-type exercise training in older (>65 years) men and women. Measurements Lean body mass (DXA), type I and type II muscle fiber size (biopsy), leg strength (1-RM on leg press and leg extension), and physical function (chair-rise time) were assessed at baseline, and after 12 and 24 weeks of resistance-type exercise training. Results Lean body mass increased by 0.9 ± 0.1 kg (range: -3.3 to +5.4 kg; P <.001) from 0 to 12 weeks of training. From 0 to 24 weeks, lean body mass increased by 1.1 ± 0.2 kg (range: -1.8 to +9.2 kg; P <.001). Type I and II muscle fiber size increased by 324 ± 137 µm2 (range: -4458 to +3386 µm2; P = .021), and 701 ± 137 µm2 (range: -4041 to +3904 µm2; P <.001) from 0 to 12 weeks. From 0 to 24 weeks, type I and II muscle fiber size increased by 360 ± 157 µm2 (range: -3531 to +3426 µm2; P = .026) and 779 ± 161 µm2 (range: -2728 to +3815 µm2; P <.001). The 1-RM strength on the leg press and leg extension increased by 33 ± 2 kg (range: -36 to +87 kg; P <.001) and 20 ± 1 kg (range: -22 to +56 kg; P <.001) from 0 to 12 weeks. From 0 to 24 weeks, leg press and leg extension 1-RM increased by 50 ± 3 kg (range: -28 to +145 kg; P <.001) and 29 ± 2 kg (range: -19 to +60 kg; P <.001). Chair-rise time decreased by 1.3 ± 0.4 seconds (range: +21.6 to -12.5 seconds; P = .003) from 0 to 12 weeks. From 0 to 24 weeks, chair-rise time decreased by 2.3 ± 0.4 seconds (range: +10.5 to -23.0 seconds; P <.001). Nonresponsiveness was not apparent in any subject, as a positive adaptive response on at least one training outcome was apparent in every subject. Conclusions A large heterogeneity was apparent in the adaptive response to prolonged resistance-type exercise training when changes in lean body mass, muscle fiber size, strength, and physical function were assessed in older men and women. The level of responsiveness was strongly affected by the duration of the exercise intervention, with more positive responses following more prolonged exercise training. We conclude that there are no nonresponders to the benefits of resistance-type exercise training on lean body mass, fiber size, strength, or function in the older population. Consequently, resistance-type exercise should be promoted without restriction to support healthy aging in the older population.
    Synergistic stiffening in double-fiber networks
    Rombouts, W.H. ; Giesbers, M. ; Lent, J.W.M. van; Wolf, F.A. de; Gucht, J. van der - \ 2014
    Biomacromolecules 15 (2014)4. - ISSN 1525-7797 - p. 1233 - 1239.
    gelation properties - living cells - hydrogels - gels - copolymers - dipeptides - strength
    Many biological materials are composite structures, interpenetrating networks of different types of fibers. The composite nature of such networks leads to superior mechanical properties, but the origin of this mechanical synergism is still poorly understood. Here we study soft composite networks, made by mixing two self-assembling fiber-forming components. We find that the elastic moduli of the composite networks significantly exceed the sum of the moduli of the two individual networks. This mechanical enhancement is in agreement with recent simulations, where it was attributed to a suppression of non-affine deformation modes in the most rigid fiber network due to the reaction forces in the softer network. The increase in affinity also causes a loss of strain hardening and an increase in the critical stress and stain at which the network fails.
    Effect of resistance-type exercise training with or without protein supplementation on cognitive functioning in frail and pre-frail elderly: Secondary analysis of a randomized, double-blind, placebo-controlled trial
    Rest, O. van de; Zwaluw, N.L. van der; Tieland, C.A.B. ; Adam, J.J. ; Hiddink, G.J. ; Loon, L.J.C. van; Groot, C.P.G.M. de - \ 2014
    Mechanisms of Ageing and Development 136-137 (2014). - ISSN 0047-6374 - p. 85 - 93.
    older-adults - alzheimers-disease - physical-activity - aerobic exercise - dietary-protein - performance - memory - strength - metaanalysis - impairment
    Physical activity has been proposed as one of the most effective strategies to prevent cognitive decline. Protein supplementation may exert an additive effect. The effect of resistance-type exercise training with or without protein supplementation on cognitive functioning in frail and pre-frail elderly people was assessed in a secondary analysis. Two 24-week, double-blind, randomized, placebo-controlled intervention studies were carried out in parallel. Subjects performed a resistance-type exercise program of two sessions per week (n = 62) or no exercise program (n = 65). In both studies, subjects were randomly allocated to either a protein (2 × 15 g daily) or a placebo drink. Cognitive functioning was assessed with a neuropsychological test battery focusing on the cognitive domains episodic memory, attention and working memory, information processing speed, and executive functioning. In frail and pre-frail elderly, resistance-type exercise training in combination with protein supplementation improved information processing speed (changes in domain score 0.08 ± 0.51 versus -0.23 ± 0.19 in the non-exercise group, p = 0.04). Exercise training without protein supplementation was beneficial for attention and working memory (changes in domain scores 0.35 ± 0.70 versus -0.12 ± 0.69 in the non-exercise group, p = 0.02). There were no significant differences among the intervention groups on the other cognitive tests or domain scores.
    Low vitamin D status is associated with reduced muscle mass and impaired physical performance in frail elderly people
    Tieland, C.A.B. ; Brouwer, E.M. ; Nienaber-Rousseau, C. ; Loon, L.J.C. van; Groot, C.P.G.M. de - \ 2013
    European Journal of Clinical Nutrition 67 (2013). - ISSN 0954-3007 - p. 1050 - 1055.
    randomized controlled-trial - parathyroid-hormone levels - nursing-home residents - older men - women - population - determinants - strength - calcium - 25-hydroxyvitamin-d
    Background/Objectives: Serum 25-hydroxyvitamin D (25(OH)D) status has been associated with muscle mass, strength and physical performance in healthy elderly people. Yet, in pre-frail and frail elderly people this association has not been studied. The objective of this study was to explore the association between vitamin D intake and serum 25(OH)D status with muscle mass, strength and physical performance in a pre-frail and frail elderly population. Subjects/Methods: This cross-sectional study included 127 pre-frail and frail elderly people in The Netherlands. Whole body and appendicular lean mass (ALM) (dual energy X-ray absorptiometry), leg strength (one repetition maximum), handgrip strength and physical performance (short physical performance battery) were measured, and blood samples were collected for the assessment of serum 25(OH)D status (liquid chromatography-tandem mass spectrometry). In addition, habitual dietary intake (3-day food records) and physical activity data (accelerometers) were collected. Results: In total, 53% of the participants had a serum 25(OH)D level below 50¿nmol/l. After adjustment for confounding factors, 25(OH)D status was associated with ALM (ß=0.012, P=0.05) and with physical performance (ß=0.020, P0.05). Conclusion: In this frail elderly population, 25(OH)D status is low and suggests a modest association with reduced ALM and impaired physical performance. In addition, vitamin D intake tended to be associated with impaired physical performance. Our findings highlight the need for well-designed intervention trials to assess the impact of vitamin D supplementation on 25(OH)D status, muscle mass and physical performance in pre-frail and frail elderly people
    Less is more: The effect of multiple implementation intentions targeting unhealthy snacking habits
    Verhoeven, A.A.C. ; Adriaanse, M.A. ; Ridder, D.T.D. de; Vet, E.W.M.L. de; Fennis, B.M. - \ 2013
    European Journal of Social Psychology 43 (2013)5. - ISSN 0046-2772 - p. 344 - 354.
    behavior-change - goal achievement - personal goals - past behavior - strength - plans - metaanalysis - breaking - number - power
    Implementation intentions have been shown to effectively change counter-intentional habits. Research has, however, almost solely been concerned with the effectiveness of a single plan. In the present research, we investigated the behavioral and cognitive implications of making multiple implementation intentions targeting unhealthy snacking habits and its underlying processes, linking multiple habitual snacking cues to healthy alternatives. Study 1 revealed that formulating multiple implementation intentions was not effective in decreasing unhealthy snacking, whereas formulating a single plan successfully induced behavior change. By using a lexical decision task in Study 2, it was found that when making a single plan, but not multiple plans, the healthy alternative became cognitively more accessible in response to a critical cue prime than the habitual response. However, when making additional plans in an unrelated domain, the negative effects of making multiple plans were absent. In sum, the current findings suggest that formulating multiple implementation intentions is ineffective when changing unwanted behavior. These reduced effects of multiple implementation intentions do not occur when making the plan but are rather due to interference in the enacting phase of the planning process.
    Self-reported attitude scales: current practice in adequate assessment of reliability, validity, and dimensionality
    Hendrick, T.A.M. ; Fischer, A.R.H. ; Tobi, H. ; Frewer, L.J. - \ 2013
    Journal of Applied Social Psychology 43 (2013)7. - ISSN 0021-9029 - p. 1538 - 1552.
    parametric statistics - resisting persuasion - implicit - model - strength - constructs - association - consistency
    The development of methods to create self-reported attitude scales has lost momentum, in part because of increased research focused on implicit measures. This paper reviews 162 papers on methodological approaches applied to the validation and assessment of attitude scales. Assessment of methodological approaches applied indicates that neither reliability, validity, nor dimensionality assessments are consistently used according to standard operating procedures or in accordance with best practice.Within current practices in the field of attitude scale development, the full potential of self-report scales is not met, in part because of such methodological issues. The improvement of existing practices and adoption of promising new developments in attitude scale construction and evaluation are discussed, together with recommendations for best practice in scale validation.
    Structures, stresses, and fluctuations in the delayed failure of colloidal gel
    Lindstrom, S.B. ; Kodger, T.E. ; Sprakel, J.H.B. ; Weitz, D.A. - \ 2012
    Soft Matter 8 (2012)13. - ISSN 1744-683X - p. 3657 - 3664.
    fracture - force - suspensions - particles - adhesion - lifetime - strength - network - fluid - bonds
    Sample-spanning networks of aggregated colloidal particles have a finite stiffness and deform elastically when subjected to a small shear stress. After some period of creep, these gels ultimately suffer catastrophic failure. This delayed yielding is governed by the association and dissociation dynamics of interparticle bonds and the strand structure of the gel. We derive a model which connects the kinetics of the colloids to the erosion of the strand structure and ultimately to macroscopic failure. Importantly, this model relates time-to-failure of the gel to an applied static stress. Model predictions are in quantitative agreement with experiments. It is predicted that the strand structure, characterized by its mesh size and strand coarseness, has a significant impact on delay time. Decreasing the mesh size or increasing the strand thickness makes colloidal gels more resilient to delayed yielding. The quench and flow history of gels modifies their microstructures. Our experiments show that a slow quenching increases the delay time due to the coarsening of the strands; by contrast, preshear reduces the delay time, which we explain by the increased mesh size as a result of shear-induced fracture of strands.
    Soil compaction effects on growth and root traits of tobacco depend on light, water regime and mechanical stress
    Alameda, D. ; Anten, N.P.R. ; Villar, R. - \ 2012
    Soil & Tillage Research 120 (2012). - ISSN 0167-1987 - p. 121 - 129.
    hydraulic-properties - seedling survival - strength - drought - maize - impedance - responses - plants - yield - availability
    Soil compaction can strongly affect plant performance as many other stress factors. In nature, many combinations of different stress factors may be found. We expect that the effects of soil compaction may be different depending of the occurrence of other stress. This has not been fully investigated; most studies have included only one stress factor together with soil compaction. In this study, we combine soil compaction with the interaction of shade, low water availability and mechanical stress. We use as a model system tobacco plants (Nicotiana tabacum), in which the effects of the combination of these factors in a greenhouse experiment were studied on their growth, biomass allocation, root morphology and anatomy. Soil compaction effects on growth and root traits depended strongly on the other factors. In unstressed conditions, plant growth increased with compaction up to 1.4 g cm-3 bulk density and then declined. However, at low water and under mechanical stress plant growth declined monotonically with compaction, while under shade, soil compaction had no effect on growth. Soil compaction reduced fine root proportion in all treatments except in shade condition, while it increased root diameter and xylem area only under mechanical stress. These results indicate that analyses of soil compaction effects on plant performance should take the levels of other stress factors into account. More generally, they illustrate the difficulty of interpreting effects of a given stress factor on plants as these effects tend to interact with presence of other stressors
    Protein supplementation increases muscle mass gain during prolonged resistance-type exercise training in frail elderly people: a randomized double-blind, placebo-controlled trial
    Tieland, C.A.B. ; Dirks, M.L. ; Zwaluw, N.L. van der; Verdijk, L. ; Rest, O. van de; Groot, C.P.G.M. de; Loon, L.C. van - \ 2012
    Journal of the American Medical Directors Association 13 (2012)8. - ISSN 1525-8610 - p. 713 - 719.
    lower-extremity function - body-composition - older-adults - physical function - men - sarcopenia - performance - disability - ingestion - strength
    Objectives Protein supplementation has been proposed as an effective dietary strategy to augment the skeletal muscle adaptive response to prolonged resistance-type exercise training in elderly people. Our objective was to assess the impact of protein supplementation on muscle mass, strength, and physical performance during prolonged resistance-type exercise training in frail elderly men and women. Design/setting/participants A randomized, double-blind, placebo-controlled trial with 2 arms in parallel among 62 frail elderly subjects (78 ± 1 year). These elderly subjects participated in a progressive resistance-type exercise training program (2 sessions per week for 24 weeks) during which they were supplemented twice daily with either protein (2 * 15 g) or a placebo. Measurements Lean body mass (DXA), strength (1-RM), and physical performance (SPPB) were assessed at baseline, and after 12 and 24 weeks of intervention. Results Lean body mass increased from 47.2 kg (95% CI, 43.5–50.9) to 48.5 kg (95% CI, 44.8–52.1) in the protein group and did not change in the placebo group (from 45.7 kg, 95% CI, 42.1–49.2 to 45.4 kg, 95% CI, 41.8–48.9) following the intervention (P value for treatment × time interaction = .006). Strength and physical performance improved significantly in both groups (P = .000) with no interaction effect of dietary protein supplementation. Conclusions Prolonged resistance-type exercise training represents an effective strategy to improve strength and physical performance in frail elderly people. Dietary protein supplementation is required to allow muscle mass gain during exercise training in frail elderly people. Trial Registration:clinicaltrials.gov identifier: NCT01110369
    A statistical analysis of fibre size and shape distribution after compounding in composites reinforced by natural fibres
    Moigne, N. Le; Oever, M.J.A. van den; Budtova, T. - \ 2011
    Composites Part A: Applied Science and Manufacturing 42 (2011)10. - ISSN 1359-835X - p. 1542 - 1550.
    flax straw fibers - mechanical-properties - polypropylene composites - polymers - length - orientation - wheat - microstructure - morphology - strength
    Using high resolution optical microscopy coupled with image analysis software and statistical methods, fibre length and aspect ratio distributions in polypropylene composites were characterized. Three types of fibres, flax, sisal and wheat straw, were studied. Number and surface weighted distributions were used to demonstrate the presence and amount of elementary fibres, fibre bundles and particles. A large number of small particles that are usually not taken into account were found. The obtained fibre length and aspect ratio distributions were successfully fitted by a two-parameter Weibull model. No significant effect of fibre concentration on the length and aspect ratio distributions was detected for 20% and 40% flax-based composite. Fibre type was found to be a very important parameter influencing size and shape distributions: flax is broken into long elementary fibres, sisal-based composite contains non-dispatched bundles and elementary fibres and wheat straw-based composite has bundles and large and small particles.
    Tensile fracture properties of seven tropical grasses at different phenological stages
    Jacobs, A.A.A. ; Scheper, J.A. ; Benvenutti, M.A. ; Gordon, I.J. ; Poppi, D.P. ; Elgersma, A. - \ 2011
    Grass and Forage Science 66 (2011)4. - ISSN 0142-5242 - p. 551 - 559.
    particle breakdown - foraging behavior - pasture grasses - residual stems - feed-intake - cattle - strength - ruminants - sward - resistance
    The intake of forage grasses by grazing ruminants is closely related to the mechanical fracture properties of grasses. The relationship between the tensile fracture properties of grasses and foraging behaviour is of particular importance in tropical reproductive swards composed of both stems and leaves. This study (i) quantified and compared the tensile fracture properties of stems and leaves of seven tropical grass species and (ii) provided insight into the underlying plant traits that explain differences in fracture properties between species. Fracture force, tensile strength, fracture energy and toughness of stems (in various phenological stages) and leaves were measured and compared among five introduced tropical grasses (Cenchrus ciliaris, Chloris gayana, Digitaria milanjiana, Megathyrsus maximus (syn. Panicum maximum), Setaria sphacelata) and two native tropical grasses (Setaria surgens and Dichanthium sericeum). Species differed significantly in fracture force and fracture energy, with stems and leaves of C. ciliaris and S. surgens requiring less force and energy to fracture and stems and leaves of M. maximus and S. sphacelata requiring more force and energy to fracture in comparison with the other species. Differences in tensile strength and toughness were less pronounced. The differences among species in fracture force and energy mainly resulted from differences in cross-sectional area of plant parts rather than from differences in tensile strength and toughness.
    Modulation of rheological properties by heat-induced aggregation of whey protein solution
    Purwanti, N. ; Smiddy, M. ; Goot, A.J. van der; Vries, R.J. de; Alting, A. ; Boom, R.M. - \ 2011
    Food Hydrocolloids 25 (2011)6. - ISSN 0268-005X - p. 1482 - 1489.
    beta-lactoglobulin - induced gelation - disulfide bonds - particle-size - isolate - ph - polymers - strength - gels
    Heat-induced protein aggregation at low protein concentrations generally leads to higher viscosities. We here report that aggregated protein can yield weaker gels than those from native protein at the same concentration. Aggregated protein was produced by heating a solution of whey protein isolate (WPI) at 3% and 9% w/w. The higher protein concentration resulted in a larger aggregate size and a higher intrinsic viscosity. The protein fraction in native WPI had the smallest size and the lowest intrinsic viscosity. The same trend was observed for the shear viscosity after concentrating the suspensions containing aggregates to around 15% w/w. Suspensions containing aggregates that were produced from a higher concentration possessed a higher viscosity. After reheating the concentrated suspensions, the suspension from the 9% w/w aggregate system produced the weakest gel, followed by the one from 3% w/w, while the native WPI yielded the strongest gel. Reactivity of the aggregates was also an important factor that influenced the resulting gel properties. We conclude that aggregation of whey protein solution is a feasible route to manipulate the gel strength of concentrated protein systems, without having to alter the concentration of the protein.
    Experimental evidence for interference competition in oystercatchers, Haematopus oastralegus. II. Free-living birds
    Rutten, A.L. ; Oosterbeek, K. ; Verhulst, S. ; Dingemanse, N.J. ; Ens, B.J. - \ 2010
    Behavioral Ecology 21 (2010)6. - ISSN 1045-2249 - p. 1261 - 1270.
    wintering oystercatchers - spatial-distribution - human disturbance - resource density - seasonal-changes - foraging waders - mytilus-edulis - turdus-merula - dominance - strength
    Field studies of interference competition in free-living animals have relied on natural fluctuations in forager density, which are often confounded with other factors. We therefore experimentally studied interference in the wild, capitalizing on 2 cockle beds in an isolated bay that were exploited by a population of individually marked oystercatchers. We successfully increased forager density by chasing birds from one cockle bed, leaving the other cockle bed as the only nearby alternative. The density increase was most pronounced on the eastern cockle bed where food stocks were poorer and initial feeding densities were lower compared with the western cockle bed. Oystercatchers residing on this eastern bed suffered a significant decline in intake rate when bird density was experimentally increased, providing evidence of interference. “Refugee” birds, that is, the birds that were displaced from their home bed, experienced an even stronger reduction in intake rate compared with the residents and compared with their intake rate on their “home bed,” probably partly due to the fact that the refugee birds were forced to feed on an unfamiliar cockle bed. The fact that disturbance at 1 site influenced both the refugees and the local resident birds indicates that human disturbance (an important conservation issue) has an effect that extends beyond the site where the disturbance takes place. The benefits and costs of site fidelity and interference competition are likely to play an important role in understanding animal distributions and how they change in response to environmental perturbations, including human disturbance of foraging sites.
    Natuurlijke vezelversterkte composieten
    Oever, M.J.A. van den - \ 2010
    natuurlijke vezels - polymeren - stijven - sterkte - biobased economy - composieten - natural fibres - polymers - sizing - strength - biobased economy - composite materials
    De specifieke eigenschappen van natuurlijke vezels maakt ze geschikt voor toepassing in vezelversterkte polymeren (composieten) met een hoge sterkte-stijfheid en een lage dichtheid. Momenteel bestaan er op commerciële schaal vier combinaties van verwerkingsmethoden en toepassingen van natuurlijke vezelcomposieten.
    Fracture and Self-Healing in a Well-Defined Self-Assembled Polymer Network
    Skrzeszewska, P.J. ; Sprakel, J.H.B. ; Wolf, F.A. de; Fokkink, R.G. ; Cohen Stuart, M.A. ; Gucht, J. van der - \ 2010
    Macromolecules 43 (2010)7. - ISSN 0024-9297 - p. 3542 - 3548.
    hydrogels - rheology - strength - gels
    We studied shear-induced fracture and self-healing of well-defined transient polymer networks formed by telechelic polypeptides, with nodes formed by collagen-like triple helices. When these gels are sheared at a rate that is higher than the inverse relaxation time of the nodes, fracture occurs at a critical stress which increases logarithmically with increasing shear rate. When a constant stress is applied, fracture occurs after a delay time that decreases exponentially with increasing stress. These observations indicate that fracture in these systems is due to stress-activated rupture of triple-helical junctions. After rupture, the physical gels heal completely.
    Extracellular Polymeric Substances (EPS) in Upflow Anaerobic Sludge Blanket (UASB) Reactors Operated under High Salinity Conditions
    Ismail, S. ; Parra, C.J. de la; Temmink, B.G. ; Lier, J.B. van - \ 2010
    Water Research 44 (2010)6. - ISSN 0043-1354 - p. 1909 - 1917.
    afvalwaterbehandeling - zoutgehalte - kweekmedia - deeltjesgrootteverdeling - anaërobe afbraak - korrelslib - waste water treatment - salinity - culture media - particle size distribution - anaerobic digestion - granular sludge - activated-sludge - waste-water - microbial granules - calcium - performance - stability - strength - impact - growth
    Considering the importance of stable and well–functioning granular sludge in anaerobic high rate reactors, a series of experiments were conducted to determine the production and composition of EPS in high sodium concentrations wastewaters pertaining to anaerobic granule properties. The UASB reactors were fed with either fully acidified substrate (FAS) consisting of an acetate medium (reactor R1) or partly acidified substrate (PAS) consisting of acetate, gelatine and starch medium (reactors R2, R3, and R4). For EPS extraction, the cation exchange resin (CER) method was used. Strength and particle size distribution were determined by assessing the formation of fines sludge under conditions of high shear rate and by laser diffraction, respectively. Batch tests were performed in 0.25 L bottles to study Ca2+ leaching from anaerobic granular sludge when incubated in 20 g Na+/L in absence of feeding for 30 days. Results show a steady increase in the bulk liquid Ca2+ concentration during the incubation period. UASB reactor results show that the amounts of extracted proteins were higher from reactors R2 and R3, fed with PAS compared to the sludge samples from reactor R1, fed with FAS. Strikingly, the amount of extracted proteins also increased for all reactor sludges, irrespective the Na+ concentration applied in the feed, i.e. 10 or 20 gNa+/L. PAS–grown granular sludges showed an important increase in particle size during the operation of the UASB reactors. Results also show that, addition of 1 gCa2+/L to the high salinity wastewater increases the granules strength
    Influencia de tres tipos de vegetación riberena en el control de la erosión fluvial en Pantanos de Centla, México
    Sepúlveda, A. ; Geissen, V. ; Ochoa-Gaona, S. ; Jarquin-Sanchez, A. ; Hernandez de la Cruz, S. ; Zamora-Cornelio, L.F. - \ 2009
    Revista biologia tropical = International Journal of Tropical Ecology and Conservation 57 (2009)4. - ISSN 0034-7744 - p. 1153 - 1163.
    streambank erosion - soil erodibility - bank erosion - detachment rate - strength - dynamics
    Influence of three types of riparian vegetation on fluvial erosion control in Pantanos de Centla, Mexico. Wetlands constitute very important ecological areas. The aim of this study was to quantify the soil losses due to fluvial erosion from 2006 to 2008 in two riverbanks under three types of vegetal coverage dominated by Haematoxylum campechianum, Dalbergia brownei and Brachiaria mutica, in the Pantanos de Centla Biosphere Reserve, SE Mexico. The relationship between the texture, organic matter and pH of soils and soil losses was evaluated. We used erosion sticks to estimate soil losses in 18 plots (three plots per type, three vegetation types, two riverbanks). Soil loss decreased in this order: H. campechianum>B. mutica>D. brownei indicating that D. brownei scrubland has the most potential to retain soil. The higher erosive impact within H. campechianum sites can be related with the low density of these trees in the study areas, as well as the lack of association with other types of vegetation that could reinforce the rooting of the soil profile. Furthermore, soil losses in H. campechianum sites were dependent on soil texture. The soils under this type of vegetal coverage were mainly sandy, which are more vulnerable to the erosive action in comparison with fine textured soils or soils with higher clay content, like the ones found in D. brownei and B. mutica sites. Soil losses of 100 % in the second year ( B. mutica plots) can be attributed to the distribution of roots in the upper soil layer and also to livestock management along riverbanks. This study recognizes the importance of D. brownei scrublands in riverbank soil retention. Nevertheless it is necessary to consider the role of an entire vegetal community in future research. Rev. Biol. Trop. 57 (4): 1153-1163. Epub 2009 December 01.
    Project SBW: waterkeringen beter leren kennen
    Frissel, J.Y. - \ 2009
    Visie : Deltares magazine 2009 (2009)1. - p. 7 - 8.
    dijken - sterkte testen - sterkte - bescherming - dykes - strength testing - strength - protection
    In februari en maart werd de Afsluitdijk bestookt met bakken water. Duizenden liters per dag stroomden over de kruin heen. Van watersnood was geen sprake, wel van een proef in het kader van het programma Sterkte & Belastingen Waterkeringen (SBW). Want hoe sterk zijn ze eigenlijk, de pakweg 3.500 km aan dijken, duinen, keringen en sluizen die Nederland beschermen tegen overstromingen? En met welke waterkrachten krijgen ze te maken?
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